Molecular arrangement of NADP-photoreducing system in chloroplasts.

铁氧还蛋白-NADP还原酶（FNR）的两种分子形式，即小形式（FNR-S）和大形式（FNR-L），已知存在于叶绿体中。当菠菜叶绿体在中性pH的Tris-HCl缓冲液中孵育时，几乎所有的FNR-L都容易在1小时内从类囊体中释放出来。FNR-L与去除FNR-L的类囊体的再结合显著恢复了其NADP光还原活性。通过FNR-S的重新结合的活性的恢复是相当小的，这表明FNR-S不参与NADP光还原反应的类囊体表面上。FNR-L位于类囊体表面，与NADP还原酶形成复合物，在非循环电子流中还原NADP，而FNR-S在提取FNR-L后仍保留在类囊体中，表明FNR位于类囊体内部。它被发现与光系统I粒子有关。FNR-S似乎在循环电子流中起铁氧还蛋白-质体醌还原酶的作用，但在非循环电子流中不起NADP还原酶的作用。

Two molecular forms of ferredoxin-NADP reductase (FNR), small from (FNR-S) and large form (FNR-L), have been known to exist in chloroplasts. When spinach chloroplasts were incubated in a dilute Tris-HCl buffer at neutral pH, almost all FNR-L was easily released from thylakoids within 1 hour. Rebinding of FNR-L to the FNR-L-removed thylakoids markedly restored its NADP photoreducing activity. Restoration of the activity by rebinding of FNR-S was considerably small, suggesting that FNR-S does not participate in the NADP photoreducing reaction on the surface of thylakoids. It was concluded that FNR-L was located on the surface of thylakoids to constitute an NADP reductase complex to reduce NADP in the non-cyclic electron flow.FNR-S remained in thylakoids after extraction of FNR-L, suggesting that this FNR is located in some internal part of thylakoids. It was found to be associated with photosystem I particles. It seems that FNR-S functions as a ferredoxin-plastoquinone reductase in the cyclic electron flow, but not as an NADP reductase in the non-cyclic electron flow.