Glyceraldehyde 3-phosphate dehydrogenases: I. The protein chains in glyceraldehyde 3-phosphate dehydrogenase from pig muscle

Abstract

Glyceraldehyde 3-phosphate dehydrogenases have been studied by methods of structural protein chemistry. Derivatives of the pig muscle enzyme prepared by oxidation of the native enzyme with performic acid, and by carboxymethylation with iodo[1- 14 C]acetic acid in 8 M -urea, have been shown to contain four unique residues of cysteic acid, and of S-[1- 14 C]carboxymethylcysteine, respectively, indicating that the structural monomer in the enzyme contains four unique cysteines and that disulphide bridges do not contribute to its molecular structure. Amino-acid and end-group analysis, and a study of tryptic digests by peptide mapping techniques have shown further that the protein monomer in the pig muscle enzyme consists of a single polypeptide chain containing approximately 330 amino acid residues, corresponding to a molecular weight of 36,000 ± 1000. It is proposed that the active enzyme comprises four such identical protein chains, each of which contains one reactive cysteine and combines with one molecule of coenzyme (NAD), to form four structurally independent catalytic sites within the quaternary structure of the active tetramer.