DELETION OF THE OMEGA-LOOP IN THE ACTIVE-SITE OF STAPHYLOCOCCAL NUCLEASE .2. EFFECTS ON PROTEIN-STRUCTURE AND DYNAMICS

It has been shown (Poole et al., 1991) that deletion of residues 44-49 from the sequence of staphylococcal nuclease (E43 SNase) results in an enzyme (E43 DELTA-SNase) that is significantly more active than D43 SNase, an enzyme that differs from the wild-type enzyme by deletion of a single methylene group. In addition, both E43 DELTA-SNase and D43 DELTA-SNase are significantly more stable than their respective parent enzymes. Herein we use high-resolution 2D and 3D NMR spectroscopy to characterize the solution conformations of the four enzymes in order to better understand their differences in stability and activity. The backbone assignments of E43 SNase were extended to the three mutant proteins (uniformly N-15-enriched) by using 2D HSQC, 3D HOHAHA-HMQC, and 3D NOESY-HMQC spectra. The NOE patterns observed for E43 and D43 SNase in solution are consistent with the crystal structures of these proteins. The NOESY data further show that the intact and deleted proteins have essentially the same structures except that (a) the disordered OMEGA-loops in the intact proteins are replaced by tight type II' turns, formed by residues 43-50-51-52, in the deleted proteins and (b) the orientation of the D43 side chain in crystalline D43 SNase differs from that found for D43 DELTA-SNase in solution. Except for regions neighboring the OMEGA-loops, the intact and deleted proteins show nearly identical amide N-15 and H-1 chemical shifts. In contrast, there are widespread, small and similar, chemical shift differences (a) between E43 SNase and D43 SNase and (b) between E43 DELTA-SNase and D43 DELTA-SNase. This observation indicates that deletion of the E43 gamma-methylene group causes small, widespread, and similar changes in the structures of E43 SNase and E43 DELTA-SNase.