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Vitamins Play Key Roles in the Citric Acid Cycle

Four of the B vitamins are essential in the citric acid cycle and hence energy-yielding metabolism: 

  1. Riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor for succinate dehydrogenase;
  2. Niacin, in the form of nicotinamide adenine dinucleotide (NAD), the electron acceptor for isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase;
  3. Thiamin (vitamin B1), as thiamin diphosphate, the coenzyme for decarboxylation in the â-ketoglutarate dehydrogenase reaction.
  4. Pantothenic acid, as part of coenzyme A, the cofactor attached to "active" carboxylic acid residues such as acetyl-CoA and succinyl-CoA. 

1. Bioenergetics:

10 ATP are produced per turn of citric acid cycle or per mole of acetyl CoA or 12.5 ATP per mole of pyruvate. 


Extra Edge

Acetyl CoA is also the precursor for synthesis of long chain fatty acids and steroids, including cholesterol and ketone bodies. 

  1. Inhibitors of TCA cycle
    TCA cycle is inhibited by a number of chemicals, which include the following
    1. Fluoroacetate: It inhibits the enzyme aconitase (non-competitively).
    2. Arsenite: It causes non-competitive inhibition of α-ketoglutarate dehydrogenase.
    3. Malonate: It causes competitive inhibition of succinate dehydrogenase.
  2. Source And Fate Of Oxaloacetic Acid
    a. Sources:

              i.        Carboxylation of pyruvate by pyruvate carboxylase.


            ii.        Transamination of amino acid aspartic acid.


          iii.        By malic dehydrogenase it can be obtained from malic acid.


          iv.        By splitting of citric acid, catalyzed by citrate lyase enzyme.


b.  Fate:

              i.        Formation of citric acid by condensation with aceryl CoA


           ii.        Converted to malic enzymes by malic dehydrogenase


         iii.        Converted to aspartate by transamination


         iv.        Converted to Phosphoenol pyruvate by PEPCK during gluconeogenesis


Anaplerotic Reactions: can increase the concentration of citric acid cycle intermediates, allowing an increased rate of oxidation of two carbon units.

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