STRUCTURALLY CONSERVED WATER-MOLECULES IN RIBONUCLEASE-T1

In the high resolution (1.7-1.9 angstrom) crystal structures of ribonuclease T1 (RNase T1) in complex with guanosine, guanosine 2'-phosphate, guanylyl 2',5'-guanosine, and vanadate, there are 30 water sites in nearly identical (+/- 1 angstrom) positions that are considered conserved. One water is tightly bound to Asp76(O-delta), Thr93(O-gamma), Cys6(O), and Asn9(N); another bridges two loops by hydrogen-bonding to Tyr68(O-eta) and to Ser35(N), Asn36(N); a loop structure is stabilized by two waters coordinated to Gly31(O) and His27(N-delta), and by water bound to cis-Pro39(O). Most notable is a hydrogen-bonded chain of 10 water molecules. Waters 1-5 of this chain are inaccessible to solvent, are anchored at Trp59(N-epsilon), and stitch together the loop formed by segments 60-68; waters 5-8 coordinate to Ca2+, and waters 9 and 10 hydrogen-bond to N-terminal side chains of the alpha-helix. The water chain and two conserved water molecules are bound to amino acids adjacent to the active site residues His40, Glu58, Arg77, and His92; they are probably involved in maintaining their spatial orientation required for catalysis. Water sites must be considered in genetic engineering; the mutation Trp59Tyr, which probably influences the 10-water chain, doubles the catalytic activity of RNase T1.