Differential regulation between photosynthetic type and non-photosynthetic type Fd:FNRs in the negative cooperativity and pH dependency of the electron transfer activity

In higher plants, ferredoxin (Fd) and Fd-NADP+ reductase (FNR) are each present as distinct isoproteins of photosynthetic type and non-photosynthetic type, which exhibit differential function despite their similarity in the 3D structures. In this study, we addressed differential regulation of Fd/FNR reaction between the two types from two perspectives and investigated the amino acid residues of Fd responsible for the differences. Firstly, pH-dependent profile of Fd/FNR electron transfer activity varied among the combinations of the two types of Fd and FNR; non-photosynthetic type FNR showed similar pattern for the two types of Fds while photosynthetic type FNR was previously shown to exhibit opposite pattern, which was explained by the different pH-dependent profile of Km for the two Fds. Secondly, the extent of the suppression of the affinity (in terms of Km value) between Fd and FNR by NADPH significantly varied among the combinations of the two types of Fd:FNR. The difference was shown to be mainly due to the different property of Fd between the two types. Kinetic analyses using site-directed mutants of Fd showed the contribution of C-terminal residues, together with that of 78th residue of Fd, on the differential profile of Fd/FNR reaction by pH and NADPH.