Competition between C-terminal Tyrosine and Nicotinamide Modulates Pyridine Nucleotide Affinity and Specificity in Plant Ferredoxin-NADP+ Reductase*

Chloroplast ferredoxin-NADP+reductase has a 32,000-fold preference for NADPH over NADH, consistent with its main physiological role of NADP+ photoreduction for de novo carbohydrate biosynthesis. Although it is distant from the 2′-phosphoryl group of NADP+, replacement of the C-terminal tyrosine (Tyr308 in the pea enzyme) by Trp, Phe, Gly, and Ser produced enzyme forms in which the preference for NADPH over NADH was decreased about 2-, 10-, 300-, and 400-fold, respectively. Remarkably, in the case of the Y308S mutant, thek cat value for the NADH-dependent activity approached that of the NADPH-dependent activity of the wild-type enzyme. Furthermore, difference spectra of the NAD+ complexes revealed that the nicotinamide ring of NAD+ binds at nearly full occupancy in the active site of both the Y308G and Y308S mutants. These results correlate well with thek cat values obtained with these mutants in the NADH-ferricyanide reaction. The data presented support the hypothesis that specific recognition of the 2′-phosphate group of NADP(H) is required but not sufficient to ensure a high degree of discrimination against NAD(H) in ferredoxin-NADP+ reductase. Thus, the C-terminal tyrosine enhances the specificity of the reductase for NADP(H) by destabilizing the interaction of a moiety common to both coenzymes, i.e. the nicotinamide.