STRUCTURE OF RIBONUCLEASE-A DERIVATIVE-II AT 2.1-ANGSTROM RESOLUTION

The crystal structure of bovine pancreatic ribonuclease A derivative I I, a covalent derivative obtained by reaction of 6-chloropurine 9-beta -D-ribofuranosyl 5'-monophosphate with the Lambda-amino group of Lys-1 , has been determined and refined at 2.1-Angstrom resolution with an a greement factor R = 0.166 for 6254 reflections in the resolution shell 8.0 to 2.1 Angstrom. Crystals are orthorhombic and belong to space gr oup C222(1) with unit cell parameters a = 75.73 Angstrom, b = 57.85 An gstrom, and c = 53.26 Angstrom. This crystal packing had never been re ported before for pancreatic ribonuclease nor its complexes. The struc ture found is in accordance with the location of p(2), B-3, and R(3) s ubsites at the N-terminal region of the protein and provides an explan ation of the catalytic behavior observed for this derivative. In parti cular, differences in kinetic parameters and in the pK(a) value of His -119 between derivative II and native ribonuclease A can be interprete d on the basis of the position of the phosphate moiety within the deri vative structure. Some uncertainty remains on the nucleotide sugar con formation determined.