Amino acid sequence around the pyridoxal 5'-phosphate binding sites of 6-phosphogluconate dehydrogenase.

6-Phosphogluconate dehydrogenase (6-phospho-D-gluconate: NADP+ 2-oxidoreductase(decarboxylating), EC 1.1.1.44) from Candida utilis is inhibited by reaction with pyridoxal 5'-phosphate. The aldehydic group of this compound forms a Schiff base with the epsilon-amino group of a lysine residue: reduction of this enamine with tritiated borohydride can label this amino acid. Two tryptic peptides, TS2 and TS3, have been isolated from the labelled protein and found to have the following amino acid sequences: TS2: Ile-Leu-Asx-Glx-Ala-Gly-Gly-Lys(P-Pxy)-Gly-Glx-Thr-Lys TS3: Thr-Val-Ser-Lys(P-Pxy)-Val-Asp-His-Phe-Ile-(Glx,Asx,Glx)-Ala-Lys where Lys(P-Pxy) indicates the modified lysine residue. The similarities between the amino acid sequences around the pyridoxal phosphate binding lysines of 38 peptides, obtained from enzymes which have pyridoxal phosphate as cofactor or inhibitor, are discussed and a prediction is made on the presence of reverse turns in these peptides.