Hemoglobin of the antarctic fishes Trematomus bernacchii and Trematomus newnesi: Structural basis for the increased stability of the liganded tetramer relative to human hemoglobin

Hemoglobins extracted from fishes that live in temperate waters show little or no dissociation even in the liganded form, unlike human hemoglobin (HbA ). To establish whether cold adaptation influences the tendency to dissocia te, the dimer-tetramer association constants (L-2,L-4) of the carbonmonoxy derivatives of representative hemoglobins from two Antarctic fishes, Tremat omus newnesi (Hb1Tn) and Trematomus bernacchii (Hb1Tb), were determined by analytical ultracentrifugation as a function of pH in the range 6.0-8.6 and compared to HbA. HbA is more dissociated than fish hemoglobins at all pH v alues and in particular at pH 6.0. In contrast, both fish hemoglobins are m ostly tetrameric over the whole pH range studied. The extent of hydrophobic surface area buried at the alpha (1)beta (2) interface upon association of dimers into tetramers and the number of hydrogen bonds formed are currentl y thought to play a major role in the stabilization of the hemoglobin tetra mer, These contributions were derived from the X-ray structures of the thre e hemoglobins under study and found to be in good agreement with the experi mentally determined L-2,L-4 values. pH affects oxygen binding of T. bernacc hii and T. newnesi hemoglobins in a different fashion. The lack of a pH eff ect on the dissociation of the liganded proteins supports the proposal that the structural basis of such effects resides in the T (unliganded) structu re rather than in the R (liganded) one.