Cytochrome c oxidase is the component of the respiratory chain that catalyzes the reduction of oxygen to water. Subunits 1-3 form the functional core of the enzyme complex. Co I is the catalytic subunit of the enzyme. Electrons originating in cytochrome c are transferred via the copper A center of subunit 2 and a low-spin heme of subunit 1 to the bimetallic center formed by a high-spin heme and copper B. 4 Fe(II)-[cytochrome c] + 8 H(+)(in) + O2 = 4 Fe(III)-[cytochrome c] + 4 H(+)(out) + 2 H2O This alternate cytochrome c oxidase consists of a subunit I and two cytochromes c. Equivalents to subunit 2 and 3 are not present in this complex. Bacteroid (nitrogen-fixing endosymbiont). Belongs to the heme-copper respiratory oxidase family.