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PubMed Abstract: The production of recombinant embryonic haemoglobins via a yeast expression system has enabled structural and functional studies to be conducted on these proteins. As part of a programme aimed at understanding the properties of the embryonic haemoglobins we have crystallized the human alpha2 epsilon2 (Gower II) embryonic haemoglobin in its carbonmonoxy form, and determined its structure by X-ray crystallography. The structure was solved by molecular replacement and refined at 2.9 A to give a final model with R-factor=0.185 and Rfree=0.235. The Gower II hemoglobin tetramer is intermediate between the adult R and R2 states, though closer to R2. The tertiary structure of the conserved alpha subunit is essentially identical when compared to that found in the adult (alpha2 beta2) and fetal (alpha2 gamma2) hemoglobins. The embryonic epsilon subunit has a structure very similar to that of the homologous adult beta and fetal gamma subunits, although with small differences at the N terminus and in the A helix. Amino acid substitutions can be identified that may play a role in the altered response of the Gower II haemoglobin to allosteric effectors, in particular chloride ions. The reduced chloride effect is thought to be the primary cause of the higher affinity of this embryonic hemoglobin in comparison to the adult molecule.