Evidence that Ala M260 is hydrogen-bonded to the reduced primary acceptor quinone Q A−. in reaction centers of Rb. sphaeroides

Abstract

The binding of the primary quinone anion radical Q A −. in reaction centers (Rcs) of Rhodobacter (Rb.) sphaeroides species was investigated with electron spin echo envelope modulation (ESEEM). The ESEEM spectra, at two microwave frequencies, of Zn-substituted RCs of Rb. sphaeroides R26 showed interactions of the unpaired electron of Q A −. with two nitrogen nuclei in the protein matrix. From an analysis of the experimental data the nitrogen nuclear quadrupole resonance parameters were determined: e 2 qQ/ h = 1.52 MHz, η = 0.82 and e 2 qQ/ h = 3.04 MHz, η = 0.66, which are assigned to the 14N δ(1)N group of His M219 and the peptide 14N of Ala M260. The ESEEM spectrum of Q A −. in reaction centers of the Rb. sphaeroides mutant W(M252)Y shows that the nitrogen of Trp M252 is not interacting with Q A −., that of the mutant H(M266)C shows that manipulating the Fe binding site strongly affects the Q A-binding site.