Entropy driven intermediates of oxygenation regulate hemoglobin allosteric behavior

Temperature dependent oxygen binding capacity of human, bovine and fallow deer (Dama-Dama) Hb's, shows that at least one of the ligation intermediates absorbs heat, i.e. it is entropy driven [Bucci,E et.al Biochemistry 30:3195,1991;.Biochemistry 32:3519,1993; Bioph.J.in press; Johnson.et al.Biochemistry 31:10074; Biophys.Chem.59;107,1995). This phenomenon is also present in human Hb crosslinked between the beta82 Lys by dicarboxyl residues of increasing length, namely fumaryl (4 carbon), adipoyi (6 carbon) and sebacoyl (10 carbon) residues. The value of "n" and the overall oxygen affinity are correlated to the absorption of heat of the intermediate steps of ligation. Fumaryl-Hb has the lowest absorption of heat, the highest affinity and lowest cooperatively index. Adipoyl-Hb has the highest absorption of heat and the highest value of "n". It appears that positive enthalpy changes at the intermediates of oxygénation are a characteristic of the allosteric transitions of Hb. There is not enough information for assigning heat absorption to any of the interplays of the various conformations described for Hb and always available to the system besides the classic T and R, as R2 [Silva et al. JBC. 267:17248,1992] ], t [Liddington et al Nature 331: 725,1988], r [Wilson et al JMB 264:1996.1996], partially oxygenated Hb [Liddington et al. JMB 228:551,1992], partially ferric sebacoyl-Hb [Bucci et al Biochemistry 35:3418,1996], trimesyl-Hb [Shumacher et al Nature 375:84,1995] 4-carboxydnnamyl-Hb and 2,6-naphtalenedicarboxyl Hb, [Schumacher et al. PNAS 94:7841,1997]. Entropy driven steps may char acterize conformational changes still to be described.