Heme

Heme is the small molecule that’s responsible for the reversible binding of oxygen and CO₂ in the bloodstream. The combination of globin + heme is called myoglobin, and four myoglobin units joined together form haemoglobin. Each hemoglobin molecule can transport four oxygen molecules, one attached to the heme groups inside each myoglobin subunit. The iron atom, the attached protein chain, and any molecules bonded to the iron, modify the wavelength of the absorption and give heme, myoglobin and hemoglobin their characteristic color. The key is the iron atom in the middle of the heme ring. This iron atom binds to the four nitrogen atoms in the center of the porphyrin ring, but this leaves two free bonding sites for the iron, one above and one below the plane of the heme ring.