The comparative structure of mammalian glyceraldehyde 3-phosphate dehydrogenases.

1. The amino acid sequences around the thiol groups of glyceraldehyde 3-phosphate dehydrogenase from badger and monkey skeletal muscle were compared with the sequences around the thiol groups in the enzyme isolated from other organisms. 2. Preliminary evidence of the existence of isoenzymes in the badger was obtained. Only the major form, however, could be purified completely. 3. The monkey enzyme contains only three cysteine residues per polypeptide chain compared with the four found in all the other mammalian enzymes so far examined, including that of badger, and the two in yeast. The missing thiol group in monkey was identified as residue 281 in the corresponding sequence of the pig enzyme. 4. These experiments rule out any essential role for cysteine-281 in the function of the mammalian enzymes. 5. Further evidence of the remarkable conservation of amino acid sequence in this enzyme during evolution is presented and discussed.