Certified Nutrition Specialist (CNS) Practice Exam 2025 - Free CNS Practice Questions and Study Guide

In the context of the citric acid cycle, citrate synthase is deemed an irreversible enzyme due to its role in catalyzing the condensation of acetyl-CoA and oxaloacetate to form citrate. This reaction is highly thermodynamically favorable, meaning it proceeds with a significant decrease in free energy. As a result, the forward reaction strongly favors the production of citrate, effectively driving the reaction towards completion and preventing a significant reverse reaction under physiological conditions.

This irreversibility is crucial for the regulation and directionality of the citric acid cycle, as it commits the substrate to enter the cycle for energy production rather than allowing it to revert easily to its precursors. The tight regulation of this enzyme also highlights the importance of acetyl-CoA concentration in the control of the citric acid cycle's activity and ultimately the metabolic fate of carbohydrates and fats.

Other enzymes in the cycle, like fumarase and malate dehydrogenase, can catalyze reversible reactions, allowing for flexibility in metabolism depending on the cell's energy demands and substrate availability. Succinylcholine CoA synthetase, while also connected to the energy production process, does not function within the direct citric acid cycle and is involved in a different metabolic