C-13 MAS NMR EVIDENCE FOR STRUCTURAL SIMILARITY OF L162YL MUTANT AND RHODOBACTER-SPHAEROIDES R26 RC, DESPITE WIDELY DIFFERENT CYTOCHROME C(2)-MEDIATED REREDUCTION KINETICS OF THE OXIDIZED PRIMARY DONOR

CP/MAS NMR data collected from L162YL mutant [4'-C-13]Tyr-enriched Rho dobacter sphaeroides RCs reveal that Tyr L162 is in a slightly heterog eneous and probably rigid section of the protein complex. The differen ces in chemical shifts of the individual components relative to those of the [4'-C-13]Tyr Rhodobacter sphaeroides R26 response are 0.2 ppm o r less. This is small compared to the total dispersion of [4'-C-13] is otropic shifts, similar to 5 ppm, which measures the shift range due t o variations in the microscopic environment between the various tyrosi nes in the protein complex. The structural changes in the mutant with respect to Rhodobacter sphaeroides R26, as probed by the labels, are t hus minimal on the scale of the NMR. This suggests that the dramatic d ecrease of re-reduction rate of the oxidized primary donor P upon muta tion (Farchaus et al., Biochemistry 32 (1993) 10885-10893) cannot be a ttributed to significant structural changes in the protein. Hence the NMR is in line with the current view that the decrease of the re-reduc tion rate in the mutant originates from slow reorientation of the dock ed cytochrome. (C) 1997 Elsevier Science B.V.