Abstract
The flavin mononucleotide (FMN) quinones in flavodoxin have two characteristic redox potentials, namely, Em(FMNH./FMNH-) for the one-electron reduction of the protonated FMN (E1) and Em(FMN/FMNH.) for the proton-coupled one-electron reduction (E2). These redox potentials in native and mutant flavodoxins obtained from Clostridium beijerinckii were calculated by considering the protonation states of all titratable sites as well as the energy contributed at the pKa value of FMN during protonation at the N5 nitrogen (pKa(N5)). E1 is sensitive to the subtle differences in the protein environments in the proximity of FMN. The protein dielectric volume that prevents the solvation of charged FMN quinones is responsible for the downshift of 130-160 mV of the E1 values with respect to that in an aqueous solution. The influence of the negatively charged 5'-phosphate group of FMN quinone on E1 could result in a maximum shift of 90 mV. A dramatic difference of 130 mV in the calculated E2 values of FMN quinone of the native and G57T mutant flavodoxins is due to the difference in the pKa(N5) values. This is due to the difference in the influence exerted by the carbonyl group of the protein backbone at residue 57.
Highlights
Since flavodoxins exist in these 4 states, they show 2 characteristic redox potentials (Em), namely, E1 and E2 [2] (Figure 1)
G57T mutants possess HQ structures that contain a mixture of 2 conformers, i.e., the proximal CO conformer (trans O-up conformation in ref. [3]) and the distal CO conformer (trans O-down conformation in ref. [3]); these differ with respect to the orientation of the backbone carbonyl (CO) group at residue 57 toward the N5 nitrogen atom of the flavin mononucleotide (FMN) quinone
In the proximal CO conformer (Figure 2, left), the CO group is oriented toward the FMN quinone so that the oxygen atom of CO can accept an H-bond from the protonated N5 nitrogen atom of the FMN quinone (O57–NFMN distance = 3.0 Å)
Summary
Since flavodoxins exist in these 4 states, they show 2 characteristic redox potentials (Em), namely, E1 and E2 [2] (Figure 1). By using crystal structures and considering the protonation states of all titratable sites (including the FMN 5′-phosphate group) in the protein, here the calculated E1 and E2 values of flavodoxins obtained from Clostridium beijerinckii are presented.
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