2 a ubiquinol + 4 H(+)(in) + O2(in) = 2 a ubiquinone + 4 H(+)(out) + 2 H2O(in) Binds 1 protoheme IX center (heme b595) per heterodimer, in conjunction with CydA. Binds 1 iron-chlorin (heme d or cytochrome d) per heterodimer, in conjunction with CydA. Heterodimer of subunits I and II. Belongs to the cytochrome ubiquinol oxidase subunit 2 family.