Sandbox Reserved 477: Difference between revisions

The mechanism using GAPDH to convert glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate in the sixth step of glycolysis is a two step process. The first reaction is the oxidation of glyceraldehyde-3-phosphate at the 1st carbon position. At this position, an aldehyde is converted into a carboxylic acid while NAD+ is simultaneously reduced to NADH. This reaction is energetically favorable with a Gibb's free energy of -50kJ/mol. The energy released by the first reaction drives the second reaction. GAPDH uses covalent catalysis and general base catalysis to decrease the very large and positive activation energy of the second step of this reaction. First, a cysteine residue in the active site of GAPDH attacks the carbonyl group of GAP, creating a hemithioacetal intermediate. Next, an adjacent, tightly bound molecule of NAD+ accepts a hydride ion from GAP, forming NADH; GAP is oxidized to a thioester intermediate using a molecule of water. Donation of the hydride ion by the hemithioacetal is facilitated by its deprotonation by a histidine residue in the enzyme's active site. Deprotonation encourages the reformation of the carbonyl group in the thioester intermediate and ejection of the hydride ion. NADH leaves the active site and is replaced by another molecule of NAD+. A molecule of inorganic phosphate is transferred to the intermediate to form the final product of this reaction that has a high phosphoryl-transfer reaction, 1,3-bisphosphoglycerate.