Structure of Reduced Human Hemoglobin

Hemoglobin is a protein of molecular weight about 65,000. It contains four polypeptide chains of approximately equal length which are identical in pairs. The two types of chain are known as the α- and β-chains respectively. Each chain is coiled around a heme group which consists of an iron atom coordinated to the four nitrogen atoms of a porphyrin ring and thus forms a planar group. The fifth coordination site of the iron atom is occupied by a nitrogenous group on the polypeptide chain (probably histidine; Kendrew et al., 1960). Each heme group is capable of combining reversibly with oxygen while the iron atom remains in a ferrous state. In this paper the protein is referred to as oxyhemoglobin when the sixth coordination site of the iron atom is occupied by an oxygen molecule and reduced hemoglobin when no oxygen is present. In the latter state there is probably...