The active Site of Ribonuclease A from the Crystallographic Studies of Ribonuclease‐A‐Inhibitor Complexes

Abstract

The results of X-ray analyses of three ribonuclase-A-nucleotide complexes, at 2.3 Å, are reproted, A modified purine mononucleotide, 8-oxo-guanosine 2′-phosphate in a synconformation binds at the pyrimidine-binding site of the catalytic cleft. Solvent molecules are expelled from the active site due to inhibitor binding. The positions of the side-chains of the active-site residues Gln-11, His-12 and Thr-45 are well defined and do not alter on inhibitor binding. The mobility of Lys-41 is greatly reduced in protein-nucleotde complexes and the terminal amino group interacts directly, with the 2′-phosphate group of the nucleotides. In the complex of the enzyme with a modified pyrimidine, cytidine-N(3)-oxide 2′-phosphate, His-119 is stabilised in the minor site of the native protein [Borkakoti, N., M, D. S. and Palmer, R. A. (1982) Acta Crystallogr. B38, 2210–2217], while in the protein-purine derivative the imidazole group is located in the major site. Inhibitor binding induces movements in the site-chains of Lys-7 and Lys-66 which also modify the conformation of the active-site cleft of ribonuclease A.