TRANSFER-RNA-GUANINE TRANSGLYCOSYLASE FROM ESCHERICHIA-COLI - STRUCTURE-ACTIVITY STUDIES INVESTIGATING THE ROLE OF THE AMINOMETHYL SUBSTITUENT OF THE HETEROCYCLIC SUBSTRATE PREQ(1)

A series of 5-substituted 2-aminopyrrolo[2,3-d]pyrimidin-4(3H)-ones ha ve been synthesized in order to study the substrate specificity of the tRNA-guanine transglycosylase (TGT) from Escherichia coli. A number o f these compounds were initially examined as inhibitors of radiolabele d guanine incorporation into tRNA catalyzed by TGT [Hoops, G. C., Garc ia, G. A., & Townsend, L. B. (1992) 204th National Meeting of the Amer ican Chemical Society, Washington, DC, August 23-28, 1992, Division of Medicinal Chemistry, Abstract 113]. The kinetic parameters of these a nalogues as substrates in the TGT reaction have been determined by mon itoring the loss of radiolabeled guanine from 8-[C-14]G34-tRNA. This s tudy reveals that the tRNA-guanine transglycosylase from E. coli will tolerate a wide variety of substituents at the 5-position. The role of the 5-substituent appears to be entirely in binding/recognition with no apparent effects upon catalysis. A correlation between N7 pK(a) and V-max suggests the deprotonation of N7 during the reaction, which mus t occur prior to subsequent glycosidic bond formation, appears to be p artially rate-determining for the natural substrate. Comparison of the K(i)s of 7-methyl-substituted competitive inhibitors to the K(m)s of their corresponding substrates suggests that some substrates (includin g preQ(1)) are kinetically ''sticky'' (i.e., K-m is equivalent to K-d) and other substrates have K(m)s that reflect catalytic rates as well as binding.