The Enzymatic Significance of S-Acetylation and N-Acetylation of 3-Phosphoglyceraldehyde Dehydrogenase

Abstract

Abstract Specific S- and N-acetylations of the protein by two model substrates, acetyl phosphate and P-nitrophenyl acetate, have been related to the catalytic activity of 3-phosphoglyceraldehyde dehydrogenase in the following manner. 1. DPN facilitates the S-acetylation of the dehydrogenase with acetyl phosphate but inhibits the reaction with p-nitrophenyl acetate. 2. The S-acetyl enzyme compound is more labile at pH 7.0 than simple thioesters such as S-acetyl glutathione or S-acetyl coenzyme A. Denaturation increases the stability of the S-acetyl enzyme, suggesting that labilizing groups such as histidine have been removed from the active center. 3. In the presence of DPN the S-acetyl enzyme is rapidly deacylated by transfer of the acyl group to Pi or arsenate. Neither DPN nor Pi alone induces this rapid deacylation. 4. At pH 8.5, a specific lysine residue is acetylated by an S-N transfer reaction. The N-acetyl group is enzymatically inert in that it cannot be removed by addition of DPN and Pi or arsenate. 5. The lysine moiety is not readily acetylated in the presence of DPN, and conversely the N-acetylated enzyme binds less DPN. This suggests that the lysine moiety is involved in the coenzyme-enzyme interaction. 6. A number of experiments involving S-N transfer and dual acetylation of both the cysteine and lysine residues indicates that the S and N sites are in close proximity, although not near neighbors on a peptide chain. 7. The above observations have been fitted into a general proposal for the mechanism of catalysis by this enzyme.