Arnon D I, Barber J
Department of Plant Biology, University of California, Berkeley 94720.
Proc Natl Acad Sci U S A. 1990 Aug;87(15):5930-4. doi: 10.1073/pnas.87.15.5930.
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.
光合产生的还原力的载体是铁硫蛋白铁氧还蛋白，它直接或通过NADP⁺提供细胞中二氧化碳同化和其他代谢反应所需的还原当量。现在人们普遍认为，在有氧光合作用中，还原型铁氧还蛋白-NADP⁺的产生需要两个光系统串联协作：光系统II（PSII），它将电子激发到中等还原电位并将其转移到光系统I（PSI），而光系统I又仅能将电子激发到还原铁氧还蛋白-NADP⁺所需的强还原电位（Z方案）。本研究测试了另一种方案的前提，该方案设想PSII在不涉及PSI的情况下也能够光还原铁氧还蛋白-NADP⁺。我们在此报告了与该替代方案一致的意外发现。分离的PSII反应中心（完全不含PSI成分），当补充铁氧还蛋白、铁氧还蛋白-NADP⁺氧化还原酶和PSII电子供体1,5-二苯基卡巴肼时，会使NADP⁺发生显著的光还原。这种从PSII供体到PSI的假定末端受体的电子转移的一个显著特征是它完全依赖于催化量的质体蓝素，质体蓝素是一种含铜的电子传递蛋白，迄今仅作为PSI的电子供体为人所知。
