Structural and mechanistic aspects of flavoproteins: photosynthetic electron transfer from photosystem I to NADP⁺

Abstract

This minireview covers the research carried out in recent years into different aspects of the function of the flavoproteins involved in cyanobacterial photosynthetic electron transfer from photosystem I to NADP+, flavodoxin and ferredoxin–NADP+ reductase. Interactions that stabilize protein–flavin complexes and tailor the midpoint potentials in these proteins, as well as many details of the binding and electron transfer to protein and ligand partners, have been revealed. In addition to their role in photosynthesis, flavodoxin and ferredoxin–NADP+ reductase are ubiquitous flavoenzymes that deliver NAD(P)H or low midpoint potential one‐electron donors to redox‐based metabolisms in plastids, mitochondria and bacteria. They are also the basic prototypes for a large family of diflavin electron transferases with common functional and structural properties. Understanding their mechanisms should enable greater comprehension of the many physiological roles played by flavodoxin and ferredoxin–NADP+ reductase, either free or as modules in multidomain proteins. Many aspects of their biochemistry have been extensively characterized using a combination of site‐directed mutagenesis, steady‐state and transient kinetics, spectroscopy and X‐ray crystallography. Despite these considerable advances, various key features of the structural–function relationship are yet to be explained in molecular terms. Better knowledge of these systems and their particular properties may allow us to envisage several interesting applications of these proteins beyond their physiological functions.

A physical picture for mechanical dissociation of biological complexes: from forces to free energies

“…This system is paradigmatic for the study of the association mechanism of redox complexes, as two proteins of different nature (Fd and Fld) interact at the same binding site of FNR 30 . Additionally, they constitute a suitable system for our analysis proposal, as they share similar thermodynamic properties 31 , while their interaction mechanisms are dissimilar, being that of Fd with FNR very specific, whereas that of Fld with FNR has low specificity, allowing multiple orientations on the FNR surface that are competent for the electron transfer process 32,48,49 .…”

“…Particularly the studied system composes a singular case, as Fd and Fld are common binding partners of FNR using the same interaction surface on the flavoenzyme, as undoubtedly verified 30,31,34 . FNR catalyzes the transfer of two electrons to reduce NADP + to NADPH from two independent Fd molecules, while in some algae and cyanobacteria, Fld replaces Fd under iron-deficient conditions 31,32 . While both share similar binding affinities 31 , they are known to display different interaction mechanisms 31,33,34,48,49 .…”

“…These differences might be attributed to the particular features concerning the formation of each complex. Showing a larger interacting interface, Fd binding to FNR is more specific, with salt bridges between certain key positive residues on the FNR surface and acidic residues on Fd 32,48,49,54 , which would contribute in addition to other non-specific interactions such as hydrogen bonds or the hydrophobic effect. On the contrary, kinetic analysis of side-directed mutants and docking studies on FNR:Fld suggest that Fld can adopt multiple orientations on the FNR surface, and that charged residues are not involved in crucial specific interactions 32,33,48 .…”

“…The method is applied to DFS-AFM experiments of the complexes formed by the flavoenzyme Ferredoxin-NADP + reductase (FNR) and its two protein partners Ferredoxin (Fd) and Flavodoxin (Fld) from cyanobacterium Anabaena PCC 7119. This system is paradigmatic for the study of the association mechanism of redox complexes, and their interaction mechanism has been carefully characterized [30][31][32][33][34][35] .…”

Fast Kinetic Methods with Photodiode Array Detection in the Study of the Interaction and Electron Transfer Between Flavodoxin and Ferredoxin NADP+-Reductase

“…Thus, two Fld hq molecules transfer two electrons to one FNR ox that gets fully reduced after formation of the FNR sq intermediate. FNR hq transfers then both electrons simultaneously to NADP + (Medina, 2009) (Figure 1). Additionally to their role in photosynthesis, Fld and FNR are ubiquitous flavoenzymes that deliver low midpoint potential electrons to redox-based metabolisms in plastids, mitochondria and bacteria in all kingdoms (Müller, 1991).…”