Khan Academy
MCAT Official Prep (AAMC)
The citric acid cycle is controlled so that the cellular respiration can be managed in the body. This takes place by controlling the enzymes involved in the cycle.
The citric acid cycle is controlled through the enzymes that catalyze the reactions that make the first two molecules of NADH. These enzymes are isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. When adequate ATP and NADH levels are available, the rates of these reactions decrease. When more ATP is needed, as reflected in rising ADP levels, the rate increases.
α-Ketoglutarate dehydrogenase will also be affected by the levels of succinyl CoA, a subsequent intermediate in the cycle, causing a decrease in the activity. A decrease in the rate of operation of the pathway at this point is not necessarily negative as the increased levels of the α-ketoglutarate not used by the citric acid cycle can be used by the cell for amino acid (glutamate) synthesis.
Practice Questions
Section Bank B/B Section Question 88
Key Points
• The citric acid cycle is controlled through the enzymes that break down the reactions that make the first two molecules of NADH.
• The enzyme isocitrate dehydrogenase and α-ketoglutarate dehydrogenase helps to regulate the citric acid cycle.
• The regulation of the cycle depends on the ATP and NADH levels.
Key Terms
isocitrate dehydrogenase: an enzyme that catalyzes the oxidative decarboxylation of isocitrate into alpha-ketoglutarate during the citric acid cycle.
α-ketoglutarate dehydrogenase: an enzyme that catalyzes the conversion of α-ketoglutarate to succinyl-CoA and produces NADH
citric acid cycle: a series of chemical reactions used by all aerobic organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats, and proteins into carbon dioxide
NADH/NAD+: an electron shuttle which delivers high energy electrons to the electron transport chain where they will eventually power the production of 2 to 3 ATP molecules; when this molecule has been oxidized (lost electrons), it is left with a positive charge and is called NAD+
adenosine triphosphate (ATP): an organic molecule that stores chemical energy and is the main energy source for cells
