PROTEIN-CHROMOPHORE INTERACTIONS - SPECTRAL SHIFTS REPORT THE CONSEQUENCES OF MUTATIONS IN THE BACTERIAL PHOTOSYNTHETIC REACTION-CENTER

Features of the low temperature absorption spectra of reaction centers from a large family of Rhodobacter capsulatus strains carrying site-s pecific mutations at the M208Tyr and L181Phe positions are presented. Through systematic analysis of the observed electronic transitions (wi th accompanying vibronic bands): the primary effects of many of the mu tations have been identified and explained in terms of the reaction ce nter structure. Some of the observed shifts are comparable to those th at have been seen in other systems as a result of formation of hydroge n bonds or hexacoordination of the central Mg atom in bacteriochloroph yll species. Shifts of the Q(y) and Q(x) bacteriochlorophyll bands are also seen as a result of distant mutations near the quinones that cha nge the charge distribution of the reaction center. We include kinetic data which show that the spectral and kinetic characteristics of diff erent cofactors can be modulated depending on the particular amino aci d that is substituted at the M208 site. (C) 1998 Elsevier Science B.V. All rights reserved.