Electrogenic reactions of cytochrome bd

Cytochrome bd is a terminal oxidase in the respiratory chain of Escherichia coli. Prior research has shown that it differs from heme-copper oxidases in its lack of proton pumping. It was already known that cytochrome bd generates membrane potential during quinol oxidation. However, the detailed electrogenic reactions of cytochrome bd remain unclear. This gap motivated a closer look at the sequence of events during oxygen interaction. No prior work had resolved the exact nature of the intermediate species formed. The absence of a clear analog to the 'peroxy' intermediate in heme-copper oxidases remains a key uncertainty. This study aims to clarify the electrogenic sequence and the role of heme d in the process.

This study aimed to investigate the electrogenic reactions of cytochrome bd during its interaction with oxygen. The specific problem addressed is the lack of clarity regarding the sequence of events in the oxidation process. The motivation stems from the need to understand how cytochrome bd generates membrane potential without proton pumping. The researchers propose to use time-resolved methods to capture the dynamics of the reaction. The study focuses on the formation of heme d intermediates and their electrogenic impact. The goal is to determine whether a 'peroxy' analog exists in cytochrome bd. The approach involves spectrophotometric and electrometric measurements. The results may clarify the mechanism of charge separation in this unique oxidase.

The study used time-resolved spectrophotometric and electrometric methods to monitor cytochrome bd reactions. The enzyme was fully reduced and exposed to oxygen to observe the electrogenic response. Flash photolysis of the CO-bound enzyme was used to induce electron backflow. Membrane potential changes were measured using electrometric techniques. Spectrophotometric analysis tracked the formation of heme d intermediates. The lag phase of the reaction was analyzed for oxygen concentration dependence. The monoexponential phase was characterized by a time constant under 60 microseconds. The backflow process was compared to the forward reaction in terms of membrane potential amplitude.

Oxidation of cytochrome bd by oxygen generated a two-step electrogenic process. The first step was oxygen-dependent and electrically silent, forming a ferrous oxy compound of heme d. The second step was monoexponential with a time constant under 60 microseconds, forming ferryl-oxo heme d. No 'peroxy' intermediate was detected in either spectrophotometric or electrometric measurements. Flash photolysis of the CO-bound enzyme caused a delta psi of opposite polarity. This backflow process involved electron transfer from h. d to h. b(558). The amplitude of the delta psi matched that of the forward reaction when normalized to yield. The observed membrane potential change suggested translocation of approximately one charge.

The study suggests that cytochrome bd generates membrane potential through a two-step process. The first step involves formation of a ferrous oxy compound of heme d. The second step forms ferryl-oxo heme d with a time constant under 60 microseconds. No evidence was found for a 'peroxy' intermediate similar to heme-copper oxidases. The backflow process produced a delta psi of opposite polarity. This suggests electron flow from h. d to h. b(558). The amplitude of the backflow delta psi matched that of the forward reaction. These findings may clarify the electrogenic mechanism of cytochrome bd. The results may also inform broader studies of membrane potential generation in bacterial oxidases.