Evidence for the proximity of two sulfhydryl groups at the active site of 6-phosphogluconate dehydrogenase

The reaction between 6-phosphogluconate dehydrogenase from Candida utilis and 5,5′-dithiobis(2-nitrobenzoate) results in the inactivation of the enzyme. At pH 6.0 the inactivation can be correlated with the modification of only one SH group per enzyme subunit. The modified SH group can react with another SH group forming an intramolecular disulfide bridge. Since the modified enzymes, either with an SH group modified or with a cystine disulfide bridge, are still able to bind the substrate and the coenzyme, gross conformational changes seem unlikely to have occurred. The results obtained suggest that the SH groups of two cysteine residues are located close to each other in the three-dimensional structure of the active site of the enzyme.