The bifunctional enzyme formiminotransferase-cyclodeaminase is a tetramer of dimers

Formiminotransferase-cyclodeaminase, an octameric protein of identical, bifunctional polypeptides of Mr = 62,000, yields a transferase-active fragment of Mr = 80,000 upon proteolysis with chymotrypsin in the presence of the inhibitor folic acid. The purified fragment contains one size of polypeptide, Mr = 39,000, on dodecyl sulfate gels. Cross-linking with the bifunctional reagent dithiobis(succinimidyl propionate) confirmed the dimeric structure of the purified fragment. Reaction of the native octamer with the very short bifunctional reagent difluorodinitrobenzene yields dimer and tetramer in excess of trimer, thereby indicating two types of subunit interaction in the protein. The isolation of a dimeric fragment after proteolysis and the results of cross-linking support a tetramer of dimers structure for the native enzyme. The purified transferase fragment has approximately 68% of the activity of the native enzyme, but has lost specificity for the naturally occurring polyglutamate derivatives of tetrahydrofolate. This is illustrated by an increase in Km for tetrahydropteroylpentaglutamate from 3.4 microM with the native transferase to 89 microM with the fragment transferase. It is suggested that the bifunctional enzyme may have only one polyglutamate binding site/pair of transferase-deaminase sites.