Escherichia coli tRNA (uracil-5-)-methyltransferase: Inhibition by analogues of adenosylhomocysteine.

Structural analogues of adenosylhomocysteine (AdoHcy) have been tested as inhibitors of a tRNA(uracil-5-)-methyltransferase preparation obtained from Escherichia coli. All analogues tested gave linear competitive inhibition kinetics with adenosylmethionine (AdoMet) as the variable substrate. Comparison of the Ki values obtained leads to the following conclusions concerning the specificity of the AdoMet-AdoHcy binding site on the enzyme: (i) the terminal amino group of the amino acid moiety is necessary for activity; (ii) both a chiral change of the asymmetric carbon atom of homocysteine and the presence of the terminal carboxyl group contribute little towards inhibitory activity; (iii) analogues in which the amino function of the adenyl moiety is modified or substituted are still potent inhibitors; (iv) inhibitor specificity is considerably reduced when adenine is replaced by a pyrimidine base.