Mechanism of guanosine recognition and RNA hydrolysis by ribonuclease T 1

The fungal ribonuclease RNase T1 has been co-crystallized with its inhibitor 2'-guanylic acid and was analyzed by X-ray diffraction. The enzyme is folded into an a-helix 4.5 turns long, which is covered by a four-stranded antiparallel B-sheet. Specific recognition of guanine occurs with two hydrogen bonds between main chain peptide groups of Asn43 and Asn44 and the 0(6) and N(1)-H of guanine, as well as by stacking with tyrosines 42 and 45 which sandwich the nucleobase. At the active site, Glu58, His92 are involved in hydrolysis of the phosphodiester in a general acid-base catalysis, and Arg77 is present to neutralize the charge on the phosphate.