Studies of the coenzyme binding site and essential sulfhydryl group of years 6-phosphogluconate dehydrogenase

Abstract

Studies of the binding of coenzyme analogs to yeast 6-phosphogluconate dehydrogenase indicate that NADP binding to the enzyme results from selective interactions between regions of the coenzyme binding site and portions of the NADP molecule. These studies suggested the existence of coenzyme binding site regions which selectively interact with the adenosine, 2'-phosphate, and pyrophosphate moieties of NADP. The importance of the 2'-phosphate to coenzyme binding was indicated by enhanced binding of adenosine derivatives possessing this moiety when compared to adenosine derivatives not phosphorylated at this position. The better binding of the 2'-phosphorylated derivatives became more pronounced with increasing resemblance of the derivative to the NADP molecule, and NAD was not inhibitory up to 70 roM. These results substantiate the concept that interaction of the enzyme with the d2'-phosphate is a key factor in the specificity of yeast 6-phosphogluconate dehydrogenase for NADP. Structural analogs of the pyridinium portion of the NADP molecule, Nl-alkylnicotinamide chlorides, did not inhibit yeast 6-phosphogluconate dehydrogenase at concentrations normally required for selective interactions with dehydrogenases; however, enzyme activity was decreased at micellar concentrations of Nl-dodecylnicotinamide chloride. Investigations of the role and environment of the essential sulfhydryl group of this enzyme were also performed. N-alkylmaleimides (N-methyl - N-hexyl, inclusive) were shown to inactivate the enzyme, but without a chainlength effect. In the presence of 6-phosphogluconate, the enzyme was protected from N-ethylmaleimide inactivation and this protection was enhanced by the addition of NADPH or AADP.