Cytochrome oxidase enzyme. Illustration of a model of the enzyme cytochrome oxidase, important in respiration. Cytochromes are widely distributed respiratory catalysts concerned with the electron transport chain of living cells. They are proteins often regarded as enzymes. Here, a cytochrome oxidase molecule (green) is located on a membrane such as in mitochondria, chloroplasts and endo- plasmic reticulum. In the biochemical pathway of respiration, cytochrome oxidase has an iron- porphyrin group which uses atmospheric oxygen to convert iron Fe2+ to Fe3+. With this electron transfer, the energy carrier ATP is produced.