How is Ferredoxin-NADP Reductase Involved in the NADP Photoreduction of Chloroplasts?

NADP photoreduction of chloroplasts was discovered in 1951, and subsequent research was conducted to elucidate the enzymatic mechanisms involved in this reaction. In 1963, ferredoxin-NADP reductase (FNR; EC 1.18.1.2, ferredoxin-NADP oxidoreductase) was isolated and purified to a crystalline form. Because the reaction mechanism of ferredoxin-NADP reducing system was clarified in the isolated enzyme system, it was generally thought that the role of FNR in the NADP photoreduction of chloroplasts had been fully elucidated. However, the results of a reconstitution study using the crystallized FNR and the depleted grana, from which 'built-in' FNR had been eliminated, showed that the NADP photoreducing activity of reconstituted FNR was much lower than the original physiological activity, and as a result, more studies had to be continued. In 1985, a protein factor called ‘connectein’ was discovered, and it was shown that this new protein binds with two FNR molecules to form an FNR–connectein complex. Then in 1991, the FNR—connectein complex was formed using purified connectein and FNR, and after eliminating 'built-in' FNR, the reconstituted complex was bound to the depleted grana having reduced NADP photoreducing activity. The results showed that NADP photoreducing activity of the reconstituted system was comparable to the original physiological activity. This proved that the FNR—connectein complex, which binds to a specific site on the surface of thylakoid membrane, is functionally responsible for NADP photoreduction in chloroplasts.