Complex of ribonuclease Sa with a cyclic nucleotide and a proposed model for the reaction intermediate

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Abstract
The structure of the complex of ribonuclease from Streptomyces aureofaciens (RNase Sa) with exo guanosine 2',3'-cyclophosphorothioate has been refined against 0.2-nm resolution synchrotron data using, as a starting model, coordinates from the RNase Sa: 2'-GMP complex. The refinement was based on all data over 1.0-0.2 nm and converged to a crystallographic R factor of 11.9%. This is the first structure of a microbial ribonuclease complexed with a 2',3'-cyclophosphorothioate, which is a thio analogue of the intermediate of the two-step reaction. However, exo guanosine 2',3'-cyclophosphorothioate is bound in a non-functional mode and is not hydrolysed. This structure therefore does not provide direct evidence on the identity of the amino acid residues responsible for catalytic cleavage of the substrate. However, based on present and previous results, a plausible model is proposed for the complex of the cyclic intermediate which acts as substrate for the second step of the catalysis.