Kinetic Studies of a Ferredoxin-Dependent Cyanobacterial Nitrate Reductase

Thermodynamics of ferredoxin binding to cyanobacterial nitrate reductase

The role of the seven negatively charged amino acids of Synechocystis sp. PCC 6803 ferredoxin (Fd), i.e., Glu29, Glu30, Asp60, Asp65, Asp66, Glu92, and Glu93, predicted to form complex with nitrate reductase (NR), was investigated using site-directed mutagenesis and isothermal titration calorimetry (ITC). These experiments identified four Fd amino acids, i.e., Glu29, Asp60, Glu92, and Glu93, that are essential for the Fd binding and efficient electron transfer to the NR. ITC measurements showed that the most likely stoichiometry for the wild-type NR/wild-type Fd complex is 1:1, a K_d value 4.7 μM for the complex at low ionic strength residues and both the enthalpic and entropic components are associated with complex formation. ITC titrations of wild-type NR with four Fd variants, E29N, D60N, E92Q, and E93N demonstrated that the complex formation, although favorable, was less energetically favorable when compared to complex formation between the two wild-type proteins, suggesting that these negatively charged Fd residues at these positions are important for the effective and productive interaction with wild-type enzyme.

Structural adaptations of photosynthetic complex I enable ferredoxin-dependent electron transfer

Photosynthetic complex I enables cyclic electron flow around photosystem I, a regulatory mechanism for photosynthetic energy conversion. We report a 3.3-angstrom-resolution cryo-electron microscopy structure of photosynthetic complex I from the cyanobacterium Thermosynechococcus elongatus. The model reveals structural adaptations that facilitate binding and electron transfer from the photosynthetic electron carrier ferredoxin. By mimicking cyclic electron flow with isolated components in vitro, we demonstrate that ferredoxin directly mediates electron transfer between photosystem I and complex I, instead of using intermediates such as NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate). A large rate constant for association of ferredoxin to complex I indicates efficient recognition, with the protein subunit NdhS being the key component in this process.

Identification of the Ferredoxin-Binding Site of a Ferredoxin-Dependent Cyanobacterial Nitrate Reductase

An in silico model for the 1:1 ferredoxin (Fd)/nitrate reductase (NR) complex, using the known structure of Synechocystis sp. PCC 6803 Fd and the in silico model of Synechococcus sp. PCC 7942 NR, is used to map the interaction sites that define the interface between Fd and NR. To test the electrostatic interactions predicted by the model complex, five positively charged NR amino acids (Arg43, Arg46, Arg197, Lys201, and Lys614) and a negatively charged amino acid (Glu219) were altered using site-directed mutagenesis and characterized by activity measurements, metal analysis, and electron paramagnetic resonance (EPR) studies. All of the charge replacement variants retained wild-type levels of activity with reduced methyl viologen (MV), but a significant decrease in activity was observed for the R43Q, R46Q, K201Q, and K614Q variants when reduced Fd served as the electron donor. EPR analysis as well as the Fe and Mo analyses showed that loss of activity observed with these variants was not the consequence of perturbation of the Mo center or [4Fe-4S] cluster. Therefore, the loss of the Fd-linked specific activity observed with these variants can be explained only by invoking a role for Arg43, Arg46, Lys201, and Lys614 in Fd binding. The R43Q, R46Q, K201Q, and K614Q NR variants also showed a decreased binding affinity for Fd, compared to that of wild-type NR, supporting a key role of these four positively charged residues in the productive binding of Fd.