test</scene’>

Function

Disease

Relevance

Structural highlights

H and O channels

Figure 3

The hydrogen and oxygen channels (Fig. 3) are essential for H⁺ and O₂ molecules to reach the active site of bd oxidase. A proton motive force generated by the oxidase^[1] allows protons from the cytoplasm flow through a water-filled hydrophilic H-channel entering at D119^A and moving past K57^A, K109^B, D105^B, Y379^B, and D58^{B [2]} where they can be transferred to the active site with the help of the conserved residues S108^A, E107^A, and S140^A. A smaller o-channel also exists that transitions from hydrophobic to hydrophilic as it gets closer to the active site. This channel allows oxygen to reach the active site, starting near W63 in CydB and passing by L101^B, I114^A, and E99^A. The o-channel channel is approximately 1.5 Å in diameter, which may help with selectivity. Inhbitors that target the o-channel by blocking its entrance are quite effective by preventing the reduction of oxygen to water.

Interestingly, the o-channel does not exist in the bd oxidase of Geobacillus thermodenitrificans; instead, oxygen binds directly to the active site. The CydS subunit found in E. coli blocks this alternate oxygen entry site, which allows oxygen to travel through the o-channel. The presence of an o-channel affects oxidase activity, as the E. coli oxidase acts as a "true" oxidase, while the G. th oxidase contributes more to detoxification.

Hemes

There are three heme molecules present in the CydA subunit that form a triangle to maximize subunit stability, which is an evolutionary conserved feature across bd oxidases. The triangle Similar to the hemes, the ubiquinone-8 (UQ-8) molecule found in CydB mimics the triangular formation to stabilize the subunit(safarian). Heme b₅₅₈ acts as the primary electron acceptor by catalyzing the oxidation of quinol. Heme b₅₉₅ and a conserved W374^A assist in transferring the electrons from heme b₅₅₈ to the active site, heme d. The distance between heme b₅₅₈ and heme d (18.3 Å) is too large for direct electron transfer, so a second heme is essential for adequate electron transfer.

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