pH‐induced change in nucleotide binding geometry in the ribonuclease T₁‐2'‐guanylic acid complex

Abstract

At pH 4.0, the RNase T 1 ‐2'GMP complex (1) crystallizes isomorphously with the isoenzyme complex (2) (Heinemann, U. and Saenger, W., 1982, Nature 299, 27‐31). The X‐ray structure of 1 was refined with 1.9 Å data to R = 0.195. Polypeptide folding is similar in 1 and 2. However, the sugar pucker of 2'‐GMP is 2'‐ endo (3' endo in 2), and guanine binding involves four hydrogen bonds in 1, which all differ from the two bonds in 2. Phosphate contacts Glu58, Arg77, Tyr38 in 1, but His40 in 2. These changes are not due to differences in sequence between the mother‐ and isoenzyme (Gln25‐Lys) but are associated with pH changes leadingto an inactive enzyme structure.