On the Cofactor Specificity of Glycinamide Ribonucleotide and 5-Aminoimidazole-4-carboxamide Ribonucleotide Transformylase from Chicken Liver

Tests of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) and glycinamide ribonucleotide (GAR) transformylase cofactor specificity were conducted with 5- and/or 8-deazafolate analogues formylated at N-10. Several of these compounds were found to serve as cofactors for both the enzymes. The finding that 10-formyl-8-deazafolate can be used by AICAR transformylase eliminates those mechanisms requiring cyclization to a methenyl derivative prior to carbon unit transfer for this transformylase. Surprisingly, a similar analogue, 10-formyl-5,8-deazafolate, is very effective as a cofactor for GAR transformylase in the presence or absence of the trifunctional protein which is required for 5,10- methenyl-H4-folate activity with this transformylase. This finding suggests that the trifunctional protein modulates GAR transformylase cofactor specificity by supplying the active cofactor as the iV10-formyl species, possibly through a transport process that avoids its dissociation into solution.