What factors might decrease the pool of oxaloacetate available for the activity

Question

What factors might decrease the pool of oxaloacetate available for the activity of the citric acid cycle? How can the pool of oxaloacetate be replenished?

Oxaloacetate is formed in the last step of the citric acid cycle by the NAD+-dependent oxidation of l-malate. Can a net synthesis of oxaloacetate from acetyl-CoA occur using only the enzymes and cofactors of the citric acid cycle, without depleting the intermediates of the cycle? Explain. How is oxaloacetate that is lost from the cycle (to biosynthetic reactions) replenished?

Mammalian liver can carry out gluconeogenesis using oxaloacetate as the starting material. How does this occur? Would the operation of the citric acid cycle be affected by extensive use of oxaloacetate for gluconeogenesis? Explain your answer.

Explanation / Answer

1. In gluconeogensis oxaloacteate is formed from pyruvate which acts as a substrate for gluconeogensis and also in aspartate synhesis.

* Oxaloacteate can be replinshed into the cycle by the enzyme pyruvate carboxylase which converts pyruvate to oxaloacteate. This reaction is called anaplerrotic reaction means which leads to replinshment of the pathway components.

B. No The net synthesis of oxaloacteate from acetyl-coA can not occur oonly by using the enzymes and cofactors used in the citric cycle because at the beginning we are using acetyl group which has 2 carbons in the citric acid cycle and 2 molecules of CO2 is released out so net synthesis of oxaloacteate can not occur. The oxaloacteate can be replinshed by the anplerotic reactions by the enzyme pyruvate carboxylase which converts pyruvate to oxaloacteate.

C. In mitochondria pyruvate is converted to oxaloacteate and is converted to malate to exit the mitochondria and again convert to oxaloacteate and then to phphosphoenol pyruvate and so on.

By excess use of oxaloacteate in gluconeogensis TCA cycle slows down and acetyl coA oxidation occur.

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