Crystallographic analysis of mutant human haemoglobins made in Escherichia coli

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Abstract
The expression of beta-globin in Escherichia coli has enabled us to study the functional role of individual amino-acid residues in haemoglobin (Hb) by site-directed mutagenesis. In contrast to mammalian Hbs, some teleost fish haemoglobins show a drastic lowering of oxygen affinity and cooperativity at low pH, a phenomenon known as the Root effect. We have produced the two mutant haemoglobins Hb Nymphéas [Cys(F9)93 beta----Ser] and Hb Daphne [His(H21)143 beta----Arg, Cys(F9)93 beta----Ser] to investigate this allosteric property. Although these substitutions were thought to be responsible for the Root effect, Hb Nymphéas and Hb Daphne show an increased oxygen affinity and a reduced effect of pH on oxygen affinity. Our X-ray crystallographic studies show that the hydroxyl group of Ser 93 beta forms a hydrogen bond with Asp 94 beta which is in equilibrium with the salt bridge between Asp 94 beta and His 146 beta. The oxygen-binding properties of Hbs Nymphéas and Daphne are accounted for by the partial disruption of the salt bridge.