A class of proteins that contain metals, unsurprisingly. All life seems to require such organometallic catalysts; perhaps because the two complement each other so well. Metals are electron rich and can adopt many different oxidation states, while the protein provides a tuned microenvironment for the metal. Often, sidechains ligand the ion(s) to modulate thier properties. Haem iron has four nitrogen ligands from the porphyrin and one from a histidine that is attached to the backbone. The oxygen makes up the sixth ligand, when it is bound.