In contrast to the oxidative citric acid cycle, the reverse or reductive cycle has a few key differences. There are three enzymes specific to the reductive citric acid cycle – citrate lyase, fumarate reductase, and α-ketoglutarate synthase.
The splitting of citric acid to oxaloacetate and acetate is in catalyzed by citrate lyase, rather than the reverse reaction of citrate synthase. Succinate dehydrogenase is replaced by fumarate reductase and α-ketoglutarate synthase replaces α-ketoglutarate dehydrogenase.
The conversion of succinate to 2-oxoglutarate is also different. In the oxidative reaction this step is coupled to the reduction of NADH. However, the oxidation of 2-oxoglutarate to succinate is so energetically favorable, that NADH lacks the reductive power to drive the reverse reaction. In the rTCA cycle, this reaction has to use a reduced low potential ferredoxin.