Porphyrin-containing compounds fulfil many different roles in biological systems. Broadly speaking, they fall into two main categories. There are the carrier molecules in which the substance carried is either oxygen, as in the case of the haemoglobins and myoglobins, or electrons, as in the cytochromes. Iron porphyrin containing enzymes also carry out oxidation reduction reactions on bound substances with the prosthetic group acting to bind the substrate via the central iron and at the same time, either alone or in concert with non-binding iron porphyrin groups or other redox active centres, act as a source of electrons for the coupled redox events.