Final answer:
Four steps in the citric acid cycle are catalyzed by dehydrogenase enzymes, namely the isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, succinate dehydrogenase, and malate dehydrogenase steps. These steps are essential for the production of NADH and are regulated by the cell's energy levels.
Step-by-step explanation:
The citric acid cycle, taking place in the mitochondrial matrix, includes a series of redox reactions catalyzed by various enzymes. Out of the eight steps of the cycle, four are catalyzed by dehydrogenase enzymes.
These steps are where isocitrate is oxidized to α-ketoglutarate by isocitrate dehydrogenase, α-ketoglutarate is further processed to succinyl-CoA by α-ketoglutarate dehydrogenase, succinate is oxidized to fumarate by succinate dehydrogenase, and malate is oxidized to oxaloacetate by malate dehydrogenase.
The isocitrate dehydrogenase and α-ketoglutarate dehydrogenase reactions are key control points in the cycle. These enzymes are particularly regulated by energy levels in the cell, and play a pivotal role as they make the first two molecules of NADH in the cycle.