Crystal Structure of the Rb. sphaeroides Zn(II)-binding Reaction Centre Mutant HC(M266)

Abstract

As both QA and QB in the Rb. sphaeroides RC are ubiquinone-10 molecules their different properties in the electron transfer chain must be the result of interactions with the protein environment. However, elucidation of the electronic stucture by EPR/ENDOR techniques of the respective radical anions is hindered by their strong magnetic coupling to the non-heme, high-spin Fe2+. Using RC isolated from the mutant HC(M266) in which the non-heme Fe2+ has been substituted with diamagnetic Zn2+, a narrow EPR line of the semiquinone anion radicals can be observed and these experiments become feasible. Hence ZnRC have become important in attempting to understand quinone function during electron transfer. We have solved the structure of the HC(266M) RC to 2.6Å resolution and compared it to the wild type-FeRC. This presentation will detail the crystal structure of HC(M266), particularly in regard to the position and co-ordination of the zinc and the hydrogen bonding pattern of the quinones. The substitution of a zinc for an iron ion causes a slight perturbation in the structure leading to small differences of the spin-density distribution in the QA anion radical and of the QA-/QA FTIR difference spectra in wild type RC and mutant HC(M266). In addition ENDOR, FTIR and kinetic data will be presented and correlated with the structural data.