Glyceraldehyde-3-Phosphate Dehydrogenase (G3PDH)

G3PDH catalyzes the oxidation/reduction of D-glyceraldehyde-3-phosphate (G3P) and 1,3-bisphosphoglycerate (1,3BPG).

> G3P + NAD+ + Pi <=> 1,3BPG + NADH + H+ ( = + 6.3 kJ/mol)

The of the enzymatic reaction is sufficiently low for it to be used in glycolysis, gluconeogenesis, and the Calvin cycle.

The reaction involves a two-electron oxidation of the carbonyl carbon of G3P to the carboxyl level, a reaction that is normally quite exergonic. However, the overall reaction is slightly endergonic (under standard conditions), because the enzyme utilizes most of the energy released to drive the synthesis of a super-high-energy compound, 1,3BPG. This compound contains a carboxylic-phosphoric acid anhydride group at position 1, a functional group with a very high standard free energy of hydrolysis, -49.4 kJ/mol.

Because the acyl-phosphate group is much more energy rich than the phosphate anhydride of ATP, 1,3BPG can drive the synthesis of ATP from ADP. Indeed, it does so in the glycolysis reaction catalyzed by the enzyme phosphoglycerate kinase.

Inhibition of glycolysis by iodoacetate and heavy metals, such as mercury, occurs because each of these compounds reacts with the free sulfhydryl groups of G3PDH. As shown in Figure 13.5, formation of a thiohemiacetal group involving a cysteine thiol group on G3PDH as an essential catalytic intermediate explains this earlier observation.

Previous step of glycolysis; Next step of glycolysis;

See also: Reactions/Energies of Glycolysis, Reaction Picture

INTERNET LINKS:

> 1. RasMol Structure (slow)

> 2. Glycolysis/Gluconeogenesis