Natural pyrrole pigments. Myoglobin, hemoglobin and bilirubin
The heme structure relates to a fundamental structure named porphin (porphine). Porphin is the parent for several types of biochemically significant compounds called porphyrins.
Protoporphyrin is a biochemical precursor for heme. Protoporphyrin in humans is biosynthesized after linear attachment of four pyrrole rings via saturated –CH2– bridges.
Hemoglobin (abbreviated Hb or Hgb) is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates. Hemoglobin consists of four globular protein subunits and four heme groups; therefore, it can bind and transport 4 O2 molecules at once. Each human red blood cell contains approximately 270 millions of these hemoglobin biomolecules.
The sixth ligand is a water molecule that binds to Fe2+ on the opposite side of the histidine residue to complete the octahedral complex. This Paramagnetic complex is called deoxyhemoglobin.
The water ligand can be replaced readily by oxygen molecule (at higher partial oxygen pressure in the lungs) to form bright red oxyhemoglobin that is present in arterial, oxygenated blood. These two binding modes of hemoglobin result in a change in the special arrangement of the protein subunits, a conformational change.
If hemoglobin has Fe3+ it is methemoglobin. It cannot bind oxygen. In binding, oxygen temporarily oxidizes Fe2+ to Fe3+, so iron must exist in the +2 oxidation state to bind oxygen.
Myoglobin is a single-chain globular protein, containing a heme. Myoglobin is the primary oxygen-carrying metalloprotein of muscle tissues. Unlike the blood-borne hemoglobin, to which it is structurally related, this protein does not exhibit cooperative binding of oxygen, because it has only one chain and one heme units.