Final answer:
The first step of the citric acid cycle is controlled by ATP concentration, which regulates the condensation of acetyl groups with oxaloacetate to form citrate. High ATP levels decrease the rate, while low ATP levels increase it.
Step-by-step explanation:
The concentration of ATP controls the rate at which the first step of the citric acid cycle occurs. This initial step involves the combination of a two-carbon acetyl group with a four-carbon oxaloacetate molecule to form six-carbon citrate, a reaction catalyzed by the enzyme citrate synthase. In the process, a molecule of CoA, which is bound to a sulfhydryl group, diffuses away to eventually combine with another acetyl group.
The reaction is subject to negative feedback: when ATP levels are high, the rate of the reaction decreases, signaling that the cell has ample energy. Conversely, when ATP is scarce, indicating a greater need for energy, the rate of the citric acid cycle increases to facilitate greater ATP production.