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a major functional difference between the succinyl coA-synthetase of plant and animal cell mitochondria
A fundamental difference between glycolysis and the citric acid cycle. Glycolysis takes place by a linear sequence of enzyme-catalyzed steps, whereas the sequence of reactions in the citric acid cycle is cyclical. To begin a turn of the cycle (Fig. 15-7), acetyl-CoA donates its acetyl group to the four-carbon compound oxaloacetate to form the six-carbon citrate. Citrate is then transformed into isocitrate, also a six-carbon molecule, which is dehydrogenated with loss of CO2 to yield the five-carbon compound aketoglutarate. The latter undergoes loss of CO2 and ultimately yields the four-carbon compound succinate and a second molecule of CO2. Succinate is then enzymatically converted in three steps into the fourcarbon oxaloacetate, with which the cycle began; thus, oxaloacetate is ready to react with another molecule of acetyl-CoA to start a second turn. In each turn of the cycle, one acetyl group (two carbons) enters as acetyl-CoA and two molecules of CO2 leave. In each turn, one molecule of oxaloacetate is used to form citrate but-after a series of reactionsthe oxaloacetate is regenerated. Therefore no net removal of oxaloacetate occurs; one molecule of oxaloacetate can theoretically suffice to bring about oxidation of an infinite number of acetyl groups. Four of the eight steps in this process are oxidations, in which the energy of oxidation is conserved, with high efficiency, in the formation of reduced cofactors (NADH and FADH2).


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