Formation of P(+)Q(sub B)(-) via B-branch electron transfer in mutant reaction211 centers

摘要

The crystallographic observation of two symmetry-related branches of electron211u001etransfer cofactors in the structure of the bacterial reaction center (RC) 13 211u001eyears ago (1) remains an enigma in light of experimental observations that show 211u001ethat only the A branch is active in the initial electron transfer steps in wild-211u001etype RCs. Unidirectional electron flow has been attributed to localized 211u001easymmetries between the A and B branches that lead to differences in: (1) the 211u001eelectronic couplings of the cofactors (2); (2) the relative electrostatic 211u001eenvironments of the cofactors, caused by amino acid differences which modulate 211u001ethe free energies of their charge-separated states (3) and/or create a higher 211u001edielectric constant on the active side, resulting in a stronger static field for 211u001estabilizing A-branch charge transfer states (4,5). Some photo-induced bleaching 211u001eof H(sub B) has been observed, in wild-type RCs following trapping of HA(sub 211u001eA)(sup (minus))(6), and in 'hybrid' RCs where the redox potentials of cofactors 211u001ewere manipulated by pigment exchange (7) or mutagenesis (8). Transient bleaching 211u001eof the 530-nm band of H(sub B) was more easily observed in the hybrid RCs because 211u001ethe H(sub A) transition at 545 nm was shifted to (approximately)600 nm due to 211u001eincorporation of a bacteriochlorophyll, designated '(beta)' at the H(sub A) site. 211u001eNo experiments to detect further electron transfer to Q(sub B) were done with 211u001eeither type of modified RCs. Many site-specific mutagenesis experiments have 211u001egiven us insight into the nature and magnitude of the effects that amino acid 211u001eside chains can exert in tuning the relative energy levels of the cofactors to 211u001eoptimize the balance between forward and reverse reactions, and the large 211u001edistances through which some of these effects are manifested. In this paper, we 211u001eshow that in mutant RCs of Rhodobacter capsulatus, P(sup +)Q(sub B)(sup (minus)) 211u001ecan be formed in the absence of prior formation of P(sup +)Q(sub A)(sup (minus)), 211u001esolely through activity of B-branch cofactors.