The number and role of histidine residues in the active site of guanyloribonuclease Sa.

The number and role of histidine residues in the active site of extracellular guanyloribonuclease Sa produced by Streptomyces aureofaciens (RNAase Sa) were studied via chemical modification by ethoxyformic anhydride by means of circular dichroism measurements. It was shown that only one of two histidines of RNAase Sa is situated in the active site of the enzyme. Ethoxyformylation of RNAase Sa in the presence of Guo-3'-P, Guo-5'-P and dGuo-5-P, all of them being competitive inhibitors of the enzyme, supported the assumption that an essential histidine residue is bound to the phosphate group in the position 3' of the ribose ring. The circular dichroism measurements of native and modified RNAase Sa and of its complex with Guo-3'-P showed that the modification of the essential histidine residue resulted in alteration of binding of RNAase Sa to Guo-3'-P; histidine thus may play a key role in the formation of such a complex.