THE ELECTRON-TRANSFER RATE FROM BPHA- TO QA IN REACTION CENTERS OF RHODOBACTER-SPHAEROIDES R-26 - INFLUENCE OF THE H-SUBUNIT, THE QA AND FE-2+ COFACTORS, AND THE ISOPRENE TAIL OF QA

The secondary electron transfer from reduced bacteriopheophytin (BPh(A)-) to the first acceptor quinone (Q(A)) in variously modified reaction centers from Rhodobacter sphaeroides was studied by transient absorption spectroscopy and compared with native reaction centers. In intact reaction centers, neither substitution of the native Q(A), Ubiquinone-10 (UQ10), with menaquinone (MK), nor shortening of the isoprene tail of ubiquinone (UQ) down to 4 and that of MK down to 2 isoprene units changed the rate of electron transfer to Q(A). However, in Fe2+-depleted reaction centers the electron transfer rate decreased by a factor of 5 upon MK-reconstitution and by a factor of 20-50 upon non-native UQ-reconstitution. In the latter particles, the electron transfer rate decreased with decreasing tail length of UQ, suggesting that the displacement of UQ within the Q(A) pocket as proposed in previous work (Liu B.-L., Van Kan P.J.M. and Hoff A.J. (1991) FEBS Lett. 289, 23-28), is tail-length-dependent.