Introduction

Bd oxidase is an integral membrane protein that catalyzes the reduction of O2 to 2H20 using quinol as the reducing substrate. The reaction is electrogenic but is not coupled to a proton pump it instead uses internal water molecules to provide the protons needed for the reduction reaction. It plays a key role in protecting the organism from high oxidative stress (ie. preventing free radicals in intracellular space in prokaryotes, more specifically gram-negative heterotrophs). There are two main types of respiratory cytochrome oxidases: the heme/copper oxidases, and the heme-only cytochrome bd quinol oxidase, which is what bd oxidase falls under. Heme-only cytochrome bd quinol oxidases are associated with microaerobic dioxygen respiration, and they have a high affinity for oxygen.

Figure 1. bd oxidase; two structurally similar transmembrane helices in blue and red; CydX subunit in teal; Q loop in lime green.

Structure

The overall structure contains 19 transmembrane helices that are arranged in a nearly oval shape. The protein contains two structurally similar subunits each containing nine helices (blue and red) and one smaller subunit, CydX, with one transmembrane helix. The subunits are interacting using hydrophobic residues and symmetry at the interfaces. The CydX subunit, whose function is not currently known, is positioned in the same way as CydS, which is found in E. coli bd oxidase. Due to its similar structure and position, it has been hypothesized to potentially stabilize heme b558 during potential structural rearrangements of the Q loop upon binding and oxidation of quinol. The Q loop is shown in lime green, and is a hydrophilic region above Cyd A. The lack of hydrogen bonding in this hydrophobic protein allows the protein to be flexible and go through a large conformational change for reduction of dioxygen.

Hemes

(CARSON)

Potential Oxygen Entry Site

Heme D is the hypothesized spot for the oxygen to enter the protein. Heme D (shown in green) is directly connected to the protein surface on CydA and contains a solvent accessible substrate channel.

Figure 2.

Electron Source

Disease

Relevance

Structural highlights

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