The nonprotein portion of hemoglobin and myoglobin, consisting of reduced (ferrous) iron bound to protoporphyrin.
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Iron is a structural component of porphyrin molecules like cytochromes, which have two parts: an iron complex called heme (or haem) and a protein. Heme is an iron (Ⅱ) complex that contains iron in Fe2+ or gets coordinated to four nitrogen atoms of a planar porphyrin. The heme in hemoglobin is also a porphyrin derivative. Physiological processes in plants have shown that chlorophyll is formed from protoporphyrin by the removal of iron from hemin. On the contrary, in animals iron is introduced into protoporphyrin to form heme Iron plays a central role in almost all living cells. In mammals, the principle source of energy comes from oxidation of carbohydrates, proteins and fats. Although oxygen is the oxidizing agent for these processes, it does not react directly with these molecules. Instead, the electrons from the breakdown of these nutrients are passed along a complex chain of molecules, called respiratory chain, eventually reaching the oxygen molecule. The principle electron-transfer molecules in the respiratory chain are these iron-containing molecules like heme.
