Effects of Co,-depletion on Proton Uptake and Release in Thylakoid Membranes

The ability of COz to stimulate the Hill reaction in thylakoid membrane is well established [ 11. When NaHCOs is added to COz-depleted thylakoids, a large (4-6-fold) increase in oxygen evolution is observed. Recent experiments [2-61 have demonstrated that a major site of inhibition of Hill reaction by CO2 depletion is between electron acceptor Q of photosystem II (PS II) and the plastoquinone (PQ) pool, and more specifically between the two electron acceptor R (or B) and PQ [4,5]. Although a major site of bicarbonate action has been clearly established, and recent experiments suggest that CO1 is the species that diffuses and may initially bind to the membrane [7], the mechanism of its action is not known. During illumination of thylakoids there is an uptake of protons from the external phase and a release of protons into the interior space [g-lo]. There are two proton uptake sites on the outer side of the membrane [ 1 I] and two sites of proton release on the inner side [ 121. One of the sites on the outer surface is attributable to the reduction of the electron acceptor used. The second site of proton uptake is associated with the reduction of plastoquinone (PQ + 2 H’ + 2 e--, PQH2) which acts as a shuttle such that its oxidation releases protons at the inner side of the membrane. The other site of proton release into the inner phase is attributed to the oxidation of water (1/2H,0~1/402+I-l++e-).Thesitesofprotonbinding from the outer phase and proton release into the inner aqueous volume have been studied with pH