3. Oxaloacetate is formed in the last step of the ctric acid cycle by the NAD\"-

Question

3. 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

3.a . A net synthesis of oxaloacetate from acetyl coA can not occur only by using enzymes and cofactors of the citric acid cycle.Intermediates should be depleted.The cycle can run in two ways - one is the normal citric acid cycle and the other is glyoxylate cycle.But in both the case - citrate and isocitrate are forming atleast from oxaloacetate and acetyl coA .For further proceed that is to make glyoxylate from isocitrate ( glyoxylate cycle) or alpha keto glutarate from isocitrate ( citric acid cycle) ,isocitrate needs to be depleted. But another fact is true - that all the reactions are in equillibrium and so the depleted amount is goint to be generated for every step and every intermediate of the cycle.So if we consider the equilibrium this can be possible otherwise it is not.

b. When oxaloacetate is lost from the cycle that is replenished by the reversible carboxylation of pyruvate by carbon di oxide to form oxaloacetate by the enzyme pyruvate carboxylase.Pyruvate is thus carboxylated to produce more oxaloacetate.This occurs in mammalian liver and kidney.

c. citrate synthase is the enzyme which forms citrate by breaking down of acetyl coA and oxaloacetate.It has a negative free energy that means the reaction is always spontaneous that is whenever oxaloacetate is present it will be converted into citrate.. In case of gluconeogenesis oxaloacetate is the starting material produced by pyruvate.If oxaloacetate is present it will be utilised first for citric acid cycle and then for gluconeogenesis.So citric acid cycle won't be affected.

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