Ribonuclease T1 with free recognition and catalytic site: crystal structure analysis at 1.5 A resolution.

The free form of ribonuclease T1 (RNase T1) has been crystallized at neutral pH, and the three-dimensional structure of the enzyme has been determined at 1.5 A nominal resolution. Restrained least-squares refinement yielded an R value of 14.3% for 12,623 structure amplitudes. The high resolution of the structure analysis permits a detailed description of the solvent structure around RNase T1, the reliable rotational setting of several side-chain amide and imidazole groups and the identification of seven disordered residues. Among these, the disordered and completely internal Val78 residue is noteworthy. In the RNase T1 crystal structures determined thus far it is always disordered in the absence of bound guanosine, but not in its presence. A systematic analysis of hydrogen bonding reveals the presence in RNase T1 of 40 three-center and an additional seven four-center hydrogen bonds. Three-center hydrogen bonds occur predominantly in the alpha-helix, where their minor components close 3(10)-type turns, and in beta-sheets, where their minor components connect the peptide nitrogen and carbonyl functions of the same residue. The structure of the free form is compared with complexes of RNase T1 with filled base recognition site and/or catalytic site. Several structural rearrangements occurring upon inhibitor or substrate binding are clearly apparent. In conjunction with the available biochemical knowledge, they are used to describe probable steps occurring early during RNase T1-catalyzed phosphate transesterification.