Cytochrome C enzyme. Illustration of a model of the enzyme cytochrome C, important in the process of respiration. Cytochromes are proteins often regarded as enzymes. Here, one cytochrome C molecule is seen, made of 104 amino acids with a heme" porphyrin ring containing iron (Fe). In the biochemical pathway of respiration, the valency of Fe2+ is converted to Fe3+ using oxygen, and with this electron transfer the energy carrier ATP is produced. Cytochromes are respiratory enzymes located on cell membranes such as in mitochondria, chloroplasts and endoplasmic reticulum."