d phase, glyceraldehyde-3-phosphate dehydrogenase catalyzes the NAD+-dependent oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate and NADH. The high-energy phosphate of 1,3-bisphosphoglycerate is used to form ATP. The remaining reactions are aimed at converting the relatively low energy phosphoacyl ester of 3-phosphoglycerate to a high energy form and harvesting the phosphate as ATP.
In the Krebs cycle (citric acid cycle), pyruvate from glycolysis and fatty acids combine with acetylCoA in a series of reactions which generates reducing equivalents for the electron transport-oxidative phosphorylation sequence to generate ATP (Devlin; Krebs). The first reaction is the condensation of oxaloacetate to produce citrate using the enzyme citrate synthetase. The second step involves a sequential dehydration and hydration reaction to form the D-isocitrate isomer using the aconitase enzyme. Next, there is oxidative decarboxylation using isocitrate dehydrogenase, which generates carbon dioxide by an NAD+ linked enzyme. A second oxidative decarboxyl`tion stdp is performed using the ?-ketoglutarate dehydrogenation complex producing succinlyCoA, a high potential energy molecule. This energy is used to form a high-energy phosphate bond in a guanine nucleotide diphosphate (GDP) molecule by succinylCoA synthetase, and most of the GTP formed is used to form ATP by nucleotide diphosphokinase. One turn of the citric acid cycle generates one high-energy phosphate through substrate-level phosphorylation, three NADH and one FADH2.
The reduced coenzymes produced by the Krebs cycle are immediately used to produce ATP, and the electrons carried on these molecules are passed down an electron transport chain (Biology). As they are passed down the chain, protons are pumped out of the inner compartment into the outer compartment of mitochondria and the ultimate proton acceptor is oxygen, which is reduced to water. Transport of pro...