Protein containing at least one heme, an iron atom coordinated to a protoporphyrin IX. In myoglobin and hemoglobin, one of the coordination positions of iron is occupied by oxygen or other ligands, such as carbon monoxide. Hemes are also found in cytochromes of the electron-transport chain where they bind electrons, in reducing peroxides (catalases and peroxidases), and act as terminal components in multienzyme systems involved in hydroxylation. Cytochrome c is the only common heme protein in which the heme is covalently bound.