METABOLISM

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Compiled and Edited byDr. Syed Ismail

VN Marathwada Agricultural University, Parbhani, Maharashtra, India

METABOLISM Metabolism Includes all the chemical processes within cells

and tissue that are concerned with their building up and breaking down and their functional operations.

Energy Metabolism is energy composition of metabolism and deals with the overall energy production as per requirement of the organisms.

Anabolism: Process for union of smaller into larger molecules or metabolism of tissue formation.

Catabolism: process of tissue breakdown obviously is primarily concerned with the splitting of the larger protoplasmic molecules into the smaller ones.

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Energy Metabolism

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Carbohydrate Metabolism Carbohydrate metabolism in the animal body is

essentially the metabolism of glucose and of the substances related to glucose in their metabolic processes.

Glucose occupies a central position in the metabolism of plant, animals and may microbes.

Glycolysis

Fate of Pyruvate

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Carbohydrate Metabolism

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Glycolysis (EMP) pathway

Almost universal central pathway of glucose

In glycolysis two ATP and two NADH molecules are generated.

Two phases

Primary Phase

Secondary phase

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Primary Phase

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Payoff phase

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Summary of Glycolysis

Summary for glycolysisATPATP

{Glucose G6P F6PF1,6BP DHAP GAP} Preparatory PhaseNADH 2ATP 2ATP

{GAP1,3BPG 3PG2PG PEP Pyruvate} Payoff Phase

(Keep in mind that TWO of these molecules will actually be reacting in the biological pathway for each glucose molecule that entered it). Hence, the net gain is of two molecules of ATP and two molecules of NADH in the process of glycolysis.

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Acetylationo Definition: Conversion of pyruvate into acetyl CoA is called

acetylation.

o Occurs in cytoplasm

o enzyme involved :

o Five Cofactors required: Thiamine pyrophosphate (TPP), Flavin adenine dinucleotide (FAD), Coenzyme A (CoA–SH), Nicotinamide adenine dinucleotide (NAD )and Lipoate.

o One NADH {Nicotinamide adenine dinucleotide (reduced)} is formed during conversion

o It contains three enzymes –

o Pyruvate dehydrogenase (E1)

o Dihydrolipoyl transacetylase (E2)

o Dihydrolipoyl dehydrogenase (E3)

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Acetylation

RememberOne NADH is produced during oxidation of pyruvate to Acetyl CoA Acetyl CoA formed during acetylation enters into mitochondria and is intermediate key

compound and acts as a connecting link between glycolysis and Kreb’s Cycle.

RememberOne NADH is produced during oxidation of pyruvate to Acetyl CoA Acetyl CoA formed during acetylation enters into mitochondria and is intermediate key

compound and acts as a connecting link between glycolysis and Kreb’s Cycle.

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TCA CYCLE (Kreb’s Cycle)o Definition: the citric acid cycle is a cyclical set of eight

reactions that accomplish the final steps of the breakdown of glucose to carbon dioxide and water. Its actual starting point is acetyl coenzyme A.

o Occurs in mitochondria

o Also called TCA or Kreb Cycle

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TCA Cycle

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SUMMARY OF TCA CYCLEThe reactions of TCA can be devided to 8 steps(1) Condensation to form citrate by citrate synthase.(2) Aconitase transformation to isocitrate through cis aconitase.(3) Isocitrate dehydrogenated A Ketoglutarate & CO2 by isocitrate dehydrogenase.(NAD+ ---------- NADH + H+)(4) A Ketoglutarate oxidative decarboxylation succinyl Co A & CO2 by dehydrogenase complex. (NAD+ ------ NADH + H+)(5) Succinyl Co A hydrolyzed to succinate by succinyl Co A synthesase (GDP ------ GTP)(6) Succinate dehydrogenated to Fumarate by succinate dehydrogenase (FAD ----- FADH2)(7) Fumarate to Malate by Fumarase. (+ H2O)(8) Malate is Dehydrogenated by Malate dehydrogenase to Oxaloacetate. (NAD+ ------ NADH + H+)

SUMMARY OF TCA CYCLEThe reactions of TCA can be devided to 8 steps(1) Condensation to form citrate by citrate synthase.(2) Aconitase transformation to isocitrate through cis aconitase.(3) Isocitrate dehydrogenated A Ketoglutarate & CO2 by isocitrate dehydrogenase.(NAD+ ---------- NADH + H+)(4) A Ketoglutarate oxidative decarboxylation succinyl Co A & CO2 by dehydrogenase complex. (NAD+ ------ NADH + H+)(5) Succinyl Co A hydrolyzed to succinate by succinyl Co A synthesase (GDP ------ GTP)(6) Succinate dehydrogenated to Fumarate by succinate dehydrogenase (FAD ----- FADH2)(7) Fumarate to Malate by Fumarase. (+ H2O)(8) Malate is Dehydrogenated by Malate dehydrogenase to Oxaloacetate. (NAD+ ------ NADH + H+)

Gluconeogenesiso Definition: formation of glucose from non–

carbohydrate precursors like pyruvate and related three and four carbon compounds.

o Important precursors of glucose in animals are three carbon compounds such as lactate, pyruvate and glycerol, as well as certain amino acids.

o Mainly occurs in liver

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Conversion of pyruvate to glucoseFirst Bypass

Conversion of pyruvate into phospho-enol-pyruvate.

Here pyruvate is first transported from cytoplasm into mitochondria. Then pyruvate is converted to oxaloacetate by action of pyruvate carboxylase.

Mitochondrial membrane has not tranporter for oxaloacetate, before export to the cytosol the oxaloacetate formed from pyruvate must be reduced to malate by mitochondrial malate dehydrogenase, at the expense of NADH:

The malate leaves the mitochondrial membrane, and in the cytosol it is reoxidized to oxaloacetate, with the production of cytosolic NADH:

The oxaloacetate is then converted to PEP by phospho-enol-pyruvate carboxykinase. This Mg++ dependent reaction requires GTP (Guanosine Tri-Phosphate) as the phosphoryl group donor.

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Reverse of glycolysis

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Conversion of F1-6DP into F6P

Second Bypass

Enzyme – Fructose 1,6 biphosphatase, carries irreversible hydrolysis of the

C-1 phosphate (not phosphoryl group transfer to ADP)

Fructose 6 phosphate is then reversibly converted to G6P.

Third bypass (conversion of G6P to Glucose)

Enzyme – G6Phosphatase

Gluconeogenesis is expensiveThus, Gluconeogenesis is not simply reversible of glycolysis

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GlycogenesisIt is biosynthesis of glycogen from glucose, occurs

especially in skeletal muscle and liver

• The glucose units of the outer branches of glycogen enter to the pathway through action of three enzymes; glycogen phosphorylase, glycogen debranching, and phosphoglucomutase.

• Glycogen phosphorylase catalyzes the reaction in which an (α1-4) glycosidic linkage between two glucose residues at a non-reducing end of glycogen undergoes attack by inorganic phosphate (Pi), removing the internal terminal glucose residue as α-D-glucose 1 phosphate.

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Contd….• Glycogen phosphorylase acts repetitively on the

nonreducing ends of glycogen branches until it reaches a point four glucose residues away from an (α 1-6) branch point. Where its action stops. Further degradation by glycogen phsophrylase can occur only after the debranching enzyme, formally known as oligo (α 1-6) to (α 1-4) glucantransferase, catalyzes two successive reactions that transfer branches.

• Once these branches are transferred and the glucosyl residue at C-6 is hydrolyzed, glycogen phosphorylase activity can continue.

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Contd….

• Glucose 1phosphate, the end product of the glycogen phosphorylase reaction, is converted to glucose 6 phosphate by phosphoglucomutase, which catalyzes the reversible reaction. The glucose 6 phosphate formed from glycogen in skeletal muscle can enter glycolysis and serve as an energy source to support muscle contraction.

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Glycogenesis

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Glycogenesis

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Protein Metabolism

o Amino Acid Metabolismo Biosynthesis o Degradationo Deaminationo Transamination, etco Urea Cycleo Protein Biosynthesis and

Degradation

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Amino Acids Which are Essential Amino Acidse.g. L2T2MVIP

Non Essential Amino Acidse.g. A4G3SPTC

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Oxidative Degradation

Why does it Occur? During normal synthesis and degradation of cellular proteins,

some amino acids are released from proteins breakdown and are not needed for new protein synthesis undergo oxidative degradation.

When amino acids exceed body’s need for protein synthesis. When carbohydrate is not available. starvation, diabetics, etc.

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Transamination What does it mean? Transfer Amino group from amino acid to α-ketoacid (oxaloacetate, α-

ketoglutarate, pyruvate, etc). Site: Cytosol and Mitochondria Enzyme: Aminotransferase or Transaminase. Cofactor: Pyradoxal Phosphate. During Breakdown all amino acids are transferred to α-ketoglutarate

because only glutamate can undergo rapid oxidative deamination.

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Deamination Amino acids are collected in liver in the form of the amino group

of glutamate molecules. These amino groups must next to be removed from glutamate to

prepare them for excretion. Glutamate is transported from cytosol to mitochondria where it

undergoes oxidative deamination catalyzed by glutamate dehydrogenase.

Dehydrogenase removed the amino group from glutamate and ammonia formed enters the urea cycle and the carbon skeletons (α-ketoacids) are all glycolytic and TCA cycle intermediate.

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Nitrogen Balance

Net daily loss of nitrogen (as urea) from the body, is usually to about 35 – 55 gm protein lost each day.

Positive nitrogen Balance: intake greater than loss. E.g. growth, pregnancy, etc.

Negative nitrogen balance: intake less than loss. E.g Starvation, etc.

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Urea Cycle Transmutation, Deamination leads to CO2+ NH4+ Interacts with water and 2ATP to form Carbamoyl Phosphate.

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Urea Cycle30

Role of proteins as an energy source

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Glucose – Alanine Cycle G-A cycle shows how carbon skeleton alternate between

protein and glucose. Alanine released by muscle is converted back to glucose in

the liver by gluconeogenesis. The glucose formed is taken back to the muscle for use.

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Biosynthesis of Non Essential Amino Acids

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Protein Degradation

Two possible ways Ubiquitin Pathway: degrade abnormal proteins and short-lived cytosolic

proteins. Located in cytosol. ATP dependent Losysomal pathway: Degrades long-lived membrane proteins and organelles,

for example mitochondria. ATP-Independent. Located in lysosome. Cathepsin

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Protein BiosynthesisInvolves Five Major StepsActivation of Amino AcidsInitiationElongationTerminationFolding and post-translation

processing

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Lipid Metabolism

What are the fatty acids

Carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons long (C4 to C36)

CH3(CH2)nCH2CO2H

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Classification of Fatty acids

Saturated fatty acids

Unsaturated fatty acids

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NomenclatureUsually referred by common names

– Carbon Skeleton Common name word origin– 12:0 lauric acid laurus plant– 14:0 myristic acid myristica genus– 16:0 palmitic acid Palm plant– 18:0 Stearic acid stear mean Hard fat– 18:1 Oleic acid oleum meaning oils – So on…………………..

Systematic Names: basis of their chain length and No. of double bond.12:0 lauric acid Dodecanoic acid14:0 myristic acid n-Tetradecanoic acid18:1(9) Oleic acid 9-octadecenoic acid18:2(9,12) linoleic acid ???????????????18:3 (9,12,15) Linolenic acid ??????????????20:4 (5,8,11,14) Arachidonic acid ???????????????16:0 palmitic acid ???????????????

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The Essential Fatty acids

LinoleicLinolenic Arachidonic acidElse– Scaly skin, stunted growth and increased dehydration.

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Oxidation of FatsFatty acids are oxidized to carbon dioxide

and water at the liberation of large unit of energy.

Oxidation is brought in mitochondria Several Theories.Mitochondrial oxidation of fatty acids takes

place in three stages.– β Oxidation– TCA– ETC

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β OxidationKnoop in year 1905In β oxidation, fatty acids are breakdown to

acetyl CoA i.e. Glycolysis of Fatty acid.Strictly AerobicOccurs in MitochondriaAcetyl CoA produced goes to Kreb’s Cycle

while over production leads to Ketosis.

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Transport of Fatty acids to Mitochondria

12 or fewer enter mitochondria without help of membrane transporters.

14 or more can directly pass through mitochondrial membrane – they must first undergo three enzymatic reactions of carnitine shuttle.

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Carnitine Shuttle1. Carboxyl group react with Coenzyme A to yield Fatty acyl-CoA.

Catalyzed by Acyl CoA synthetase.

2. Transesterification: catalyzed by carnitine acyltransferase I, leads to formation of fatty acyl carnitine.

1. Occurs in outer membrane. Then transferred to inter-mitochondrial membrane.

3. Transfer of fatty acyl group from carnitine to inter-mitochondira Coenzyme A by Acyltransferase II.

FA + HS-CoA FA~CoAAcyl CoA synthetase

FA~CoA + Carnitine Fatty Acyl-CarnitineAcyl-transferase I

Fatty Acyl-Carnitine + HS-CoA Fatty Acyl CoAAcyl-transferase II

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Mechanism of Beta oxidation

5 steps1.Activation of fatty acids by formation of

thioester of coenzyme A2. Dehydrogenation in α and β position by Acyl-CoA

dehydrogenase. (FAD to FADH2)

3. Addition of water to double bond by Enoyl CoA hydratase

4. Dehydrogenation of β Carbon

5. Thiolysis

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β Oxidation of Saturated Fatty Acids (Even Number - Palmitate)

Dehydrogenation

Addition of water

Dehydrogenation of β Carbon

Thiolysis

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β Oxidation of Saturated Fatty Acids (ODD Number)

Odd in plant and Marine

In same way and Even, but final product is Propionyl-CoA which enter different pathway.

Propionyl Co A to D- methylmalonyl CoA by carboxylase.

To L – methyl malonyl CoA by Epimerase

Final product is Succinyl CoA.

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β Oxidation of Mono- unsaturated Fatty Acids (Oleic Acid)

Cis double bond between 9 and 10.

Three cycle Passes without problem.

hydratase can not act on cis but trans

Isomerase Acts to convert cis form to trans form.

Rest of the reaction occurs same as that of saturated fatty acids.

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Oxidation of Poly-Unsaturated Fatty Acids

In β Oxidation of polyunsaturated fatty acids, such as linoleic acid, translocation of double bond is carried out with various enzymes and further it is oxidized in same way as that of monounsaturated fatty acids.

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Ketogenesis

Acetyl Co A formed can either enter in TCA or undergo conversion on the Ketone Bodies.

Major ketones produced are Acetone, acetoacetate and β-hydroxylbutyrate.

Acetone is exhaled.

Liver continuously produced while not utilized by TCA.

Usually during starvation and diabetes mellitus, overproduction of ketone bodies occurs.

Increased blood levels of acetoacetate and β-hydroxylbutyrate lowers blood pH, causing the condition known as acidosis.

Blood Normally contains < 3 mg/100 ml of ketone bodies, while in diabetics it can reach to extraordinary level of 90 mg/100 ml, this condition called ketosis

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Thank You!

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