CRYSTAL STRUCTURE OF THE GOWER II HUMAN EMBRYONIC HEMOGLOBIN ([[alpha]]₂[[epsilon]]₂). A.J. Sutherland-Smith, H.M. Baker, and E.N. Baker, Department of Biochemistry, Massey University, Palmerston North, New Zealand and R.M. Mould, O.M Hofmann and T. Brittain, School of Biological Sciences, University of Auckland, New Zealand

Three hemoglobin molecules (Gower I, Gower II and Portland) are synthesised by the human embryo between weeks two and twelve of gestation. These hemoglobins appear to function as scavengers of O₂ from the mother's interstitial fluid before the placenta has developed. Functional Gower II hemoglobin is a tetramer comprising the adult [[alpha]] chain and the embryonic [[epsilon]], which has 79% sequence identity to the adult [[beta]]. Binding studies have indicated that Gower II hemoglobin binds O₂ cooperatively, with a higher affinity than the adult molecule, and displays similar allosteric behaviour towards H⁺, C1^- and 2,3 DPG.

Gower II hemoglobin with carbon monoxide bound was crystallised. The crystals proved to be tetragonal (spacegroup P4₃2₁2) a=b=62.8, c=320.8Å with one [[alpha]]₂[[epsilon]]₂ tetramer in the asymmetric unit. The 3D structure has been solved at 2.9Å resolution by molecular replacement and refined to a crystallographic R-factor of 0.204 (R_free of 0.279) with good geometry.

The quaternary structure is very similar to that of the adult molecule. Within the [[epsilon]] subunit the main difference from the adult [[beta]] is a small shift of the N terminal helix over the central cavity of the tetramer. The environment of the heme pocket is like that of the adult with the major variation being a closer packing of Ser 70 (Ala in [[beta]]). Clear density is visible for the bound CO ligand.