The primary and secondary acceptors in bacterial photosynthesis: II. The structure of the Fe2+-Q− complex

Abstract

The primary processes of bacterial photosynthesis take place in a membrane-bound protein called the reaction center (RC). The process involves light-induced electron transfer from a primary electron donor to a sequence of electron acceptors. The terminal acceptors, operating sequentially, are two ubiquinones, QA and QB, that couple magnetically to a high-spin (S = 2) Fe2+ forming an Fe2+-Q− complex. We have used a variety of techniques to investigate the electronic as well as some features of the spatial structure of the Fe2+-Q− complex in RCs fromRb. sphaeroides. These include: (a) static magnetization measurements in the temperature range of 0.7 to 190 K and magnetic fields up to 8 kG, (b) EPR spectroscopy at helium temperatures at 1.2, 9, and 35 GHz, (c) Mossbauer spectroscopy and (d) extended X-ray fine structure (EXAFS) determinations. The results of these measurements showed that Fe2+ resides in an asymmetric ligand field environment forming 6 ligands with a combination of oxygens and nitrogens. From the EXAFS results the distances of the Fe2+ to the first and third coordination shells were determined. These spatial features were subsequently corroborated by the X-ray structure of the RC, which showed the environment of the Fe to be a distorted octahedron, the base plane of which is formed by three Nɛ’s of histidines and one carbonyl oxygen; the apex is formed by a fourth Nɛ and a second carbonyl oxygen. The distances from the Fe2+ to the first and third shell were in good agreement with the values obtained from EXAFS. The most detailed information of the electronic structure of the Fe2+-Q− complex was obtained from the EPR spectra using the spin Hamiltonian formalism. The following spin-Hamiltonian parameters were obtained: for the crystalline field parameters,D = 7.60 K,E/D = 0.25; for the electronicg-values, gFe, x = 2.16, gFe, y = 2.27, gFe, z = 2.04 and for the antiferromagnetic exchange interaction,J x = −0.13 K,J y = −0.58 K,J z = −0.58 K. The use of the spin Hamiltonian was validated by comparing the results obtained from it with those obtained from an exact numerical solution of the 25 lowest energy levels of the orbital Hamiltonian. Possible roles that Fe2+ may play in the function of the RC are discussed. They include a structural role in which Fe2+ liganded to four histidine nitrogens imparts a rigid structure to a four α-helix bundle and a role in the electron transfer kinetics, most notably from the intermediate acceptor I− (bacteriopheophytin) to QA. Replacement of Fe2+ by diamagnetic Zn2+ retained all observed native characteristics of the RC.