Electron transfer from flavin to iron in the Pseudomonas oleovorans rubredoxin reductase-rubredoxin electron transfer complex

Rubredoxin reductase (RR) and rubredoxin form a soluble and physiological eT complex. The complex provides reducing equivalents for a membrane-bound ω-hydroxylase, required for the hydroxylation of alkanes and related compounds. The gene (alkT) encoding RR has been overexpressed and the enzyme purified in amounts suitable for studies of eT by stopped-flow spectroscopy. The eT reactions from NADH to the flavin of RR and from reduced RR to the 1Fe and 2Fe forms of rubredoxin have been characterized by transient kinetic and thermodynamic analysis. The reductive half-reaction proceeds in a one-step reaction involving oxidized enzyme and a two-electron- reduced enzyme-NAD+ charge-transfer complex. Flavin reduction is observed at 450 nm and charge-transfer formation at 750 nm; both steps are hyperbolically dependent on NADH concentration. The limiting flavin reduction rate (180 ± 4 s-1) is comparable to the limiting rate for charge-transfer formation (189 ± 7 s-1) and analysis at 450 and 750 nm yielded enzyme-NADH dissociation constants of 36 ± 2 and 43 ± 5 μM, respectively. Thermodynamic analysis of the reductive half-reaction yielded values for changes in entropy (ΔS(+) = - 65.8 ± 2.2 J mol-1 K-1), enthalpy (ΔH(+) = 37.8 ± 0.6 kJ mol-1) and Gibbs free energy (ΔG(+) = 57.5 ± 0.7 kJ mol-1 at 298 K) during hydride ion transfer to the flavin N5 atom. Spectral analysis of mixtures of 1Fe or 2Fe rubredoxin and RR suggest that conformational changes accompany eT complex assembly. Both the 1Fe (nonphysiological) and 2Fe (physiological) forms of rubredoxin were found to oxidize two electron-reduced rubredoxin reductase with approximately equal facility. Rates for the reduction of rubredoxin are hyperbolically dependent on rubredoxin concentration and the limiting rates are 72.7 ± 0.6 and 55.2 ± 0.3 s-1 for the 1Fe and 2Fe forms, respectively. Analysis of the temperature dependence of eT to rubredoxin using eT theory revealed that the reaction is not adequately described as a nonadiabatic eT reaction (H(AB) >> 80 cm-1). eT to both the 1Fe and 2Fe forms of rubredoxin is therefore gated by an adiabatic process that precedes the eT reaction from flavin to iron. Possible origins of this adiabatic event are discussed.