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Page 1: A Quick Look at Biochemistry Carbohydrate Metabolism(ELENA1)

This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/authorsrights

Page 2: A Quick Look at Biochemistry Carbohydrate Metabolism(ELENA1)

Author's personal copy

Review

A quick look at biochemistry: Carbohydrate metabolism

Monireh Dashty ⁎Department of Cell Biology, University Medical Center Groningen, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands

a b s t r a c ta r t i c l e i n f o

Article history:Received 17 October 2012Received in revised form 18 April 2013Accepted 20 April 2013Available online 14 May 2013

Keywords:CarbohydrateGlycolysisPyruvate decarboxylationOxidative pathwayPentose phosphate pathwayGlycogenesisGlycogenolysisGluconeogenesis

In mammals, there are different metabolic pathways in cells that break down fuel molecules to transfer theirenergy into high energy compounds such as adenosine-5′-triphosphate (ATP), guanosine-5′-triphosphate(GTP), reduced nicotinamide adenine dinucleotide (NADH2), reduced flavin adenine dinucleotide (FADH2)and reduced nicotinamide adenine dinucleotide phosphate (NADPH2). This process is called cellular respira-tion. In carbohydrate metabolism, the breakdown starts from digestion of food in the gastrointestinal tractand is followed by absorption of carbohydrate components by the enterocytes in the form of monosaccha-rides. Monosaccharides are transferred to cells for aerobic and anaerobic respiration via glycolysis, citricacid cycle and pentose phosphate pathway to be used in the starvation state. In the normal state, the skeletalmuscle and liver cells store monosaccharides in the form of glycogen. In the obesity state, the extra glucose isconverted to triglycerides via lipogenesis and is stored in the lipid droplets of adipocytes. In the lipotoxicitystate, the lipid droplets of other tissues such as the liver, skeletal muscle and pancreatic beta cells alsoaccumulate triacylglycerol. This event is the axis of the pathogenesis of metabolic dysregulation in insulinresistance, metabolic syndrome and type 2 diabetes. In this paper a summary of the metabolism of carbohy-drates is presented in a way that researchers can follow the biochemical processes easily.

© 2013 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1340Descriptions of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1342Carbohydrate digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1342Fructose metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1342Galactose metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1343Glucose oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1343

Non-oxidative pathway (glycolysis) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1343Pyruvate decarboxylation (oxidative decarboxylation reaction) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1343Oxidative pathway (Krebs cycle and OXPHOS reaction (electron transport chain)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1344

Glycolysis (Embden–Meyerhof–Parnas pathway) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345Pyruvate dehydrogenase (PDH) or oxidative decarboxylation reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345

Regulation of PDH complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1346Krebs cycle (citric acid or tricarboxylic acid (TCA) cycle) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1346

Malate-aspartate shuttle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1346Energy balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1347TCA regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1347

Glycogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1347Glycogenolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1349

Phosphorylase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1349Pentose phosphate pathway (PPP), phosphogluconate pathway or hexose monophosphate shunt . . . . . . . . . . . . . . . . . . . . . . . . 1349

Clinical Biochemistry 46 (2013) 1339–1352

⁎ Department of Cell Biology, University Medical Center Groningen, University of Groningen, Antonius Deusinglaan 1, 9713 AV, Groningen, The Netherlands. Fax: +31 503632522.

E-mail address: [email protected].

0009-9120/$ – see front matter © 2013 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.clinbiochem.2013.04.027

Contents lists available at ScienceDirect

Clinical Biochemistry

j ourna l homepage: www.e lsev ie r .com/ locate /c l inb iochem

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Gluconeogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1350Cori cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1350Glucose-alanine cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1350Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1351Glycerol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1351Role of renal gluconeogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1351Regulation of gluconeogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1351

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1351Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1352References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1352

Introduction

The study of biochemistry is always one of the most complicatedtopics among medical and biology students in spite of being one of the

main subjects during their study. In addition, there seems to be a shortageof papers on pure biochemistry for biology students to be used as refer-ence in their papers. In this paper, the main topics and definitions of car-bohydrate pathways are summarized and the relatedfigures are shown in

Graphic abstract figure. Presentation of the association between 6 biochemical pathways. The biochemical reactions happen in the cytoplasm or mitochondria;therefore, a close interaction between these two parts always exists. These interactions are mostly, regulated by transporters that are in themitochondrial membrane(red ovals). The energetic pathways in the cell metabolize energetic molecules such as glucose and lipids. These pathways are glycolysis, gluconeogenesis, lipolysis,lipogenesis and the electron transport chain. Pyruvate (product of glycolysis) can be used in amino acid reactions as well. The energetic products of the oxidative re-action of glucose in themitochondria (NADH2 and FADH2) are delivered to the inner mitochondrial intermembrane (IMM) for complete conversion of their potentialenergy to chemical energy in the form of ATP. ATP is the energetic parcel in cells that is used in different biochemical reactions. In this figure, the main energetic bio-chemical reactions and their association to each other are illustrated. These reactions are 1. Glycerol phosphate shuttle, 2. NADH2 shuttle into IMM, 3. Transhydrogenasecycle (blue), 4. Citrate/malate cycle (pink), 5. Krebs cycle (red) and 6.Malate-aspartate shuttle (green). The correlative function of the lipid and carbohydrate biochemicalpathways together with the electron transport chain of the mitochondria for maintenance of glucose is also shown. Abbreviations are explained in figure 2.

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a way to facilitate the study of this topic and its link to other pathways.The subjects that are investigated here are listed below:

1. Carbohydrate digestion (in the intestine)2. Fructose metabolism (in the liver)

3. Galactose metabolism (in the liver)4. Glucose oxidation via glycolysis (in the cytoplasm), oxidative

decarboxylation reaction (in the mitochondria), citric acid cycle

Fig. 2. Abbreviations.

Fig. 1. Formulas of biochemical molecules. Abbreviations are explained in figure 2.

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(in the mitochondria) and the electron transport chain (ETC) (inthe inner mitochondrial membrane (IMM))

5. Glycogenesis (in the liver and skeletal muscle)6. Glycogenolysis (in the liver, skeletal muscle and kidney)7. Pentose phosphate pathway (in the liver, adipose tissue, adrenal cor-

tex, testis, milk glands, phagocyte cells and red blood cells (RBCs))8. Gluconeogenesis (in the liver, kidney, brain, testes and erythro-

cytes) [1].

Descriptions of figures

In this paper, the biochemical figures are shown in a new style inorder to facilitate the study and follow up of the pathways. Substratesand products are presented in brown, enzymes in blue, number of car-bons in each molecule in red, utilized materials at the left side andyielded materials at the right side. S is the stimulators of the enzymesand I is the inhibitors of the enzymes. Red arrows represent the inhibi-tory effects of agents. For interpretation of the references to color in thefigure legends, the reader is referred to the web version of this article.Graphic abstract figure presents the association between pathways.The general information of the pathways including formulas, abbrevia-tions and a summary of the biochemical reactions in carbohydratemetabolism are shown in figures 1, 2, 3 and table 1.

Carbohydrate digestion

Dietary carbohydrates of greatest importance are composed ofhexoses such as sucrose (saccharose or table sugar), lactose (milksugar), galactose (derived from fermented products) and maltose(derived from hydrolysis of starch) and also pentoses such as xyloseand arabinose (from fruits) [2]. Food digestion starts in the mouththrough secretion of salivary alpha-amylase (or ptyalin) that hydro-lyses alpha-1,4 (α-1,4) linkage of starch (or amylum) and converts ittomaltose. The next enzyme is pancreatic-amylase (or amylopsin) inthe small intestine that digests 60% of starches. Intestinal epithelialcell enzymes degrade 6-carbon (6C-) carbohydrates [3–5]. These en-zymes are lactase for degradation of lactose to glucose and galactose,

sucrase for degradation of sucrose to its constituent components(glucose and fructose) [6] and maltase for degradation of maltoseto two glucose molecules. The 5C-carbohydrates such as xylulose,arabinose, ribose and ribulose easily diffuse into the intestinal ab-sorptive cells and do not need degradation.

Absorption of the 6C-carbohydrates from intestinal epitheliumhappens in two ways: passive and active transport systems. In thepassive diffusion form, phosphorylation of carbohydrate (e.g., glucoseor galactose) in the intestinal cells leads to their facilitated transferto the circulation. Glucose-6-phosphate (G6P) and galactose-6-phosphate (Gal6P) are then dephosphorylated and enter the liver.In the active diffusion form, carbohydrates utilizing a mobile carrierprotein coupled with the sodium/potassium (Na+/K+) pump andagainst the gradient together with Na+ ion enter enterocytes [7].Therefore, the Na+/K+ pump using ATP as its source of energy ex-changes 3 Na+ ions with 3 K+ ions [7].

Galactose and fructose are converted to glucose in the liver. Glucoseis used in differentmetabolic pathways for (i) stability of blood sugar inthe hypoglycemic state, (ii) energy supplier of the peripheral tissues,(iii) energy storage in the liver and skeletal muscle in the form of gly-cogen to be used in exercise [8,9], (iv) energy storage in the adiposetissue following conversion to triglycerides (TG, TAG, triacylglycerolor triacylglyceride) [10] in the case of excess glucose and (v) stabilityof body temperature [11,12].

Fructose metabolism

Glucose is the main source of energy in cells; however, with highconsumption of sucrose (composed of glucose and fructose) cellscan use fructose as well. In the muscles, adipose tissue and kidney,which contain hexokinase (HK), fructose gets phosphorylated to be-come fructose-6-phosphate (F6P) to be directly used in the glycolysispathway. However, in the liver, which contains glucokinase (GK),fructose must first be converted to glucose for consumption in theglycolysis pathway. Therefore, fructose first gets phosphorylated andis then divided to two 3-carbon glycolysis intermediates: glyceraldehydeand dihydroxyacetone phosphate (DHAP). These intermediates can easily

Fig. 3. Summary of biochemical reactions in carbohydrate metabolism.

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be converted to each other. Thereafter, glyceraldehyde is phosphorylatedto become glyceraldehyde-3-phosphate (GA3P) and is again joinedto DHAD to form the double-phosphorylated product, fructose-1,6-bisphosphate (F1,6BP). Dephosphorylation of F1,6BP by phospha-tase produces F6P. F6P is either used as the substrate of the glycolysispathway or is converted to G6P using isomerase. G6P is used intwo ways. It either loses its phosphate with glucose-6-phosphatase(G6Pase) activity and is converted to glucose to enter the circulationor via utilizing mutase is converted to glucose-1-phosphate (G1P)(substrate of glycogen synthesis pathway). G6Pase exists only in theliver and kidney cells and not in the muscle cells (Figs. 4 and 5).

Galactose metabolism

Galactose enters the body following consumption of milk. Milksugar or lactose is composed of galactose and glucose. Lactose isconverted to its constituents with lactase activity in the brush borderof the small intestine. Galactose enters the blood stream following ab-sorption by enterocytes and enters the liver through the portal vein tobe metabolized and converted to glucose for consumption as energy.Glucose and galactose are the sugars whose active forms are transferredby the uridine diphosphate (UDP) coenzymes in the form of uridinediphosphate-glucose (UDPG) and UDP-galactose (UDPGal). Galactosemetabolism starts from phosphorylation and conversion of galactoseto galactose-1-phosphate (Gal1P). Thereafter, galactose-1-phosphateuridylyltransferase (GALT) and its UDPG coenzyme, exchange thegalactose of Gal1P with glucose to form G1P and as a result UDPGal is

released. UDPGal can be converted to UDPG with UDP-galactose-4-epimerase activity (Figs. 4 and 5).

Glucose oxidation

There are two non-oxidative and oxidative pathways that oxidizeglucose to prepare the energy source of cells. Oxidative decarboxylationreaction is the linker reaction between these two pathways.

Non-oxidative pathway (glycolysis)

This is an anaerobic respiration or pyruvic acid (pyruvate) fer-mentation that happens in the cytoplasm in the absence of oxygen.In glycolysis, partial oxidation of glucose produces pyruvic acid(Fig. 6). In anaerobic states (heavy exercise or cells withoutmitochondria), pyruvate is converted to lactate through a bilateralreaction catalyzed by lactate dehydrogenase (LDH) and consump-tion of one molecule NADH2 (Fig. 3/12). Lactate is transferred tothe liver for gluconeogenesis and glucose production to be usedagain in the glycolysis pathway (Cori cycle) (Fig. 7).

Pyruvate decarboxylation (oxidative decarboxylation reaction)

This reaction is the linker between glycolysis and the Krebs cycleand is catalyzed by pyruvate dehydrogenase complex (PDC) (Fig. 8).In this process, two pyruvic acids of the glycolysis pathway are con-sumed to produce two molecules of acetyl coenzyme A (acetyl-CoA)

Table 1A summary of carbohydrate biochemical pathways. Numbering in glycolysis, PDH reaction and the Krebs cycle is based on breakdown of one glucose molecule. VR: Vital reaction,C: Catabolic or exergonic reaction, A: Anabolic or endergonic reaction, Cyt: Cytoplasm,Mit: Mitochondria, 4C-P: Phosphorylated form of 4C-carbohydrate (erythrose-4P), 5C-P: Phosphor-ylated form of 5C-carbohydrate (xylulose-5P, riboluse-5P and ribose-5P), 7C-P: Phosphorylated form of 7C-carbohydrate (sedoheptulose-7P).

Pathway VR Place Substrate Product Utilized Yielded

Glycolysis (aerobic) C Cyt 1 Glucose 2 Pyruvate Mg2+ 2 ATP, 2 NADH2Glycolysis (anaerobic) C Cyt 1 Glucose 2 Lactic acid Mg2+ 2 ATPPDH reaction (aerobic) C Mit 2 Pyruvate 2 Acetyl-CoA Lipoic acid, 2 CoA 2 NADH2Krebs cycle (aerobic) C Mit 2 Acetyl-CoA, 2 OAA 2 Citrate 4 H2O, Fe2+, Mg2+, Mn2+, Fe/S 6 NADH2, 4 CO2, 2 GTP, 2 FADH2, 2 CoAElectron transport chain C Mit 1 NADH2 & 1 FADH2 3 ATP & 2 ATP Oxygen H2OGlycogenesis A Cyt Glucose Glycogen 2 ATP, Mg2+

Glycogenolysis C Cyt Glycogen G6P, glucose, F6PGluconeogenesis A Both 2 Pyruvate (or 2 lactate) G6P, glucose, F6P 2 HCO3

−, 4 NADH2, 4 ATP, 2 GTP, 2 H2O 2 NADPH2, 2 CO2, 6 PiFructose metabolism A Cyt Fructose G6P, glycogen 2ATP PiGalactose metabolism A Cyt Galactose G6P, lactose 1 ATP, UDPGPentose phosphate A Cyt G6P F6P, GA3P, 4C-P, 5C-P, 7C-P 1 H2O, TPP 2 NADPH2, 1 CO2, 1 H+

Fig. 4. Presentation of the association between carbohydrate metabolisms. Galactose, fructose, mannose and glucose monosaccharides have a close association with each other and alsowith the polysaccharide pathways including glycogenesis and glycogenolysis. In this figure, the roles of the main metabolic hormones on these pathways are also shown.Abbreviations are explained in figure 2.

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and 2 NADH2. NADH2 is consumed in the oxidative phosphorylation(OXPHOS) reaction. Acetyl-CoA is either used in the Krebs cycle(Fig. 9) and OXPHOS reaction in mitochondria for full oxidization ofglucose or is converted to citrate. Citrate is exported to the cytosol forbiosynthesis of fatty acid (FA) and isoprenoid.

PDC is composed of multiple copies of 3 separate enzymes, namelypyruvate dehydrogenase (PDH), dihydrolipoamide S-acetyltransferase(DLAT) and dihydrolipoamide dehydrogenase (DLD) and 5 coenzymes,namely coenzyme A (CoA or CoASH), NAD+, FAD+, lipoic acid andthiamine pyrophosphate (TPP). TPP, lipoic acid and FAD+ are tightlybound to the enzymes of the complex and CoA and NAD+ are thecarriers of the products of PDC.

Oxidative pathway (Krebs cycle and OXPHOS reaction (electron transportchain))

It is an aerobic respiration that happens in the mitochondria inthe presence of oxygen. In the Krebs cycle, two pyruvates of glycolysisare degraded to release their energy in the form of 2 GTP and the re-duced forms of high energy molecules (6 NADH2 and 2 FADH2).NADH2 and FADH2 are consumed in the respiratory chain process

of the mitochondria (OXPHOS reaction). These reduced coenzymes(NADH2 and FADH2) release their hydrogen (proton), which at theend of the process combines with the oxygen atom to producewater. The electron of the hydrogen has a high transfer potential en-ergy that gradually is transferred to oxygen through a series of en-zymes (complexes I to IV). In each step a small amount of its energy isreleased and the level of its energy fall down to a lower energy state. En-zymes of the electron transport system use the energy released fromthe oxidation of NADH2 and FADH2 to pump protons across the IMMto the intermembrane space (IMS). This generates a proton gradientacross the IMM. Reverse movement of protons to the mitochondrialmatrix across the IMM releases energy, which is used for creation ofATP (Graphic abstract figure). Therefore, the reducing potentials ofthe NADH2 and FADH2 are converted to chemical energy in the formof ATP. The energy, which is released from the OXPHOS reaction ofNADH2 is equal to 3 ATP and the energy of FADH2 is equal to 2 ATP.As a result, the final products of complete glucose oxidation are twowaste products (CO2 and H2O) and energy (Fig. 3/1). Succinate is alsooxidized by the electron transport chain, but feeds into the pathwayat a different point. In conclusion, during glucose oxidation most ofthe ATP, which is produced via aerobic cellular respiration, is made byOXPHOS reaction through a series of complex electron transfer process-es. Aerobic metabolism is 19 times more efficient than anaerobicmetabolism.

Fig. 5. Fructose and galactose metabolism. In overconsumption of sucrose and lactose,cells can also use fructose and galactose as their source of energy. These productscan be converted to G6P and F6P (the intermediates of the glycolysis pathway) andG1P (the substrate of glycogenesis). The figure and abbreviations are explained inthe text and in figure 2.

Fig. 6. Glycolysis. The non-oxidative glycolysis pathway is a cytoplasmic anaerobicpathway that happens in the absence of oxygen. It degrades the glucose molecule to two3C-pyruvate molecules and yields 2 ATP and one energetic molecule, NADH2. The figureand abbreviations are explained in the text and in figure 2.

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Glycolysis (Embden–Meyerhof–Parnas pathway)

Degradation of glucose for releasing its energy for the anabolic path-ways starts from glycolysis and continues to the Krebs or tricarboxylicacid (TCA) cycle in the mitochondria. Glycolysis is a cytoplasmicnon-oxidative reaction for glucose degradation that is composed of 9 pro-cesses. A non-specific HK enzyme by using ATP phosphorylates glucosefollowing entrance to the cell and converts it to G6P. In the liver both

HK as well as GK (the specific kinase for glucose substrate) exist. There-fore, in high concentration of glucose (>100mg/100ml) GK is also active.These enzymes are stimulated by insulin, adenosine monophosphate(AMP) and adenosine diphosphate (ADP) and inhibited by long chainfatty acids (LCFAs) and their ownproducts, G6P andATP.G6P is convertedto one 6C-F1,6BP molecule. Aldolase divides F1,6BP to two 3C-molecules,namely DHAP or glyceraldehyde 3 phosphate (GA3P). These two mole-cules can be converted to each other using triosephosphate isomerase(TPI). The high energy products of glycolysis pathway are ATP,1,3-di(or bis)phosphoglyceric acid (1,3D(B)PGA) and phosphoenolpyr-uvate (PEP) that produce 8 kcal, 10 kcal and 14 kcal energy respective-ly. Thefinal product of the glycolysis pathway is twopyruvatemoleculesthat are used in the PDH reaction (Fig. 3/2). The allosteric enzymes ofglycolysis are phosphofructokinase (PFK), pyruvate kinase (PK) andeither GK or HK depending on the tissue.

One glucose produces 8 ATP during glycolysis. Glycolysis hap-pens in two phases: preparatory and pay-off phases. In the prepa-ratory phase, HK and PFK consume 2 ATP molecules. In the pay-offphase glyceraldehyde-3-phosphate dehydrogenase (GA3PDH),phosphoglycerate kinase (PGK) and PK produce 10 ATP via pro-duction of 2 NADH2 (equal to 6 ATP) and 4 ATP (Fig. 6).

In anaerobic states, each pyruvate produces lactate and 2 ATP byutilizing one NADH2. Therefore, it produces 6 ATP less than the aerobicstate. Pyruvic acid is used in each of the following pathways and its finalfate is regulated by acetyl-CoA (Fig. 10).

1. Acetyl-CoA synthesis inside themitochondria during the PDH reaction2. Lactate synthesis inside the cytoplasm following LDH activity3. Alanine (Ala) synthesis during the transamination reaction4. Acetaldehyde synthesis after decarboxylation in bacteria, mice and

moles (Fig. 3/3)5. Ethanol synthesis via using alcohol dehydrogenase and NADH2

(Fig. 3/4)6. Oxaloacetic acid (OAA) synthesis after carboxylation by pyruvate

carboxylase (PC). OAA can be consumed in either the Krebs cycleor the gluconeogenesis (Figs. 3/5 and 3/7).

Pyruvate dehydrogenase (PDH) or oxidative decarboxylationreaction

This mitochondrial reaction is the process that happens followingglycolysis and preceding the Krebs cycle. PDH reaction utilizes

Fig. 7. Cori cycle and glucose-alanine cycle. These are the cycles that link glucose production in the liver to energy production in other tissues. In the Cori cycle, bilateral associationbetween glycolysis in the skeletal muscle cells with gluconeogenesis in the hepatocytes is shown. Waste product of the skeletal muscles (lactate) is used in the hepatocytes to pro-duce glucose for consumption as energy in the skeletal muscle. In the glucose-alanine cycle, the nitrogen product of non-hepatic tissues, produced from transamination reaction andamino acid production, is transferred to the hepatocytes to be used for either gluconeogenesis and glucose production or excretion as urea. Abbreviations are explained in figure 2.

Fig. 8. Pyruvate dehydrogenase (PDH) reaction. Thismitochondrial oxidative decarboxylationreaction is the linker between glycolysis and the Krebs cycle and produces acetyl-CoA.Acetyl-CoA can be used in different pathways including the Krebs cycle. In this pathway,one molecule of NADH2 is produced. NADH2 enters the OXPHOS process and releases 3ATP. The figure and abbreviations are explained in the text and in figure 2.

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coenzyme A (CoA or CoASH), thiamine pyrophosphate (TPP) and PDCto oxidize cytoplasmic pyruvate, which is transferred to the mitochon-dria by a carrier protein, to acetyl-CoA and CO2. In this reaction, onemolecule NAD+ is reduced to NADH2 (Fig. 3/6). In the absence of TPP,pyruvate is concentrated in the cytoplasm and converted to lactate(Fig. 10).

Regulation of PDH complex

The fate of pyruvate in the mitochondria depends on the level ofintracellular energy. In the absence of energy, pyruvate is used in theKrebs cycle to produce ATP, while in the excess of energy (resultedfrom high concentration of acetyl-CoA) pyruvate can be used in thegluconeogenesis pathway and the excess acetyl-CoA can be consumedin the lipogenesis pathway. Therefore, acetyl-CoA and PDC are the reg-ulators of glycolysis, gluconeogenesis, fatty acid oxidation (FAO thatproduce acetyl-CoA) and the Krebs cycle (Fig. 10) [13].

PDH kinase (PDK) phosphorylates and inhibits the PDC (Fig. 3/8).The products of the PDC (acetyl-CoA and NADH2) and ATP are the ac-tivators of PDK and the substrates of the PDC (pyruvate, NAD+ andCoASH), ADP, Mg2+ and Ca2+ are the inhibitors of PDK. In high con-centrations of ADP (or in shortage of energy), the phosphorylated

PDC loses its phosphate using PDH phosphatase and becomes active.Insulin stimulates this reaction and consequently the Krebs cycle.Mg2+ and Ca2+ are the stimulators of PDH phosphatase (Fig. 8).

The level of acetyl-CoA (product of PDH reaction) is the regulatorof PDK as well as PC (enzyme of gluconeogenesis) and the PDH itself(enzyme of PDH reaction). Acetyl-CoA is also the substrate of FA syn-thesis and the Krebs cycle. Therefore, in the high level of acetyl-CoA(or in excess of intracellular energy), gluconeogenesis (through OAAproduction) and FA synthesis pathways are activated.

Translocation of the acetyl-CoA from the mitochondria to the cyto-plasm happens via conversion to citrate. First, acetyl-CoA is convertedto citrate using citrate synthesis, and then citrate translocates to the cyto-plasm and is degraded back to acetyl-CoA using citrate lyase. In the cyto-plasm, acetyl-CoA carboxylase (ACC) converts acetyl-CoA to malonyl-CoA and proceeds toward the FA synthesis (Graphic abstract figure).Malonyl-CoA is the inhibitor of carnitine palmitoyltransferase I (CPT-I)and consequently FAO. This is the way that increased rate of glucose ox-idation results in inhibition of FAO. The reason for high regulation of thePDC is that there is no pathway in the body to convert acetyl-CoA to glu-cose. Adenosine monophosphate-activated protein kinase (AMPK) thatis the energy sensor of cells has the opposite effect on the function ofACC. Low energy in cells stimulates both glucose degradation and thefunction of AMPK. AMPK consequently inhibits ACC and decreases theconcentration of malonyl-CoA. As a result, FAO and production ofacetyl-CoA is increased.

Carbohydrates, lipids and amino acids are the substrates whichproduce acetyl-CoA. Acetyl-CoA is used in different biochemicalpathways including (i) the Krebs cycle to produce H2O, CO2 and energy,(ii) biosynthesis of cholesterol, squalene, bile acids, vitamin D3 andsteroidal hormones such as estrogen, progesterone, testosterone andadrenal hormones, (iii) synthesis of FAs, (iv) synthesis of ketone bodiessuch as acetone, beta-hydroxybutyric acid (BHB) and acetoacetic acid,in the liver in starvation and the diabetic states and (v) synthesis ofacetylcholine that is the carrier of nerve impulses (Fig. 10).

Krebs cycle (citric acid or tricarboxylic acid (TCA) cycle)

The Krebs cycle is an aerobic biodegradation process that starts fromcatabolism of acetyl-CoA to produce the reduced coenzymes (NADH2and FADH2) and CO2. OAA (derived from pyruvate carboxylation) isthe first substrate of the Krebs cycle. OAA joins to acetyl-CoA to formcitric acid (CA). Therefore, the source of OAA is sugar. In diabeticpatients, who have less sugar and pyruvate in their cells, the level ofOAA and consequently the activity of the Krebs cycle is low.

In the higher energy level, the rate of citrate synthesis increasesand the rate of flux via the Krebs cycle decreases. This excess citrateis then transferred to the cytosol and degraded to acetyl-CoA. There-after, acetyl-CoA can be used in the FA and cholesterol synthesis.Citrate also has a positive regulatory effect on the enzymes of lipo-genesis (e.g., ACC), and a negative regulatory effect on PFK1 (the en-zyme of glycolysis) (Fig. 6). Citrate is also required for ketogenesis inthe non-hepatic tissues.

Alpha-ketoglutarate (αKG) converts to succinyl-CoA via severalprocesses of the alpha-ketoglutarate dehydrogenase (αKGDH) com-plex (Fig. 9). Succinyl-CoA is a high energetic molecule and producesthe only GTP molecule of this process under the influence of succinatethiokinase. The energetic molecules produced in this oxidative decar-boxylation cycle are NADH2 and FADH2. These products release theirenergy during the OXPHOS reaction and consequently their energy isreserved as high energy phosphate in ATP.

Malate-aspartate shuttle

Themalate-aspartate shuttle is a process, which transfers electrons ofthe reducing equivalents of the glycolysis pathway (NADH2) fromcytoplasm to the mitochondria in the form of malate. In this process,

Fig. 9. Tricarboxylic acid (TCA) cycle or Krebs cycle. Themitochondrial oxidative Krebs cycle isa process, whereby the product of the PDH complex (acetyl-CoA) joins to OAA. In this pro-cess glucose is degraded to produce energetic molecules such as GTP, NADH2 and FADH2.These reduced products release their energy in the IMM and via the OXPHOS reaction.The figure and abbreviations are explained in the text and in figure 2.

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inter-conversion of the malate and OAA in the cytoplasmic membraneand OAA and aspartate in the mitochondrial membrane lead to transferof NADH2 from the cytosol to themitochondria (Graphic abstractfigure).

Energy balance

Theoretically, 38 ATP molecules are made from one molecule ofglucose during cellular respiration. 8 ATP are directly formed from theanaerobic glycolysis via degradation of one glucose molecule to twopyruvate molecules. In the pyruvate decarboxylation reaction, each3C-pyruvate produces one NADH2 (equal to 3 ATP). Therefore, each6C-glucose in this reaction produces 6 ATP. In the Krebs cycle, thetotal amount of energy that is made from each 3C-pyruvate is 12 ATP.This amount is produced at the level of isocitrate dehydrogenase(NADH2 = 3 ATP), α-ketoglutarate dehydrogenase (NADH2 = 3ATP), malate dehydrogenase (MDH) (NADH2 = 3 ATP), succinate de-hydrogenase (FADH2 = 2 ATP) and succinate thiokinase (GTP). Sinceeach 6C-glucose molecule produces two 3C-pyruvate molecules, thetotal amount of energy, which is made from one glucose is 24 ATP.The 8 ATP of glycolysis, 6 ATP of the PDH complex and 24 ATP of theKrebs cycle together produce 38 ATP.

Since the energy of each ATP is equal to 8 kcal, therefore the totalamount of the released energy is 304 kcal. This amount of obtainedenergy in the chemical form is equal to 44% of the total amount ofenergy which is measured by a calorimeter. Therefore, 56% of theenergy of glucose is released as heat to stabilize the body tempera-ture. This wasting energy is through proton loss via the leaky mito-chondrial membranes that flow back the protons from the IMS to thematrix. Moreover, moving pyruvate and ADP into the mitochondrialmatrix requires energy. Therefore, in reality each glucose molecule

produces 29 to 30 ATP molecules and this maximum amount is neveryielded.

TCA regulation

TCA is regulated through different ways including (i) allostericregulation by Ca2+, ATP and ADP, (ii) the level of acetyl-CoA (theentry substrate of the cycle), (iii) substrate availability (e.g., reducedOAA regulates citrate synthase), (iv) NAD+/NADH ratio (e.g., theTCA cycle needs NAD+ as cofactor) and (v) product inhibition (e.g.,citrate inhibits citrate synthase as well as NADH2 and succinyl-CoAinhibit αKGDH).

Glycogenesis

Glycogenesis is the process of glycogen synthesis. Glycogen is apolymer of glucose residues that is linked byα-1,4 andα-1,6 glycosidicbonds. Therefore, it is the glucose storage molecule in the hepatocytesand skeletal muscle cells. The total amount of glycogen storage amongthese two tissues depends on the mass of the hepatocytes and skeletalmuscle cells. Glycogen amount per mass unit of the liver is higherthan the skeletal muscle; however, since in body the total mass of theskeletal muscle is more than that of the liver, thus 2/3 of the totalamount of glycogen is stored in the muscle cells. Liver glycogen is forthe stability of the blood sugar to supply enough energy for other tissues(mainly the brain that consumes 75% of the blood glucose). However, gly-cogen in the skeletalmuscle is used just as their sources of energy becausemuscle cells don't have G6Pase. In glycogenesis, glucose is joined to a coreof glycogen via α-1,4 linkage. G1P converts to the active form of glu-cose (UDPG) using Glucosyl-1-phosphate uridyl-transferase (UGP2or UDPG pyrophosphorylase) and one uridine triphosphate (UTP)

Fig. 10. Presentation of the linking role of pyruvate, G6P and acetyl-CoA in carbohydrate and lipid pathways. In this figure, the interrelation between glycogenesis, glycogenolysis, glycolysis, glu-coneogenesis, the Krebs cycle, the urea cycle and hexose monophosphate (HMP) shunt (blue rectangle) is illustrated. In HMP shunt, G6P (product of glycogenolysis) is used in the pentosephosphate pathway to produce Xu5P. Xu5P is also formed from F6P andGA3P by transketolase activity. Xu5P activates PP2A, which is the inducer of PFK2 and ChREBP. ChREBP is a lipogenictranscription factor that which can induce triglyceride synthesis. Here also the linking role of pyruvate (product of glycolysis) and acetyl-CoA (product of PDH reaction) in carbohydrate andlipid pathways are highlighted. As is shown, acetyl-CoA regulates the fate of pyruvate, gluconeogenesis and lipogenesis and is the substrate for acetylcholine, ketone bodies, biliary acids,cholesterol, steroids and triglyceride synthesis. PDC is the main regulatory enzyme of acetyl-CoA. This complex is regulated by the PDH phosphatase and PDK enzymes. Ca2+ and Mg2+

are the activators of PDH phosphatase and the inhibitors of PDK. ATP, NADH2 and acetyl-CoA are the stimulators of PDK. The figure and abbreviations are explained in the text and figure 2.

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[14]. Consequently, one group of pyrophosphate is made. Active glu-cose joins to the core of glycogen via α-1/4 linkage using glycogensynthase to elongate the glycogen molecule. By this way, C1 of

glucose joins to the C4 of glycogen and UDP molecule is released.When the number of the joined glucose increases till 10–12 glucosemolecules in each branch, amylo-α-(1,4 ➔ 1,6)-transglycosylase

Fig. 12. Influence of glucagon on carbohydrate pathways. In the fasted state, glucagon stimulates both liver glycogenolysis via activation of glycogen phosphorylase and gluconeogenesis viastimulation of FBPase2 in order to elevate the blood sugar. Glucagon also inhibits glycogenesis via phosphorylation of glycogen synthase. Glucagon influences beta receptors and usesadenylate cyclase to stimulate conversion of ATP to cAMP. PKA is a protein kinase, which is cAMP-dependent and is activated through this receptor-mediated mechanism. The cAMP/PKA complex phosphorylates (and activates) phosphorylase kinase (a form), which phosphorylates (and activates) phosphorylase (a form). Both phosphoprotein kinase and phosphor-ylase are dephosphorylated (and inhibited-b form) by phosphoprotein phosphatase. The inhibitor of phosphoprotein phosphatase is PPI1. PPI1 dephosphorylates (and inhibits) phos-phorylase kinase (b form), phosphorylase (b form) and itself (b form). Phosphorylase kinase, phosphorylase and PPI1 are all activated (a-form) via phosphorylation by using ATP. PPI1is phosphorylated (and activated) by cAMP/PKA and dephosphorylated (and inhibited-b form) by phosphoprotein phosphatase. Phosphorylase kinase is also activated by calciumeven without phosphorylation. This allows acetylcholine to increase glycogenolysis via neuromuscular stimulation and without receptor stimulation. In the normal state and excessglucose, glycogenesis is activated and glycogenolysis is inhibited to reserve the glucose in the body. The figure and abbreviations are explained in the text and in figure 2.

Fig. 11. Glycogenesis and glycogenolysis. Regulation of blood glucose is in close associationwith the biochemical regulation of glycogen pathways. G6P and F6P (products of glycolysis) are thelinkers of glycolysis and glycogen pathways. The figure and abbreviations are explained in the text and in figure 2.

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(branching enzyme) picks 6- to 7-glucose molecules up from the branchand via α-1/6 linkage makes a new branch in the glycogen molecule(Fig. 11). Insulin is the activator of glycogen synthase.

Glycogenolysis

Glycogenolysis is the process of glycogen degradation. Glycogenoly-sis happens in the liver and kidney to produce glucose for balancing theblood sugar; however, it produces G6P in muscle cells to be used as theenergy supplier of myocytes.

Glycogen phosphorylase using inorganic phosphate group (Pi)hydrolyzes the α-1,4 glycosidic linkages of glycogen and produces G1P(or glucose in the heart). G1P is converted to G6P and thereafter toglucose using G6Pase in the liver and kidney. In the skeletal muscle dueto the absence of G6Pase, G6P converts to F6P to be used in glycolysis.

Phosphorylase continues degradation of the α-1,4 linkages untilonly four glucose molecules remain on the branch. Then threeglucose molecules are transferred to another glucose-branch viaglucan transferase (amylo-α-(1,4 ➔ 1,6) transferase) and the lastglucose molecule (α-1,6 linkage) is degraded with glucosidase(amylo-α-1,6-glucosidase) activity (Fig. 11). During exercise, adrena-line in the skeletal muscle and during hypoglycemia, glucagon in theliver is the stimulator of glycogenolysis. This happens by activating theglycogen phosphorylase. Therefore, glycogenolysis results in nutritionof the skeletal muscle cells and maintenance of blood glucose. Adrena-line and glucagon affect beta receptor activity and use adenylate cyclaseto convert ATP to cyclic AMP (cAMP) (Fig. 12).

Phosphorylase

Glycogen phosphorylase is a lytic enzyme in glycogenolysis. Phos-phorylase joins a phosphate group to the glucose residue of glycogenand catalyzes the phosphorylation reaction and consequently the cleav-age of glucose from glycogen in the form of G1P (phosphorolysis

reaction). Phosphorylase is in either the active (R) form or the inactive(T) form. Liver phosphorylase is in dimeric form and skeletal musclephosphorylase is in tetrameric form (Fig. 3/9). Each monomer of thisenzyme is composed of the C-terminal domain, N-terminal domain, ac-tive site and pyridoxal phosphate (PLP, derived from vitamin B6) cofac-tor, which binds near the active site to facilitate the reaction. The activesite contains a serine amino acid that can be affected by the phosphor-ylase kinase and phosphorylase phosphatase. Phosphorylation of serineby phosphorylase kinase and using ATP produces an activated form,whereby the monomers are attached to each other. This happens afterchange in the shape of phosphorylase and production of an ester link-age between OH of serine and the phosphoric acid. In addition, serinecan join to PLP (B6-al-PO4) and form an active phosphorylase. Phos-phorylase phosphatase inactivates phosphorylase via losing its phos-phate group.

Pentose phosphate pathway (PPP), phosphogluconate pathway orhexose monophosphate shunt

In the liver, adipose tissue, adrenal cortex, testis, milk glands,phagocyte cells and RBCs another glucose oxidation pathway existsthat is called pentose phosphate pathway (PPP). G6P is the substrate

Fig. 13. Pentose phosphate pathway (PPP). PPP is a cytoplasmic pathway and is composed oftwo oxidative and non-oxidative (highlighted) stages. This glucose oxidation pathway con-sumes G6P as substrate to produce F6P, Xu5P and NADPH2. NADPH2 is used in the lipid bio-chemical pathways. The figure and abbreviations are explained in the text and in figure 2.

Fig. 14. Gluconeogenesis. Gluconeogenesis is a mitochondrial–cytoplasmic pathwaythat happens mainly in the liver and kidney. The substrates of this pathway are pyruvate(product of glycolysis), lactate (produced frompyruvate), alanine, glutamic acid and glycerol(product of triglyceride breakdown). Glycerol is used in gluconeogenesis via glycerolphosphate shuttle. Shifting of oxaloacetate from the mitochondria to cytoplasm happensvia transition to malate. PEP is the substrate of glycolysis and G6P is the substrate ofglycogen synthesis. The figure and abbreviations are explained in the text and in figure 2.

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of PPP to produce ribose-5-phosphate (R5P), riboluse-5-phosphate(Ru5P) and reduced coenzyme NADPH2. R5P is either used for nucle-otide synthesis of nucleic acids or is recycled in the PPP for more pro-duction of NADPH2. The reduced equivalent (NADPH2) is consumedfor the reductive biosynthesis reactions such as synthesis of FA andcortical steroidal hormones of the adrenal gland. In RBCs, NADPH2is consumed for glutathione reduction. Oxidized glutathione (GSSG)converts to the reduced form of glutathione (GSH) using glutathionereductase and NADPH2 as coenzyme. In the absence of the GSH, thephospholipids of RBCs that contain unsaturated FAs become oxidizedand lyse the RBC membrane (Fig. 3/10).

Therefore, NADPH2 has three functions in RBC: (i) supporting the RBCmembrane, (ii) supporting the ferrous (Fe2+) group of hemoglobin and(iii) reduction of enzymes which have cysteine (Cys) in their active site(Cys has SH). In the phagocytic cells (e.g., neutrophils andmacrophages),NADPH2 is used to produce superoxide radicals from oxygen and also re-active oxygen species (ROS) to kill microorganisms. Therefore, duringphagocytosis the phagocytic cells increase their oxygen utilization dra-matically in order to attackmicroorganisms (it is called oxygen burst). Re-ductive and oxidative enzymes utilize the NADP+/NADPH and NAD+/NADH cofactor pairs respectively. 50% of glucose oxidation in adipose tis-sue and 30% of glucose oxidation in liver happens through PPP. Thereby,adipocytes, hepatocytes and also mammalian gland cells can producefats due to the existence of NADPH2 in their tissues. NADPH2 can be pro-duced both in PPP and gluconeogenesis (Figs. 3/11 and 13).

Gluconeogenesis

Gluconeogenesis is the process inwhich non-carbohydratemolecules(pyruvate, lactate, glycerol, alanine and glutamine) are converted to glu-cose in the liver, kidneys, brain, testes and erythrocytes. Gluconeogenesisis the reverse process of glycolysis and happensmostly in the cytoplasm.It starts from conversion of pyruvate to oxalate in the mitochondriausing PC and biotin as its coenzyme. Oxalate is required to beused in the cytoplasm; therefore, it has to be transferred across themitochondrial membrane. The mitochondrial membrane is not diffus-ible to oxalate. Therefore, OAA is transported to the cytoplasm inone of the following ways (i) conversion to PEP through functionof the mitochondrial isoform of phosphoenolpyruvate carboxykinase

(PEPCK-M),which is themost important allosteric enzyme in the kidney(part of the transhydrogenase cycle), (ii) transamination to aspartateand (iii) reduction to malate [15]. In reaction (iii), oxaloacetate isconverted to malate using MDH and NADH2. This process happens inthe mitochondria and is the opposite of the Krebs cycle. Malate passesthrough the mitochondrial membrane and is reconverted to oxaloace-tate in the cytoplasm using NADPH2 as coenzyme. The NADH2 of thisprocess is used in theGA3PDH reaction of glycolysis. Therefore, couplingof these two oxidation-reduction reactions is necessary for the function-ality of gluconeogenesis. Conversion of OAA to malate predominateswhen pyruvate is the substrate of gluconeogenesis. Gluconeogenesis isactive in the shortage of glucose supply, for instance during a longtime starvation or diabetic states where the body uses fats and aminoacids instead of carbohydrates.

The regulators of this pathway are adrenocorticotropic hormone(ACTH), ATP, AMP, ADP and insulin. ATP by stimulation of phos-phatase and ACTH by stimulation of cortisol secretion enhancesgluconeogenesis. In contrast, AMP, ADP and insulin by inhibition ofphosphatase regulate gluconeogenesis negatively. Thyroxin, which in-creases the basal metabolic rate and the catabolism of fats and proteins,stimulates gluconeogenesis. In this state, the excess amount of aminoacids, FAs and glycerol convert to carbohydrate derivatives via gluco-neogenesis (Figs. 3/7 and 14).

Cori cycle

Cori cycle is a hepatic gluconeogenesis that consumes lactate as itssubstrate. During the Cori cycle, the non-hepatocyte cells including skele-tal muscle cells and erythrocytes consume glucose to produce lactate viaglycolysis. Lactate is then transferred to the hepatocytes for gluconeogen-esis. Since during hepatic gluconeogenesis 6 ATP are consumed and dur-ing the Cori cycle in the non-hepatic tissues 2ATP are produced, thereforethe final result is consumption of 4 ATP in each cycle. Thus, the Cori cyclecannot be a permanent process for energy production (Figs. 3/12 and 7).

Glucose-alanine cycle

Alanine is another source of gluconeogenesis in the liver. Alanineis produced from the transamination process in non-hepatic tissues.

Fig. 15. Association between glycolysis, gluconeogenesis and pentose phosphate pathways and the role of Xu5P (product of PPP) in PP2A activation and FA synthesis. Xu5P-activated PP2Aby using ATP and H2O and following activation of phosphoprotein phosphatase and hydroxylation of the PFK2/FBPase2 complex phosphorylates F6P to form F2,6BP. F2,6BP acti-vates PFK1 (regulatory enzyme of glycolysis) and inhibits FBPase1 (regulatory enzyme of gluconeogenesis). These effects lead to increase in the level of F1,6 BP and glycolysis.Xu5P-activated PP2A also activates ChREBP and gene expression of lipogenic enzymes, including ACL, ACC, and FAS. F1,6BP, PFK1 and FBPase1 are regulated by F2,6BP and the ratio ofATP/ADP. In high level of F2,6BP and low level of this ratio, glycolysis will proceed maximally. The figure and abbreviations are explained in the text and figure 2.

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During transamination, one α-amino acid donates the amino groupand generates one α-keto acid. During the glucose-alanine cycle, thenon-hepatic tissues send the amino part of the catabolized aminoacids to the liver to be excreted as urea; therefore, nitrogen is elimi-nated from muscles while replenishing its energy supply (Fig. 7).

Amino acids

Except for leucine and lysine, the carbon skeleton of all otheramino acids can be degraded to the intermediates of the TCA cycle(e.g., oxaloacetate and pyruvate) and are used in gluconeogenesis. Bythis way, during starvation amino acids are used for the maintenanceof glucose in circulation.

Glycerol

FAs are highly energetic molecules; however, because theircarbons cannot be used for glucose synthesis, body cannot use FAs di-rectly as a source of energy. The glycerol backbone of lipids is linkedto glycolysis via the glycerol-phosphate shuttle.

In the glycerol-phosphate shuttle, glycerol is phosphorylatedto glycerol-3-phosphate (G3P) and then dehydrogenated to DHAPby Glycerol-3-phosphate dehydrogenase (G3PDH) in the cytoplasm.G3PDH is the same enzyme which is used for transporting electronsfrom the cytosolic NADH2 (produced by glycolysis) into themitochondri-al oxidized carriers (FAD+) to be used in the OXPHOS reaction. Therefore,continuous conversion of DHAP and G3P (products of glycolysis) to eachother leads to transfer of electron from the cytosolic reduced NADH2 tothe mitochondrial oxidized FAD+ (Graphic abstract figure).

Role of renal gluconeogenesis

Gluconeogenesis happens in the liver as well as in the kidney. There-fore, in liver failure, the kidney is involved in the maintenance of bloodglucose. The substrate of kidney gluconeogenesis is glutamine. Glutamine

is produced in the skeletal muscles during the prolonged fasting state,when muscles use amino acids as their source of energy. In this state,kidney gluconeogenesis removes the waste nitrogen that is produced bythe catabolism of amino acids [16–18].

Regulation of gluconeogenesis

Glycolysis and gluconeogenesis are regulated simultaneously andthrough the same parameters, however, in the opposite direction toeach other. The main regulatory enzymes of glycolysis and gluconeogen-esis are PFK1 and F1,6BPase respectively. Both of these enzymes are reg-ulated by the bifunctional PFK2/F2,6BPase2 enzyme. Insulin and glucagonregulate glycolysis and gluconeogenesis via influence on the componentof this enzyme. Glucagon (and also catecholamine) reduces the level ofF2,6BP and insulin increases the activity of PFK2 and consequently in-creases the function of F2,6BP. F2,6BP has an inhibitory effect on F1,6BPand a stimulatory effect on PFK1. Therefore, glucagon stimulates gluco-neogenesis and insulin stimulates glycolysis (Fig. 15). Insulin also acti-vates phosphodiesterase (PDE), which hydrolyses cAMP to AMP [13,19].

Another point for gluconeogenesis regulation is at the level ofpyruvate to PEP bypass. PK (the glycolysis enzyme, which converts PEPto pyruvate) is negatively regulated through phosphorylation by gluca-gon and epinephrine. This modifies the pathway to the gluconeogenesisdirection. ATP and alanine are the other inhibitors of PK. This meansthat in the high level of energy and availability of the gluconeogenesissubstrate the gluconeogenesis pathway is active.

The other regulatory enzymes of gluconeogenesis are G6Pase, PC andPEPCK (first control point). Cortisol is the activator of these enzymes andADP (or low energy level in the cell) is the inhibitor of them. Duringprolonged starvation, glucagon is secreted. Glucagon increases thecAMP level and stimulates PEPCK and gluconeogenesis and produces glu-cose for organs like the brain, which are dependent on glucose.

cAMP (activated by glucagon and epinephrine) phosphorylates thetranscription factor cAMP response element-binding protein (CREB) of atarget gene and results in the recruitment of the transcriptional P300/CBP (CREB-binding protein) coactivator. The CREB–CRE–P300/CBP com-plex can alone or by recruitment of other coactivator molecules stimulatethe gene expression of the gluconeogenesis enzymes. These coactivatormolecules are peroxisome proliferator-activated receptor-gammacoactivator-1 alpha (PGC1α), glucocorticoid receptor (GR or GCR), hepat-ic nuclear factor-4 alpha (HNF4α) and forkhead transcription factor(FOXO).

Insulin has the opposite effect on gluconeogenesis. It activates PDEand hydrolyzes cAMP to AMP and switches off the gluconeogenesis tran-scription factors. Insulin/phosphoinositide 3-kinase (PI3K) phosphory-lates CBP, which is adjacent to CREB-binding domain (CREB-BD). Insulinalso excludes FOXO from the nucleus to the cytoplasm via FOXO phos-phorylation and inhibits its stimulatory effect on the gluconeogenesisgene expression (Fig. 16) [19,20,21].

Conclusion

Energy homeostasis is one of the main tasks of the body. Regulationand retour of energeticmolecules like glucose and FAs is a complex pro-cess in the body in which all cells are involved. Adipose tissue, skeletalmuscle and liver are themainmetabolic organs in the body. The normalfunction of these metabolic organs is represented as normoglycemicand normolipidemic states in the circulation. Understanding thebiochemistry of carbohydrates and lipids is one of the main steps to-wards comprehending the pathophysiology of metabolic diseases likemetabolic syndrome, diabetes and dyslipidemia. This review summa-rizes and illustrates a general view on the biochemistry of carbohy-drates. The goal of this attempt is to facilitate the understanding ofcomplex biochemical pathways for researchers.

Fig. 16. Presentation of gluconeogenesis transcription factors. PGC1α is the key regulator ofgluconeogenesis. Glucagon/PKA phosphorylates CREB transcription factor and recruitsCBP coactivator to induce PGC1α coactivator. PGC1α coactivates and forms complexeswith other gluconeogenesis transcription factors including FOXO1, GR, HNF4α (as wellas PPARα) to induce the gluconeogenesis promoters. PEPCK and G6Pase are the gluconeo-genesis genes (the same name of the related enzyme). AF2 is the PEPCK promoter and IRUis theG6Pase promoter. Post-translationalmodification of PGC1α by reversible acetylationand phosphorylation of PGC1α regulates the function of PGC1α. Insulin/Akt inhibitsPGC1α via induction of PGC1α phosphorylation and degradation of the hepatic PGC1α.SIRT1 induces PGC1α activity via deacetylation of PGC1α. SIRT1 stimulates gluconeogen-esis through coactivation of FOXO1 and HNF4α. GCN5 acetylates and inhibits PGC1α andgluconeogenic activity. GCN5 is amember of theN-acetyltransferase superfamily. Thisfigureis mainly the idea of Sugden et al. The figure and abbreviations are explained in the textand figure 2.

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Acknowledgment

I would like to extend my appreciation to Dr Nessa DashtyRahmatabady for her useful comments in this paper.

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