Induced fit and equilibrium dynamics for high catalytic efficiency in ferredoxin NADP(H) reductases

PALADINI, D H; MUSUMECI, M; CARRILLO, N; CECCARELLI, E

BIOCHEMISTRY

AMER CHEMICAL SOC

Lugar: Washington DC; Año: 2009 vol. 48 p. 5760 - 5768

0006-2960

Ferredoxin-NADP(H) reductase (FNR) is a FAD-containing protein that catalyzes the reversibletransfer of electrons between NADP(H) and ferredoxin or flavodoxin. This enzyme participates in the redox based metabolism of plastids, mitochondria, and bacteria. Plastidic plant-type FNRs are very efficient reductases in supporting photosynthesis.They have a strong preference for NADP(H) overNAD(H), consistent with the main physiological role of NADP+ photoreduction. In contrast, FNRs from organisms with heterotrophic metabolisms or anoxygenic photosynthesis display turnover rates that are up to 100-fold lower than those of their plastidic and cyanobacterial counterparts. With the aim of elucidating the mechanisms by which plastidic enzymes achieve such high catalytic efficiencies and NADP(H) specificity, we investigated themanner in which the NADP(H) nicotinamide enters and properly binds to the catalytic site. Analyzing the interaction of different nucleotides, substrate analogues, and aromatic compounds with the wild type and the mutant Y308S-FNR from pea, we found that the interaction of the 20-P-AMP moiety from NADP+ induces a change that favors the interaction of the nicotinamide, thereby facilitating the catalytic process. Furthermore, the main role of the terminal tyrosine,Y308, is to destabilize the interaction of the nicotinamide with the enzyme, inducing product release and favoring discrimination of the nucleotide substrate. We determined that thisfunction can be replaced by the addition of aromatic compounds that freely diffuse in solution and establish a dynamic equilibrium, reversing the effect of the mutation in the Y308S-FNR mutant.