Oxaloacetate is formed in the last step of the ctric acid cycle by the NAD+ -dep

Question

Oxaloacetate is formed in the last step of the ctric acid cycle by the NAD+ -dependent oxidation of L-malate. a) Can a net synthesis of oxaloacetate from acetyl-CoA occur using only the enzymes and cofactors of the ctric acid cycle, without depleting the intermediates of the cycle? Explain. (5 points)

b) How is oxaloacetate that is lost from the cycle (to biosynthetic reactions) replenished? (5 points)

c) The mammalian liver can carry out gluconeogenesis using oxaloacetate as the starting material. Would the operation of the ctric acid cycle be affected by extensive use of oxaloacetate for gluconeogenesis? What role does the free energy of citrate synthase (-32.2 kJ/mol) play in this regulation? (10 points)

Explanation / Answer

[I am going to answer part 'a' asked by you. I hope it is helpful. Thanks]

In the citric acid cycle, the acetyl coA combines with oxaloacetate to form citrate. If one turn of this citric acid cycle occurs, it causes the regeneration of oxaloacetate and two molecules of carbon dioxide. So, there is no net syntehsis of the oxaloacetate molceule. If any of the intermediate of the citric acid cycle is channeled into biosynthetic reactions, there must be replenishemnt of the oxaloacetate molecule. Oxaloacetate depletion will inhibit the citric acid cycle. Four enzymes produces oxaloacetate from pyurvate and phosphoenolpyurvate.

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