[Topochemical aspects of pyrimidine specificity of ribonuclease A].

To get insight into the origin of pyrimidine specificity of ribonuclease A, a study of the enzyme interaction with the substrate analogs having a modified nucleobase was undertaken. Pyridine and pyrimidine cyclophosphates were obtained by phosphorylation of 2'(3')-O-dibutyl stannylidene derivatives of nonprotected nucleosides in high yields. The results of kinetic and NMR studies suggested that a substrate should be locked in anti-conformation in the productive enzyme-substrate complex as it was shown for the crystalline complexes of the enzyme with pyrimidine nucleotides by X-ray analysis. The interaction between carbonyl group in position 2 of substrate nucleobase and proton accepting group of the protein (NH of Thr45) was found to be a prerequisite for the specific recognition of a substrate by the enzyme. The rate constants for transformation of lactam form (slow) and lactim form (fast) of pyrimidine substrates were estimated.