Final answer:
b) After oxaloacetate; before acetyl CoA The citrate synthase mechanism in the citric acid cycle involves oxaloacetate binding before acetyl CoA, forming citrate which then leaves before CoA is released.
Step-by-step explanation:
The order of events in the citrate synthase mechanism of the citric acid cycle is best described as oxaloacetate binding before acetyl CoA.
The reaction initiated by citrate synthase is catalytic, combining oxaloacetate with the acetyl group from acetyl CoA, forming citrate. Oxaloacetate must be present first for the enzyme to undergo a conformational change ready to bind acetyl CoA tightly.
Then, the six-carbon citrate is formed and leaves before Coenzyme A (CoA) is released.
In this condensation reaction, the enzyme citrate synthase plays a crucial role. The process begins with the binding of oxaloacetate to the active site of the enzyme, followed by the subsequent binding of acetyl CoA.
This leads to the formation of citrate, which is released, and then CoA is released last. This ordered sequence ensures that the citric acid cycle continues efficiently, contributing to the cell's energy production.
The citrate synthase mechanism is an irreversible reaction and is a key regulatory point of the citric acid cycle.