A new metal-binding site in photosynthetic bacterial reaction centers that modulates Q{sub A} to Q{sub B} electron transfer

Isolated reaction centers (RCs) from Rhodobacter sphaeroides were found to bind Zn(II) stoichiometrically and reversibly in addition to the 1 equiv of non-heme Fe(II). Metal and EPR analyses confirm that Zn(II) is ligated to a binding site that is distinct from the Fe site. When Zn(II) is bound to this site, electron transfer between the quinones Q{sub A} and Q{sub B} (Q{sub A}{sup -}Q{sub B} {yields} Q{sub A}Q{sub B}{sup -}) is slowed and the room-temperature kinetics become distributed across the microsecond to millisecond time domain. This effect of metal binding on the kinetics is similar to the more global effect of cooling RCs to 2 C in the absence of Zn(II). This suggests that Zn(II) binding alters localized protein motions that are necessary for rapid Q{sub A}{sup -}Q{sub B} {yields} Q{sub A}Q{sub B}{sup -} electron transfer. Inspection of the RC crystal structure suggests a cluster of histidine ligands located beneath the Q{sub B} binding pocket as a potential binding site.