The active site of 6-phosphogluconate dehydrogenase: A phosphate binding site and its surroundings

Tetrahedral anions bind to a phosphate binding site of 6-phosphogluconate dehydrogenase from Candida utilis, inhibit the enzyme competitively with the 6-phosphogluconate, decrease the reactivity of the SH groups, and mimic the protective effect of 6-phosphogluconate against some inactivating agents. The reaction of the enzyme with butanedione results in the inactivation of the enzyme associated with the modification of a single arginine residue per subunit. This arginine residue may be involved in the binding of the phosphate to the enzyme. Inactivation of the enzyme, upon reaction with permanganate, appears to be due to the oxidation to cysteic acid of a single cysteine residue per enzyme subunit. The reaction of the enzyme with either periodate or hexachloroplatinate causes the loss of the catalytic activity. This inactivation, due to an affinity labeling, is correlated with the oxidation of two SH groups per subunit to an S-S bridge. Photoinactivation of the enzyme by pyridoxal 5′-phosphate is also restricted to the active site of the enzyme. The lysine and the histidine residues involved in this photoinactivation should thus be in the vicinity of the phosphate binding site.