C-13 MAS NMR evidence for structural similarity of L162YL mutant and Rhodobacter sphaeroides R26 RC, despite widely different cytochrome c(2)-mediated re-reduction kinetics of the oxidized primary donor

CP/MAS NMR data collected from L162YL mutant [4'-C-13]Tyr-enriched Rhodobacter sphaeroides RCs reveal that Tyr L162 is in a slightly heterogeneous and probably rigid section of the protein complex. The differences in chemical shifts of the individual components relative to those of the [4'-C-13]Tyr Rhodobacter sphaeroides R26 response are 0.2 ppm or less. This is small compared to the total dispersion of [4'-C-13] isotropic shifts, similar to 5 ppm, which measures the shift range due to variations in the microscopic environment between the various tyrosines in the protein complex. The structural changes in the mutant with respect to Rhodobacter sphaeroides R26, as probed by the labels, are thus minimal on the scale of the NMR. This suggests that the dramatic decrease of re-reduction rate of the oxidized primary donor P upon mutation (Farchaus et al., Biochemistry 32 (1993) 10885-10893) cannot be attributed to significant structural changes in the protein. Hence the NMR is in line with the current view that the decrease of the re-reduction rate in the mutant originates from slow reorientation of the docked cytochrome. (C) 1997 Elsevier Science B.V.