CRYSTAL-STRUCTURE OF A HUMAN EMBRYONIC HEMOGLOBIN - THE CARBONMONOXY FORM OF GOWER-II (ALPHA(2)EPSILON(2)) HEMOGLOBIN AT 2.9 ANGSTROM RESOLUTION

The production of recombinant embryonic haemoglobins via a yeast expre ssion system has enabled structural and functional studies to be condu cted on these proteins. As part of a programme aimed at understanding the properties of the embryonic haemoglobins we have crystallized the human alpha(2)epsilon(2) (Gower II) embryonic haemoglobin in its carbo nmonoxy form, and determined its structure by X-ray crystallography, T he structure was solved by molecular replacement and refined at 2.9 An gstrom to give a final model with R-factor = 0.185 and R-free = 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 cons erved alpha subunit is essentially identical when compared to that fou nd in the adult (alpha(2)beta(2)) and fetal (alpha(2)gamma(2)) hemoglo bins. The embryonic epsilon subunit has a structure very similar to th at of the homologous adult beta and fetal gamma subunits, although wit h small differences at the N terminus and in the A helix. Amino acid s ubstitutions can be identified that may play a role in the altered res ponse of the Gower II haemoglobin to allosteric effecters, in particul ar chloride ions. The reduced chloride effect is thought to be the pri mary cause of the higher affinity of this embryonic hemoglobin in comp arison to the adult molecule. (C) 1998 Academic Press.