Heme, C34H32FeN4O4, is a subunit of the hemoglobin molecule. It
consists of an
organic part, called protoporphyrin IX, and an iron
atom. The organic part is made up of four rings that are linked together
to form a tetra structure. The iron atom is in the center of the four rings
and binds to oxygen. Structurally it resembles hematin with the iron in
the Fe(II) state and with no chlorine. Heme combines with the protein globin
to form the blood pigment hemoglobin.
|The heme unit of which there are four in each hemoglobin
molecule is planar porphyrin structure, linked to the globin proteins
which make up the bulk of the hemoglobin molecule via the amino acid
histidine, part of which is shown here above the plane of the porphyrin
ring. The histidine coordinates with the iron (II) atom of the heme,
which also, in oxyhemoglobin, loosely binds the oxygen.
Heme serves as the prosthetic group of
hemoglobin, of cytochromes, and of some
peroxidases. The iron may be oxidized to Fe(III) during reactions involving
cytochromes or peroxidases, i.e. it becomes hematin.