Structural elements contribute to the calcium/calmodulin dependence on enzyme activation in human endothelial nitric-oxide synthase

Two regions, located at residues 594-606/614-645 and residues 1165-1178, are present in the reductase domain of human endothelial nitric-oxide synthase (eNOS) but absent in its counterpart, inducible nitric-oxide synthase (iNOS). We previously demonstrated that removing residues 594-606/614-645 resulted in an enzyme (Delta45) containing an intrinsic calmodulin (CaM) purified from an Sf9/baculovirus expression system (Chen, P.-F., and Wu, K.K. (2000) J. Biol. Chem. 275, 13155-13163). Here we have further elucidated the differential requirement of Ca2+/CaM for enzyme activation between eNOS and iNOS by either deletion of residues 1165-1178 (Delta14) or combined deletions of residues 594-606/614-645 and 1165-1178 (Delta45/ Delta14) from eNOS to mimic iNOS. We measured the catalytic rates using purified proteins completely free of CaM. Steady-state analysis indicated that the Delta45 supported NO synthesis in the absence of CaM at 60% of the rate in its presence, consistent with our prior result that CaM-bound Delta45 retained 60% of its activity in the presence of 10 mm EGTA. Mutant Delta14 displayed a 1.5-fold reduction of EC50 for Ca2+/CaM-dependence in l-citrulline formation, and a 2-4-fold increase in the rates of NO synthesis, NADPH oxidation, and cytochrome c reduction relative to the wild type. The basal rates of double mutant Delta45/Delta14 in NO production, NADPH oxidation, and cytochrome c reduction were 3-fold greater than those of CaM-stimulated wild-type eNOS. Interestingly, all three activities of Delta45/ Delta14 were suppressed rather than enhanced by Ca2+/CaM, indicating a complete Ca2+/CaM independence for those reactions. The results suggest that the Ca2+/CaM-dependent catalytic activity of eNOS appears to be conferred mainly by these two structural elements, and the interdomain electron transfer from reductase to oxygenase domain does not require Ca2+/CaM when eNOS lacks these two segments.