In the presence of saturating amounts of oxaloacetate, the activity of citrate synthase from pig heart tissue shows a sigmoid dependence on the concentration of acetyl‑CoA. When succinyl‑CoA is added, the curve shifts to the right and the sigmoid dependence is more pronounced. Choose the statements that are reasonable explanations for the right-ward shift of the velocity curve caused by succinyl-CoA.

a. Succinyl-CoA competes with dissociation of CoA as a product of the reaction.
b. Succinyl-CoA binds covalently to the enzyme.
c. Succinyl-CoA binds at a regulatory site other than the active site.
d. Succinyl-CoA competes with acetyl-CoA for binding at the active site.
e. Succinyl-CoA competes with oxaloacetate for binding at the active site.

c. Succinyl-CoA binds at a regulatory site other than the active site

Explanation:

The sigmoid saturation curve is a property of of allosteric enzymes which reflects the cooperaive interactions between the protein subunits in the enzymes.

Allosteric enzymes function through reversible, non-covalent binding of regulatory compounds called allosteric modulators.

Allosteric enzymes  have different binding site for the substrate and for their allosteric modulator.

In the citric acid cycle, succinyl-CoA serves as as allosteric inhibitor of citrate synthase which catalyses the production of citrate from oxaloacetate and acetyl-CoA.

From the given options:

a. Succinyl-CoA competes with dissociation of CoA as a product of the reaction is wrong because it has its own binding site as an allosteric modulator.

b. Succinyl-CoA binds covalently to the enzyme is wrong because allosteric enzymes functions through non-covalent interactions.

c. Succinyl-CoA binds at a regulatory site other than the active site is true because citrate synthase is an allosteric enzyme

d. Succinyl-CoA competes with acetyl-CoA for binding at the active site is wrong because acetyl-CoA binds to the active site while succinyl-Coa binds to the regulatory site.

e. Succinyl-CoA competes with oxaloacetate for binding at the active site is wrong because oxaloacetate binds to the active site while succinyl-Coa binds to the regulatory site.

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