Introduction

Structure

Subunits

E. coli bd oxidase is made up of four individual subunits.^[1] The two major subunits, CydA and CydB, are each composed of one peripheral helix and two bundles of four transmembrane helices. The plays the most important role in the oxygen reduction reaction as it contains the Q-loop as well as all three heme groups. The harbors the molecule which provides structural support to the subunit that mimics the three hemes found in CydA.^[2][3] The remaining two subunits, CydS and CydX, are both single helix structures that assist in the oxygen reduction reaction. Unique to E. coli, the binds to CydA to block oxygen from directly binding to heme b595. The promotes the assembly and stability of the oxidase complex. CydX is composed of 37 mostly hydrophilic amino acid residues, including that is exposed to the cytoplasm and prevents the helix from fully entering the membrane. ^[1]

Q-Loop

Another significant structural feature of bd oxidase is the which is located between TM helices 6 and 7 of the CydA subunit.^[1] The periplasmic Q-loop in E. coli stretches over a length of 136 amino acid residues, making it much longer than the Q-loop in Geobacillus thermodentrificans.^[2] The Q-loop is likely involved in quinone binding and oxidation. The N-terminal end of this Q-loop is very flexible and likely functions as the hinge that allows for quinone binding while the C-terminal end is much more rigid which provides stabilization for the enzyme.^[1]

Hemes

Function

Disease

Relevance

Structural highlights