Photoreduction of NADP+ by isolated reaction centers of photosystem II: requirement for plastocyanin.
The carrier of photosynthetically generated reducing power is the iron-sulfur protein ferredoxin, which provides directly, or via NADP+, reducing equivalents needed for CO2 assimilation and other metabolic reactions in the cell. It is now widely held that, in oxygenic photosynthesis, the generation of reduced ferredoxin-NADP+ requires the collaboration in series of two photosystems: photosystem II (PSII), which energizes electrons to an intermediate reducing potential and transfers them to photosystem I (PSI), which in turn is solely competent to energize electrons to the strong reducing potential required for the reduction of ferredoxin-NADP+ (the Z scheme). This investigation tested the premise of an alternative scheme, which envisions that PSII, without the involvement of PSI, is also capable of photoreducing ferredoxin-NADP+. We report here unexpected findings consistent with the alternative scheme. Isolated PSII reaction centers (completely free of PSI components), when supplemented with ferredoxin, ferredoxin-NADP+ oxidoreductase, and a PSII electron donor,1,5-diphenylcarbazide, gave a significant photoreduction of NADP+. A striking feature of this electron transfer from a PSII donor to the perceived terminal acceptor of PSI was its total dependence on catalytic quantities of plastocyanin, a copper-containing electron-transport protein hitherto known only as an electron donor to PSI.
Arnon
D I
DI
Department of Plant Biology, University of California, Berkeley 94720.
Barber
J
J
D002734:Chlorophyll / Q000302:isolation & purification / Q000378:metabolism*; D007887:Fabaceae / Q000378:metabolism*; D007700:Kinetics; D045342:Light-Harvesting Protein Complexes; D009249:NADP / Q000378:metabolism*; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D010940:Plant Proteins / Q000302:isolation & purification / Q000378:metabolism*; D010946:Plants, Medicinal; D010970:Plastocyanin / Q000378:metabolism*; D013053:Spectrophotometry
D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D002734:Chlorophyll; D009249:NADP; D010970:Plastocyanin