Oxidation-reduction potential studies on p-hydroxybenzoate hydroxylase from Pseudomonas fluorescens

Understanding the redox properties of enzymes is crucial for elucidating their catalytic mechanisms. Prior research has shown that flavin-containing oxidoreductases often exhibit complex redox behavior due to the variable oxidation states of their cofactors. However, the specific redox dynamics of p-hydroxybenzoate hydroxylase remain unclear. This gap motivated a detailed investigation into the oxidation-reduction potential of this enzyme. The study's focus is on how substrate binding influences the enzyme's redox state. No prior work had resolved the thermodynamic stabilization of flavin semiquinone in this system. The enzyme's sensitivity to oxygen and its thiol groups are known but not fully characterized. This uncertainty drove the use of spectrocoulometry to measure potential shifts. The enzyme's role in aromatic compound metabolism is well established, but its redox behavior is less understood.

This study aimed to investigate the oxidation-reduction potential of p-hydroxybenzoate hydroxylase from Pseudomonas fluorescens. The specific problem addressed was the enzyme's redox behavior in the presence and absence of its substrate. The motivation stemmed from the need to clarify how substrate binding affects the enzyme's redox properties. The enzyme's air-sensitive thiol groups were suspected to influence its redox state. The study sought to determine whether substrate binding alters the midpoint potential of the enzyme. The researchers also aimed to assess whether flavin semiquinone stabilization occurs during redox titrations. This work builds on prior findings about flavin cofactor interactions. The results could provide insights into the enzyme's catalytic mechanism.

The researchers used spectrocoulometry to measure the oxidation-reduction potential of the enzyme. The native enzyme was analyzed in both the presence and absence of its substrate. Reductive and oxidative titrations were performed to observe potential shifts. The enzyme's midpoint potential was measured under varying conditions. The study also included redox titrations with the substrate p-hydroxybenzoate. Spectral and electrochemical data were collected to detect flavin semiquinone stabilization. The enzyme's affinity for different flavin forms was calculated from the data. The experimental design focused on quantifying redox changes and enzyme-substrate interactions.

The native enzyme showed a two-electron midpoint potential of -129 mV during initial reductive titration. Subsequent oxidative and reductive titrations revealed a midpoint potential of -152 mV. This hysteresis is attributed to the enzyme's air-sensitive thiol group. In the presence of substrate, the midpoint potential was -177 mV. No evidence of flavin semiquinone stabilization was observed. The apoenzyme's affinity for hydroquinone FAD was 150-fold higher than for oxidized flavin. Substrate binding to the reduced enzyme had a 3-fold lower affinity than to the oxidized form. These findings suggest that enzyme reduction by NADPH is geometry-dependent rather than potential-driven.

The authors propose that the enzyme's hysteresis arises from the oxidation and reduction of its thiol group. The data suggest that substrate binding does not significantly alter the flavin's oxidation-reduction potential. The enzyme's affinity for hydroquinone FAD is much greater than for oxidized flavin. Substrate binding to the reduced enzyme has lower affinity than to the oxidized form. The stimulatory effect of substrate on enzyme reduction is likely due to the geometry of bound FAD and NADPH. No large perturbation of the flavin's redox potential was observed. The findings support a model where enzyme geometry influences NADPH-driven reduction. These conclusions align with the observed redox behavior and substrate interactions.