Co

ch2opo3~

Dihydroxyacetone phosphate triose phosphate isomerase ch2opo|-

Glyceraldehyde 3-phosphate ch2opo|-

Glyceraldehyde 3-phosphate aG"

7.5 kJ/mol

The reaction mechanism is similar to the reaction promoted by phosphohexose isomerase in step (2 of glycolysis (Fig. 14-4). After the triose phosphate isomerase reaction, C-1, C-2, and C-3 of the starting glucose are chemically indistinguishable from C-6, C-5, and C-4, respectively (Fig. 14-6), setting up the efficient metabolism of the entire six-carbon glucose molecule.

This reaction completes the preparatory phase of glycolysis. The hexose molecule has been phosphory-lated at C-1 and C-6 and then cleaved to form two molecules of glyceraldehyde 3-phosphate.

The Payoff Phase of Glycolysis Yields ATP and NADH

The payoff phase of glycolysis (Fig. 14-2b) includes the energy-conserving phosphorylation steps in which some of the free energy of the glucose molecule is conserved in the form of ATP. Remember that one molecule of glucose yields two molecules of glyceraldehyde 3-phos-phate; both halves of the glucose molecule follow the same pathway in the second phase of glycolysis. The conversion of two molecules of glyceraldehyde 3-phos-phate to two molecules of pyruvate is accompanied by the formation of four molecules of ATP from ADP. However, the net yield of ATP per molecule of glucose degraded is only two, because two ATP were invested in the preparatory phase of glycolysis to phosphorylate the two ends of the hexose molecule.

6 Oxidation of Glyceraldehyde 3-Phosphate to 1,3-Bisphos-phoglycerate The first step in the payoff phase is the oxidation of glyceraldehyde 3-phosphate to 1,3-bis-phosphoglycerate, catalyzed by glyceraldehyde 3-phosphate dehydrogenase:

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