Photoreduction of NADP⁺ by a chloroplast photosystem II preparation: effect of light intensity

Abstract

Recent work in this and other laboratories has demonstrated that, contrary to the favored Z scheme hypothesis, photosystern II (PS II) can photoinduce electron transfer from water to NADP +, without the participation of photosystem I (PS I). One proposed explanation for this conflict between hypothesis and observation was that PS II can reduce NADP ÷ but only at high light intensities. We report here findings at variance with this proposal. A PS II preparation made from spinach chloroplasts by the two-phase aqueous polymer partition method photoreduced NADP ÷ without the involvement of PS I, at varying light intensities ranging from limiting to saturating.

Photoreduction of NADP+ by isolated reaction centers of photosystem II: requirement for plastocyanin.

“…This PSII preparation photoreduced ferredoxin-NADP+ at various light intensities ranging from limiting to saturating, under experimental conditions that indicated no involvement of PSI (24). A striking finding in this and earlier work (21) was the dependence of the oxygenic photoreduction of ferredoxin-NADP+ by PSII on added plastocyanin, the coppercontaining electron-carrier protein of chloroplasts that is lost during the preparation of inside-out vesicles.…”

“…We found that isolated PSII reaction centers, completely devoid of PSI components, photoreduced ferredoxin-NADP+ and that this photoreduction was totally dependent on added plastocyanin, hitherto thought to donate electrons exclusively to PSI reaction centers (18). However, these findings are relevant to an alternative concept of photosynthetic electron transport (19)(20)(21) and its recent refinements (22)(23)(24) proposed by one of us (D.I.A. ), which envisions that PSII (renamed the oxygenic photosystem) drives the complete noncyclic electron transport from water to ferredoxin without the collaboration of PSI (renamed the anoxygenic photosystem), whose physiological role centers on cyclic electron flow and the resultant cyclic phosphorylation.…”

“…When tested (24), the proposed effect of high light intensity (34) was not supported by experiments with inside-out vesicles enriched in PSII and made from spinach chloroplasts by the two-phase aqueous polymer partition method (21). This PSII preparation photoreduced ferredoxin-NADP+ at various light intensities ranging from limiting to saturating, under experimental conditions that indicated no involvement of PSI (24).…”

“…-610 mV) was found to be the primary electron acceptor in PSII (31)(32)(33). Reduced pheophytin is thermodynamically competent to reduce ferredoxin, and such a role for pheophytin was proposed in the alternative scheme (19,20) and rationalized as advantageous from the standpoint of energy conservation (24). On the other hand, Klimov et al (31)(32)(33) ydomonas mutant lacking PSI was found capable of photoreducing NADP+ (and hence ferredoxin) solely by PSII, such activity by PSII was explained as a special case possible only at high light intensities (34).…”

“…Despite the limited electron-transport capacity of the PSII reaction centers, we considered it worthwhile to test the remote possibility that under favorable experimental conditions, especially in the presence of plastocyanin (21,24), the reaction centers might conform to the postulates of the alternative scheme of photosynthetic electron transport and catalyze a photoinduced electron transport from a PSII donor (other than water) to the primary PSII acceptor, pheophytin, then to ferredoxin, and, enzymatically, to NADP+. This tenuous expectation was experimentally substantiated by using 1,5-diphenylcarbazide as electron donor.…”