Glycolysis

• Glucose is one of the most important energy sources as it serves as a fuel and

is ready to be oxidized to provide energy for other metabolic processes.

• Both galactose and fructose are converted into glucose in the liver.

First: glucose metabolism:

Fates of glucose:

a) Oxidation either for energy production as in case of glycolysis and CAC or

for other purposes as in HMS and uronic acid pathway.

b) Conversion to other biologically important substances as galactose which is

essential for lactose formation, glycolipids and mucopolysaccharides.

c) Storage: in the form of glycogen or triglyceride.

d) Maintaining blood glucose level (60-110 mg/dl)

e) Excretion in urine in case of hyperglycemia (>180 mg/dl) which is then

called glucosuria.

Glycolysis:

Definition: it’s the first step of glucose oxidation to produce energy….its the

degradation of glucose to Generate ATP and to provide intermediates for other

synthetic and metabolic pathways. Aerobically, it ends with pyruvate.

In RBCs, there is no mitochondria so anaerobic glycolysis takes place where

lactate is the end product.

Site: it occurs in the cell cytosol

• Some cells are permeable to glucose carried on proteins as liver, kidney,

brain and intestine.

• While others need insulin as adipose tissue and skeletal muscles.

Steps:

• Hexokinase can act on all hexoses but mainly on glucose. It has high affinity

to glucose so it can act on very low concentrations and it can also stimulate

its phosphorylation.

• Glucokinase can act only on glucose but it has low affinity to it so it needs

high glucose concentration to stimulate phosphorylation.

• Phosphofructokinase (PFK) is a rate limiting enzyme and this step is called

the committed step as it catalyzes the first irreversible reaction unique to

the glycolytic pathway.

• When glyceraldehyde 3 phosphates is changed into 1, 3 bisphosphoglycerate,

oxidative phosphorylation takes place.

• When 1,3 bisphosphoglycerate is converted into 3 phosphoglycerate , ATP is

produced known as energy at substrate level at the site of the chemical

reaction.

Importance of glycolysis:

A) Energy production:

• Under aerobic conditions, 2 pyruvate molecules are produced with 8

ATP……NADH is oxidized in ETC to give 3 ATP.

• Under anaerobic conditions( as in RBCs and lens or during high activity in

skeletal muscles), 2 lactate molecules are produced with 2 ATP where lactate

can then change into pyruvate by lactate dehydrogenase enzyme (LDH) using

NAD.

B) Synthetic functions:

• Dihydroxyacetone phosphate can be converted by (glycerol-3-phosphate

dehydrogenase) into glycerol 3 phosphate which can be used in TG formation

and phospholipids.

• Pyruvate can change into acetyl Co-A that is used in FA and steroids as well

as ketone bodies.

• Synthesis of different amino acids.

• E.g. Pyruvate ……….Alanine by ALT (Alanine aminotransferase)

• Phosphoglycerate can be converted into serine that can change either into

cysteine or glycine.

C) In RBCs:

• Some types of hemolytic anemia are due to inherited deficiency in glycolytic

enzymes especially pyruvate kinase.

• NADH reduces methemoglobin by NADH-cytb5 –reductase enzyme.

• 1,3 bisphosphoglycerate is converted into 2,3 bisphosphoglycerate that

combines with oxyhaemoglobin to help oxygen transfer among tissues.

Regulation:

• It’s according to body physiological state.

• Regulation occurs according to certain levels.

• Induction by stimulating the increase of protein production or repression by

stimulating decreasing protein production.

• Allosteric modification is all about Substrate and Product…..As S Inc. the

enzymes is activated while when P Inc. the enzymes are deactivated.

• Constitutive enzymes are enzymes that is neither induced nor repressed.

• It’s controlled by 3 irreversible enzymes which are hexokinase or

glucokinase, PFK and PK.

• It’s achieved by induction and repression besides allosteric and covalent

modifications.

inducer repressor Allosteric

activator

Allosteric

inhibitor

Covalent

phosphorylation

Covalent

dephosphorylation

glucokinase Insulin Glucagon ….. ….. .…. …..

hexokinase Const. Const. …... G6P ….. …..

PFK Insulin Glucagon AMP and

F6P

ATP and

citrate

Glucagon in

hypoglycemia

Insulin in fed

state

PK Insulin Glucagon F1,6 P ATP Glucagon Insulin

-Inhibitors:

If they are present they inhibit glycolytic enzymes

i) 2-deoxyglucose inhibits hexokinase.

ii) Mercury and iodoacetate bind to active site of enzyme and react with

sulfhydrile group so it inhibits the enzymatic action.

iii) Flouride changes Mg+2 into MgF2…….no enolase action

iv) Arsenate is an uncoupler of oxidation as it reacts with glyceraldehyde 3

phosphate forming 1 aresno 3 phospho glycerate instead of 1,3

bisphosphoglycerate so no ATP at substrate level is formed.

*Fates of pyruvate:

-It can be converted into:

a) Oxaloacetate: By carboxylation using biotin and ATP with CO2 and water to

give ADP and oxaloacetate.

b) Acetyl co-A: By oxidative decarboxylation using a complex enzyme (5 co-

enzymes which are TPP, Lipoic acid, Co-ASH, NAD and FAD).

*Regulation of puruvate dehydrogenase:

Arsenite and mercury inhibit it covalently by reacting with sulfhydrile group of

lipoic acid leading to change of pyruvate into lactate which may lead to lactic

acidosis.

Second: Citric Acid Cycle(CAC):

-Definition: It’s a series of reactions in mitochondria that catabolizes acetyl

residues to provide energy.

-Site: Mitochondria

-Steps:

-Importance of CAC:

1) Energy production: every one mole of acetyl Co-A releases 12 ATP

2) It has amphibolic functions:

1. Catabolic functions: it’s the final common metabolic pathway for oxidation

of CHO, Fat and Proteins.

2. the most important anabolic functions are:

1) Citrate in the cytosol by ATP citrate lyase gives acetyl Co-A which is

used for synthesis of fatty acids and cholesterol.

2) By transamination alpha ketogluteric acid is converted into glutamate

and oxaloacetate is converted into aspartate.

3) Oxaloacetate in the cytosol is converted to PEP which is converted into

glucose by gluconeogenesis.

4) Succinyl co-A is used for heme synthesis, oxidation of ketone bodies

and detoxification.

5) Malate gives pyruvate by malic enzymes in the cytosol.

6) CO2 produced is used in many important reactions including different

CO2 fixation reactions, purine and pyrimidines and urea synthesis and

synthesis of H2CO3/BHCO3 buffer system.

*Regulation of CAC:

• Substrate availability: citrate synthase is activated by high concentrations of

acety Co-A.

• High concentration of citrate causes feedback inhibition of citrate synthase

while high concentration of succinyl co-A causes competitive inhibition of the

same enzyme.

o Ex1: alpha ketoglutarate dehydrogenase is allosterically inhibited by

succinyl co-A.

o Ex2: high NADH/NAD+ and ATP/ADP ratio produces allosteric

inhibition of citrate synthase, isocitrate dehydrogenase and alpha

ketoglutarate dehydrogenase while increase in ADP allosterically

stimulates the 3 enzymes.

*Unlike pyruvate dehydrogenase, it's not regulated covalently.

*Inhibitors of CAC:

citrate

isocitrate

α-ketoglutarate

succinyl co-A

fumarate

Arsenite and Mercury

flourocitrate

Malonic acid Inhibitors of

CAC

Relation between Citric acid cycle and Gluconeogenesis, Transamination, &

Deamination

Hexose Monophosphate pathway

(HMP)

Alternative names:-

• Pentose phosphate pathway (PPP).

• Pentose pathway

• phosphogluconate oxidative pathway

• Direct oxidation of glucose

1-Definition:-

Another route (alternative route) of glucose oxidation without

direct consumption or generation of ATP.

2-Site:-

Cytosol of many cells e.g. liver, adrenal cortex, adipose tissue,

testis, ovaries, retina, lactating mammary gland and RBCs

(erythrocytes).

3-steps:-

Occurs in 2 phases; oxidative and non-oxidative.

#OXIDATIVE phase:-

• It is irreversible;

• Glucose-6-phosphate is converted to ribulose-5-phosphate.

• We get 2 NADPH from this phase.

#NON-OXIDATIVE phase:-

• It is reversible.

• And it is catalyzed by transketolase and transaldolase.

• Pentoses in phase I are converted to glucose-6-phosphate

again.

That means ------->

6 molecules of glucose-6-phosphate are converted to ribulose-5-

phosphate

THEN

Converted to 5 molecules of glucose-6-phosphate.

THE SUM OF THESE two REACTIONS:-

3 RIBOSE-5-P ����- ����2 FRUCTOSE-6-PHOSPHATE + GLYCERALDHYDE-3-P

Thus, excess ribose-5-p formed by the pentose phosphste pathway

can be completely converted into GLYCOLETIC INTERMEDIATES.

So, transketolase and transaldolase create a REVERSABLE LINK

between the "PPP" and "GLYCOLESIS".

HMP:-

(A) It is another way for oxidation of glucose without generating

energy.

(b) It provides the cell with ribose-5-phosphate (active ribose)

which is needed for nucleosides, nucleotides, nucleic acid and

coenzymes biosynthesis.

N.B:-HMS is the only way of R-5-P production in our body due to

absence of ribokinase enzyme.

(c) It is the main generator of reduced co II (NADPH+H) which is

needed in reductive biosynthesis.

(d) IMPORTANCE of HMS:-

• Red cells are liable for oxidative damage by H2O2 due to their

role in O2 transport.

• In RBCs, H2O2 causes both:

• -oxidation of iron in hemoglobin to form methemoglobin and lipid

perioxidation.

• Lipid perioxidation increases the cell membrane fragility.

• The major role of HMS in red blood cells, is the production of

NADPH which protect these cells from oxidative damage by

providing reduced glutathione for removal of H2O2

(detoxication).

(e)It serves a mechanism for synthesis or / and degradation of

sugars other than hexoses (erytherose, xylulose, sedoheptulose).

REGULATION:-

• Synthesis of G6PD is induced during feeding and repressed during

fasting.

• Insulin, which is secreted in response of hyperglycemia, increases

the rate of glucose oxidation by HMS.

• Insulin induces the synthesis of G6PD and 6-phosphogluconate

dehydrogenase.

• The reaction catalyzed by G6P dehydrogenase is the RATE

LIMITING in the oxidative branch (the control site).

• The most important regulatory factor is the level of NADP+.

[NADPH is a competitive inhibitor of NADP+].

• NADPH accumulation produces feedback inhibition of glucose-6-

phosphate dehydrogenase.

• The flow of G-6-P in PPP depends on the need for NADPH+, R-5-

P and ATP.

Glucose-6-p dehydrogenase deficiency:-

• It is an inherited disease characterized by hemolytic anaemia

as G-6-P dehydrogenase deficiency impairs to form NADPH+H+

that is essential in the detoxification of peroxides formed

within the sell.

• G-6-P dehydrogenase deficiency is most severe in RBCs where

HMS provides the only means of generating NADPH+H+.

Precipitating factors in G6PD deficiency:-

A. Drugs���� sulpha drugs, antimalarial drugs, and antipyretic drugs,

which stimulate production of H2O2.

B. Fava Beans����ingestion of fava beans will lead to acute haemolytic

anaemia (Favism), as it contains oxidizing agents.

C. Severe infection���� in viral and bacterial infections due to release

of oxidation from active phagocytes.

URONIC ACID PATHWAY:-

It is an alternative oxidation pathway for glucose, that doesn't lead

to the generation of ATP.

URONIC ACID PATHWAY:-

It is an alternative oxidation pathway for glucose, that doesn't lead

to the generation of ATP.

Site:-mainly in LIVER CYTOPLASM.

Importance:-

UDP glucuronic acid (the active form) is needed in:-

(a) Conjugation to less polar compounds as bilirubin, steroids, and

some drugs making them more H2O soluble (detoxicated).

(b)Synthesis of glycosaminoglycans (mucopolysaccharides) as heparin,

hyalouronic acid, Etc.

(c)In plants and some animals (not man) glucuronic acid serves as

precursor of L-ascorbic acid.

Gluconeogenesis

Definition: synthesis of glucose (&/or glycogen) from non-CHO precursors

such as: lactate, glucogenic amino acids, glycerol, and propionate

Site: mainly in the cytoplasm, partly in the mitochondria

90% in the liver (liver glycogen can meet these needs for 10-18 hours in the

absence of dietary CHO) 10% in the kidney

Steps:

Glycolytic key enzymes Gluconeogenic key enzymes

Glucokinase , Hexokinase Glucose-6-phosphate

Phosphofructo kinase-1 Fructose-1,6-bisphosphatase

Pyruvate kinase Pyruvate carboxylase (PC)

Phosphoenolpyruvate carboxykinase

(PEPCK)

Summary diagram for Gluconeogenesis:

PK: pyruvate carboxylase (liver & kidney mitochondrial enzyme , not present in

muscle )

MD: malate dehydrogenase

PEPCK: phosphoenol pyruvate carboxylase (decarboxylation & phosphorylation

of oxaloacetate, rate limiting step)

PK: pyruvate kinase

NB:

• Fructose-1,6-bisphosphatase is present in liver & kidney

• Free glucose formed by the action of glucose-6-phosphatase in liver &

kidney , while it is absent in muscles & adipose tissues , thus glucoe

cannot be formed by these organs

The glycogenic substrates:

1-Lactate: 6 high energy phosphate bonds are spent (from 2 molecules of

pyruvate )

2-

glucogenic amino acids: after 18 hours fasting proteins are considered as a

source of energy

3-glycerol:

In liver & kidney, glycerol cannot be utilized in adipose tissue which lacks

glycerol kinase

4-propionyl -CoA: converted to succinyl CoA

Importance of gluconeogemesis

1. Maintenance of blood glucose

2. Synthesis of lactose ( of milk in mammary gland )

3. Supply the basal glucose requirements for optimal activity of CAC to extra

hepatic tissues ( FA & keton bodies oxidation require a minimal supply of

glucose )

4. Removal of glycerol ( produced by lipolysis )

5. Removal of lactic acid ( produced by anaerobic oxidation ) thus prevent

metabolic acidosis

Regulation of gluconeogenenesis

1. Insulin : repressor � synthesis of gluconeogenic key enzymes

Inducer � glycolytic key enzymes

2. Anti-insulin ( glucagon , adrenaline , cortisol ) : secreted during

fasting , stress , severe exercise

• Glucocorticoids : inducer � synthesis of gluconeogenic key enzymes

• Glucagon , adrenaline: inhibitor � glycolytic enzymes

3. Substrate availability & alosteric modifiers :

• Protein break down : � glucogenic amino acids � stimulation

of Gluconeogenesis

• Excessive lipolysis � glycerol � stimulation of Gluconeogenesis

� Fatty acids � B- oxidation � acetyl CoA

Acetyl CoA: stimulates gluconeogenesis, inhibit glycolysis by increasing the rate

of CAC

- Excessive citrate � inhibits PFK & activates F1,6 diphosphatase

- ATP � inhibits PFK & activates F1,6 diphosphatase

Acetyl CoA: allosterically stimulates pyruvate carboxylase via inhibition of

pyruvate dehydrogenase

Glycogen metabolism

• Glucose is converted into glucose 6 phosphate which could be used in many

processes such as HMS And glycogenesis.

• By phosphoglucose-mutase glucose 1 phosphate is formed……then a UTP

molecule is added to form UDPglucose by the help of UDP pyrophosphorolase.

UTP…………UMP + PPi (pyrophosphate)

• The UDP glucose is the active nucleotide form of glucose which can then

take:

A) The uronic acid pathway (mainly in liver cytoplasm):

By UDP-glucose dehydrogenase and H2O…..UDP glucuronic acid is formed and its

used in:

• Conjugation to bilirubin resulting from RBCs degradation….steroids such as

sex hormones.

• synthesis of GAGs as heparin and hyaluronic acid.

• It serves as a precursor for L-ascorbic acid but only in plants and animals.

B) Glycogenesis:

…10% of liver is glycogen which is used to maintain blood glucose level in case of

fasting while glycogen In muscles is stored to be used in case of prolonged

muscular exercise.

…By glycogen synthase, glucose from UDP glucose are added to glycogen primer

forming alpha 1, 4 Glucosidec bonds between the original primer and the new

added glucose molecule.

…Up to 11 residues can be added.

…Then a new enzyme is used which is branching enzyme (to form mature

glycogen) or as called (amylo 1, 4 to 1, 6 transglucosidase).

…It adds 6-8 residues to the branches with 1, 6 glucosidic bonds and this

branch is elongated using glycogen synthase enzyme.

Glycogenolysis:

…glycogen phosphorylase enzyme with a phosphate group acts on 1, 4 glucosidic

bond leading to formation of G1P.

…It stops when there is 3-4 residues before the branching point then another

enzyme works which is Alpha 1, 4 to alpha 1, 4 glucan tranferase where the

trisaccharide units move to join the straight chain leaving the 1,6 glucosidic

bond exposed

…A debranching enzyme is used called amylo 1, 6 glucosidase….by adding water

the glucose residue is removed and then phosphorylase enzyme completes its

action.

…The G1P is converted into G6P by phosphoglucomutase which is the changed in

the liver and kidneys into free glucose by G6-phosphatase…………liver send

glucose to blood to maintain its level.

…In muscles G6P can't be transformed into free glucose so it's utilized to

produce energy in case of prolonged muscular exercise.

N.B.:

Phosphorylation is better than hydrolysis as the released units are G6P

which can be used directly in the muscle for glycolysis without using

ATP…..also G6P cant diffuse out of the muscle while glucose can.

*Regulation:

-Allosterically:

• G6P activates the glycogenesis process and stimulated the glycogen synthase

enzyme where it inhibits the glycogenolysis.

• ATP inhibits glycogen phosphorilase enzyme and inhibits the glycogenolysis

process while it activates Glycogenesis.

• AMP inhibits glycogenesis while it stimulates glycogen phosphorylase in

muscles.

-Covalently:

• Glucagon (only in liver), epinephrine and nor epinephrine (in liver and muscles):

• They stimulate adenylcyclase which converts ATP to cAMP which activates

protein kinase A which allow conversion of glycogen phosphorylase

(dephosphorylated) to the active phosphorylated form.

• The opposite occurs in case of glycogen synthase where the active

dephosphorylated form is changed into inactive phosphorylated.

• Insulin stimulates the phosphodiesterase enzyme to change cAMP to AMP..It

also stimulates protein phosphatase enzyme to change phosphorylated to de

phosphorylated form so:

1. -It activates glycogen synthase (glycogenesis)

2. -It inhibits glycogen phosphorylase (glycogenolysis)

Pathways of glycogenesis and of glycogenolysis in the liver. ( , Stimulation; , inhibition.) Insulin

decreases the level of cAMP only after it has been raised by glucagon or epinephrine; ie, it

antagonizes their action. Glucagon is active in heart muscle but not in skeletal muscle. *Glucan

transferase and debranching enzyme appear to be two separate activities of the same enzyme

Other Sugar metabolism

Fructose metabolism.

Utilization of fructose

In liver.

• Fructose is mainly metabolized in liver by the fructokinase enzyme (also in

kidney and intestine) to form F-1-P

• Then by the action of fructose-1-Phosphate aldolase (B), F-1-P is

converted to DHAP and glyceraldehydes

• Glyceraldehydes is converted to glyceraldehyde-3-Phosphate by thiokinase

The utilization of fructose by fructokinase then aldolase B can bypass the steps

of glucokinase and PFK-1 which are activated by insulin

This explains why fructose disappears from blood more rapidly than glucose in

diabetic patients

Synthesis of free fructose from glucose (polyol pathway)

• By this mechanism, glucose is converted into fructose in seminal vesicles and

secreted in seminal fluid

• Also, conversion of glucose to sorbitol is increased in diabetic subjects,

sorbitol produces osmotic damage of cells, which may account for production

of diabetic cataract, retinopathy, neuropathy and nephropathy

• N.B. aldose reductase may be irreversible since sorbitol given I.V. is

converted to fructose rather than to glucose.

Hereditary defects in fructose metabolism

These metabolic inborn errors include mainly

1- essential fructosuria: due to hereditary deficiency of fructokinase

2- hereditary fructose intolerance: due to hereditary deficiency of liver

enzyme aldolase B

Galactose metabolism

• The major dietary source for galactose is lactose obtained from milk and milk

products

• The digestion of lactose is by lactase (β- Galactosidase) of intestinal mucosal

cell membrane

• Some galactose may be obtained by lysosomal degradation of complex CHO

such as glyco-proteins and glyco-lipids, which are important membrane

components as well as from the turnover of body’s own cells

• Like fructose, entry of galactose into the cell isn’t insulin dependent

N.B.

Glucose can be converted to galactose

Thus performed galactose isn’t essential in the diet

Galactosemia

1- Inherited disease due to

a. Deficiency of the uradil transferase (most commonly)

b. Inherited defects of galactokinase or 4-epimerase

2- Characterized by mental retardation, diarrhea and cataract