Question

In the citric acid cycle, what does the ATP do?

Answer 1

Answer:

ATP acts as a regulatory molecule in multiple steps of the citric acid cycle:

1. An abundance of ATP inhibits the pyruvate dehydrogenase complex that converts pyruvate into Acetyl Co-A.
2. ATP acts as an inhibitor for the enzymes, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.
3. In bacteria, ATP is an allosteric inhibitor of the enzyme, citrate synthase.

Explanation:

In the citric acid cycle, the 2 pyruvate molecules formed can either be converted to Acetyl Co-A or stored as lipids. As the enzyme, pyruvate dehydrogenase has to make this decision, this step of the TCA cycle is a crucial control point of the pathway. Pyruvate dehydrogenase is activated when phophorylated by a kinase but deactivated in the presence of high amount of ATP by a phosphatase enzyme. On the contrary, the presence of high ADP activates the complex.

ADP or a low energy signal allosterically activates the isocitrate dehydrogenase that converts isocitrate to a five carbon alpha-ketoglutarate. High ATP inhibits the enzyme.

The enzyme, α-ketoglutarate dehydrogenase converts alpha ketoglutarate to a four carbon compound, succinyl Co-A. Here, ATP also acts as an inhibitor.