Hemoglobin

The 3-dimensional structure of hemoglobin was solved using X-ray crystallography in the year 1959 by Max Perutz. This exposed that hemoglobin is made  up of four  polypeptide  chains,  each  of which  has  a very  similar  three- dimensional  structure  to  the  single  polypeptide  chain  in  myoglobin  despite  the fact that their  amino  acid sequences  differ  at 83 percent of the residues. This  highlights   a  common   theme  in  protein  structure:   that  the different  primary  sequences  can  specify  similar  3-dimensional structures. The basic type of hemoglobin which is found in adults (HbA) is made up of two types of   polypeptide   chains:   the α-chain   which comprises of   141 amino   acid residues, and the β-chain of 146 residues. Every chain, as in myoglobin,   comprises of eight α-helices and each contains a heme prosthetic group as shown in figure b. Thus, hemoglobin can bind four molecules of O₂. The 4 polypeptide chains, two α and two β, are tightly packed together in a tetrahedral array to form a spherically  shaped molecule  which is held together by multiple noncovalent interactions.

Figure Structure of (a) myoglobin and (b) hemoglobin, showing α and β polypeptide chains. Cα-backbone traces of (c) human myoglobin and (d) human hemoglobin, showing α-helices, heme prosthetic group in space ?lling representation and how monomer of myoglobin maps onto structure of hemoglobin (circle).