Deformability of the hemoglobin molecule as the basis of its functional behavior

It has been proposed that the process of ligand binding to hemoproteins is governed by sequential potential barriers as depicted in the following scheme [Austin et al., Biochemistry 14, pp. 5355-5373, 1975; Ansari et al., Proc. natn. Acad. Sci. USA. pp. 5000-5004, 1985]: (formula; see text) At sufficiently high temperatures (approximately 300 K), the binding process is exponential in time and its velocity proportional to the concentration of the ligand in the solvent. At low temperatures (typically less than 210 K), the combination velocity is not correlated with the amount of free ligand in the solvent and the time course is nonexponential (approximately power law). This implies that (a) proteins are flexible structures and fluctuate between many substates, each having a different ligand binding velocity, and (b) that the rate of interconversion between the various conformational substates is high compared to that of ligand binding at and above room temperature and extremely slow below approximately 200 K. We have investigated the temperature dependence of CO binding after a photolysing pulse to a variety of myoglobins and hemoglobin subunits. The results show that for all myoglobins investigated, the most probable activation energy for the inner process (B----A) is in the order of 10 kJ/mol, while for the hemoglobin subunits it varies between 2.5 and 5 kJ/mol. The distribution of barrier heights is very broad in myoglobins, somewhat narrower in the non-alpha-chains and narrowest in the alpha-hemoglobin subunits. This finding implies that different degrees of flexibility exist between storage and transport proteins and that hemoglobin cooperativity may be related to the presence of subunits with different flexibility within the tetramer.