Please answer question c Oxaloacetate is formed in the last step of the ctric ac
ID: 269000 • Letter: P
Question
Please answer question c
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
Oxaloacetate depletion would tend to inhibit the citric acid cycle. Oxaloacetate is present at relatively low concentrations in mitochondria, and removing it for gluconeogenesis would tend to shift the equilibrium for the citrate synthase reaction toward oxaloacetate. However, anaplerotic reactions counter this effect by replacing oxaloacetate. Citrate synthase catalyzes the condensation reaction of the two-carbon acetate residue from acetyl coenzyme A and a molecule of four-carbon oxaloacetate to form the six-carbon citrate.