Metabolism Of Carbohydrates Biochemistry Note -V

Afza.Malik GDA

Carbohydrates Metabolism Biochemistry for Nurses

Metabolism Of Carbohydrates Biochemistry Note -V

Fate of pyruvic acid depends on the redox state of the tissues,Biomedical Importance Of Citric Acid Cycle, TCA cycle has dual role,Shuttle Systems, Glycerophosphate shuttle, Malate shuttle,Metabolism Of Glycogen,Role of Liver Glycogen.

Fate of pyruvic acid depends on the redox state of the tissues:

• In Presence of O2: Pyruvic acid is oxidatively decarboxylated to two-carbon unit “Acetyl-CoA”

• In absence of O2: Pyruvic acid is converted to Lactic acid (LA)


    One molecule of glucose produces two molecules of PA which in turn by oxidative decarboxylation produces 2 molecules of Acetyl-CoA and 2 NADH. Two molecules of NADH will be oxidized to 2 molecules of NAD+ producing 6 ATP molecules in respiratory chain. + 6 A T P

Citric Acid Cycle Synonyms:
TCA cycle (tricarboxylic acid cycle), Krebs’ cycle, Krebs’ citric acid cycle.
Biomedical Importance Of Citric Acid Cycle

• Final common pathway for carbohydrates, proteins and fats, through formation of 2-carbon unit acetyl-CoA.

• Acetyl-CoA is oxidised to CO2 and H2O giving out energy (III pase of catabolism)-catabolic role.

• Intermediates of TCA cycle play a major role in synthesis also like heme formation, formation of non-essential amino acids, FA synthesis, cholesterol and steroid synthesisanabolic role.

Role Of Vitamins In Tca Cycle

Five B vitamins are associated with TCA cycle essential for yielding energy.

• Riboflavin: In the form of flavin adenine dinuleotide (FAD)— a cofactor for succinate dehydrogenase enzyme.

• Niacin: In the form of nicotinamide adenine dinucleotide (NAD) the electron acceptor for isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase.

• Thiamine: As “thiamine diphosphate”—required as coenzyme for decarboxylation in the α-ketoglutarate dehydrogenase reaction.

• Lipoic acid: It is required as coenzyme for α-ketoglutarate dehydrogenase reaction.

• Pantothenic acid: As part of coenzyme A, the cofactor attached to “active” carboxylic acid residues such as acety CoA and succinyl-CoA

TCA cycle has dual role

• catabolic,

• anabolic.

(a) Catabolic role: The two carbon compound acetyl-CoA produced from metabolism of carbohydrates, Lipids and Proteins are oxidised in this cycle to produce CO2, H2O and energy as ATP.

(b) Anabolic or synthetic role:Intermediates of TCA cycle are utilised for synthesis of various compounds.

Transamination: Synthesis of non-essential amino acids

Formation of glucose: (Gluconeogenesis)

Fatty acid synthesis

Synthesis of cholesterol and steroids

Haem synthesis

Formation of acetoacetyl-CoA

Regulation of TCA Cycle

1. As the primary function of TCA cycle is to provide energy, respiratory control via the ETC and oxidative phosphorylation exerts the main control.

2. In addition to this overall and coarse control, several enzymes of TCA cycle are also important in the regulation.

Shuttle Systems

    NADH produced in the glycolysis is extramitochondrial, whereas the electron transport chain, where NADH has to be oxidised to NAD+ is in the mitochondrion. NADH is not permeable to mitochondrial membrane. 

    It is envisaged that NADH produced in cytosol transfer the reducing equivalents through the mitochondrial membrane via substrate pairs, linked by suitable dehydrogenases by shuttle systems. It is important that the specific dehydrogenases which act as “shuttle” be present on both sides of mitochondrial membrane. Two such shuttle systems are there:

1. Glycerophosphate shuttle

2. Malate shuttle

Metabolism Of Glycogen

    Glycogen is the storage form of glucose, which is stored in animal body specially in liver and muscles. It is mobilised as glucose whenever body tissues require. Students should revise their knowledge regarding chemistry of glycogen. Why body stores glucose as glycogen and not as glucose itself?

Possible Reasons

1. Being insoluble it exerts no osmotic pressure, and so does not disturb the intracellular fluid content and does not diffuse from its storage sites.

2. It has a higher energy level than a corresponding weight of glucose (though energy has to be expended to make it from glucose). 3. It is readily broken down under the influence of hormones and enzyme:

• Into glucose in liver (to maintain blood glucose level).

• Into lower intermediates in skeletal muscle and other tissues for energy.

Role of Liver Glycogen

• It is the only immediately available reserve store of blood glucose.

• A high liver glycogen level protects the liver cells against the harmful effects of many poisons and chemicals, e.g. CCl4, ethyl alcohol, arsenic, various bacterial toxins.

• Certain forms of detoxication, e.g. conjugation with glucuronic acid; and acetylation reactions, are directly influenced by the liver glycogen level.

•.The rate of deamination of amino acids in the liver is depressed as the glycogen level rises, so that amino acids are preserved longer in that form and so remain available for protein synthesis in the tissues.

• Similarly, a high level of liver glycogen depresses the rate of ketone bodies formation.

Biomedical Importance

• Liver glycogen is largely concerned with storage and supply of glucose-1-P, which is converted to glucose, for maintenance of blood glucose, particularly in between meals.

• Muscle glycogen on the other hand, is to act as readily available source of intermediates of glycolysis for provision of energy within the muscle itself. Muscle glycogen cannot directly contribute to blood glucose level.

• Inherited deficiency of enzymes in the pathway of glycogen metabolism produces certain inherited disorders called as Glycogen storage diseases (GSDs).

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