A terminal oxidase that produces a proton motive force by the vectorial transfer of protons across the inner membrane. It is the component of the aerobic respiratory chain of E.coli that predominates when cells are grown at low aeration. Generates a proton motive force using protons and electrons from opposite sides of the membrane to generate H(2)O, transferring 1 proton/electron. 2 a ubiquinol + 4 H(+)(in) + O2(in) = 2 a ubiquinone + 4 H(+)(out) + 2 H2O(in) Binds 1 protoheme IX center (heme b595, originally called cytochrome a1) per heterodimer, in conjunction with CydA. Binds 1 iron-chlorin (heme d or cytochrome d) per heterodimer, in conjunction with CydA. 90% inhibited by cyanide and 2-heptyl-4-hydroxyquinoline N-oxide, at 1 mM and 40 uM respectively. pH 7.0, 37 degrees Celsius. Energy metabolism; oxidative phosphorylation. Heterodimer of subunits I and II. Probably interacts with CydX, and overexpressed AppX. The displayed topology is based on (PubMed:15013751) not the large scale studies (PubMed:15919996). Under conditions of low aeration, in stationary phase (at protein level). The N-terminus is blocked. Loss of cytochrome b595 and d from enzyme preparations (PubMed:3013298). A double cydA/cydB deletion shows increased sensitivity to reductant (beta-mercapoethanol) (PubMed:23749980). Greatly increased resistance to hydroxyurea, probably due to decreased OH radical formation as an electron transport chain is disrupted (PubMed:20005847). Belongs to the cytochrome ubiquinol oxidase subunit 2 family.