RESONANCE RAMAN CHARACTERIZATION OF RHODOBACTER-SPHAEROIDES REACTION CENTERS BEARING SITE-DIRECTED MUTATIONS AT TYROSINE M210

Resonance Raman (RR) spectroscopy and low-temperature absorption spectroscopy have been used to investigate the structural changes in the reaction centers (RCs) of Rhodobacter sphaeroides induced by site-specific mutations on the tyrosine (Y) M210 residue. RCs in which Y M210 has been genetically replaced with phenylalanine (F) or leucine (L) exhibit a 5-fold decrease in their primary electron-transfer kinetics (Finkele et al., 1990). The general similarity of RR spectra of the wild-type RCs as compared to those of the two mutant RCs indicates that no significant global structural changes occur upon mutation at the level of any of the six bacteriochlorin pigments. In the RR spectra of the two mutant RCs there is a conspicuous absence of contributions from the BPhe(M) prosthetic group, which is interpreted in terms of a change in the resonance enhancement conditions of this chromophore. Low-temperature absorption spectroscopy reveals marked shifts in the Q-chi absorption band of BPhe(M). This shift is interpreted as arising from a destabilization of the protein in the vicinity of BPhe(M) and accounts for the change in resonance condition for this chromophore in its RR contributions. As well, there is a 3-nm red shift of the Q-gamma absorption band of the BChls from 803 to 806 nm for the mutant RCs. Difference RR spectra yielding structural information concerning, selectively, the primary donor (P) indicate that the structure of the P binding pocket is conserved for these mutant RCs. The tyrosine M210 is not observed to be engaged in a hydrogen bond with either of the acetyl or keto carbonyls of P.