COMPARISON OF CONFORMATIONAL FEATURES OF STAPHYLOCOCCAL NUCLEASE IN TERNARY COMPLEXES WITH PDTP, PDGP, AND NITROPHENYL-PDTP

The conformations of wild-type staphylococcal nuclease (SNase) in the ternary complexes with thymidine 3',5'-bisphosphate (pdTp), 2'-deoxyguanine 3',5'-bisphosphate (pdGp), and thymidine 3'-phosphate 5'-(p-nitrophenylphosphate) (NpdTp) with Ca2+ were examined by two-dimensional NMR NOESY and ROESY experiments. The results of these experiments indicate that the conformational features of the SNase are quite similar in the three ternary complexes. This suggests that the conformational features of SNase, in these ternary complexes, are not strongly dependent on whether the 5'-phosphate is a mono- or diester. This is in contrast to our prior studies on substitutions of active site charged amino acids which indicated that the conformational features of SNase in the ternary complex are quite sensitive to substitutions for active site charged amino acids (Hibler et al., 1987; Wilde et al., 1988; Pourmotabbed et al., 1990). The similarity of the SNase conformational features in the ternary complexes with pdTp and pdGp indicates that the features of the nucleotide bound at the active site are not strong determinants of the enzyme conformation in the ternary complexes. These conclusions are in general agreement with the results on pdApdT ternary complexes with SNase which suggested that it is the conformational features of the bound nucleic acid which determine the differences in catalysis observed for SNase with different substrates (Weber et al., 1991), more so than the conformational features of the enzyme.