Influence of Polar Mutations on the Electronic and Structural Properties of QA- . in Bacterial Reaction Centers

Reaction centers from Rhodobacter sphaeroides with residue M265 mutated from isoleucine to threonine, serine, and asparagine (M265IT, M265IS, and M265IN, respectively) in the Q(A)(-.) state are studied by high-resolution electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance spectroscopy methods to investigate the structural characteristics of these mutants influencing the redox properties of the Q(A) site. All three mutants decrease the redox midpoint potential (E-m) of Q(A) by similar to 0.1 V, yet the mechanism for this drop in E-m is unclear. In this work, we examine (i) the hydrogen bonding interactions between Q(A)(-.) and residues histidine M219 and alanine M260, (ii) the electron spin density distribution of the semi-quinone, and (iii) the orientations of the ubiquinone methoxy substituents. C-13 measurements show no significant contribution of methoxy dihedral angles to the observed decrease in E-m for the Q(A) mutants. Instead, N-14 three-pulse ESEEM data suggest that electrostatic or hydrogen bond formation between the mutated M265 side chain and His-M219 N-delta may be involved in the observed lowering of the Q(A) midpoint potential. For mutant M265IN, analysis of the proton hyperfine couplings reveals a weakened hydrogen bond network, resulting in an altered Q(A)(-.) spin density distribution. The magnetic resonance study presented here is most consistent with an electrostatic or structural perturbation of the His-M219 N-delta hydrogen bond in these mutants as a mechanism for the similar to 0.1 V decrease in Q(A)E(m).

Automated summary

Using high-resolution electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance spectroscopy methods, the structural characteristics of mutants with residue M265 mutated from isoleucine to threonine, serine, and asparagine (M265IT, M265IS, and M265IN, respectively) in the Q(A)(-) state were studied. The mutants were found to decrease the redox midpoint potential (E-m) of Q(A) by approximately 0.1 V. The study suggests that the decrease in E-m may be attributed to the electrostatic or structural perturbation of the hydrogen bond between the mutated M265 side chain and His-M219 N-delta, as well as the altered Q(A)(-) spin density distribution.