Oxidation-reduction potential dependence of formate binding to Photosystem II in maize thylakoids

Abstract

The affinity of Photosystem II for formate is less when thylakoid membranes are incubated with ferricyanide in complete darkness. A redox titration of this effect showed that the affinity for formate is not determined by the oxidation state of the non-heme iron (Q 400) associated with the reaction center complex, nor does formate influence the mid-point potential, E m, of Q 400. Rather, the formate affinity is determined by the oxidation state of a component with an E m of about +480 mV at pH 7.0. The E m of this single-electron redox mediator, here referred to as D 480, has a pH-dependence of approx. 60 mV/pH. D 480 is tentatively identified as an auxiliary electron donor ( E m = + 475 mV at pH 7.6), first described by Bearden and Malkin (Bearden, A.J. and Malkin, R. (1973) Biochim. Biophys. Acta 325, 266–274). When D 480 is oxidized in the dark by ferricyanide, the affinity of Photosystem II for other monovalent anions such as bicarbonate and chloride is low. A single saturating flash reverses the effect of ferricyanide, indicating that oxidized D 480 can be reduced in the light. The binding of bicarbonate ions to formate-inhibited thylakoids shows a period-of-two oscillation with flash number. Oxidation of D 480 by ferricyanide has a strong damping effect on these oscillations.