Subunit dissociation and reassociation leads to preferential crystallization of haemoglobin Bart's (gamma 4) from solutions of human embryonic haemoglobin Portland (zeta(2)gamma(2)) at low pH

A variety of human haemoglobins (Hbs) are produced at different stages of h uman development, including three embryonic Hbs, foetal Hb and adult Hb. Al l are heterotetramers. During crystallization experiments on human embryoni c Hb Portland (zeta (2)gamma (2)), it was discovered by crystallographic an d biochemical analysis that the homotetramer Hb Bart's (gamma4) preferentia lly crystallizes from zeta (2)gamma (2) solutions below pH 5. This results from dissociation of Hb Portland into gamma2 dimers and zeta monomers and h as interesting implications for subunit interactions and tetramer stability in Hbs. It also makes possible a full crystallographic analysis of Hb Bart 's, which is of considerable medical significance because of its presence i n the red blood cells of millions of people worldwide who suffer from alpha -thalassaemia.