Frank Anthony Ferrone

Hemoglobin is a tetrameric protein that exists in two quaternary conformations: a T structure which is found in deoxygenated Hb, and an R structure that is found in liganded Hb. At concentrations above a well defined solubility, sickle hemoglobin in the T structure forms long, multistranded polymers (which generate the pathophysiology of the disease). The molecules in these polymers make significant contacts along the polymer axis (axial), which do not involve the point mutation of the disease, as well as diagonally directed contacts (lateral) that involve the mutation site docking into a non-mutant receptor region. We have conducted light scattering measurements to probe initial steps of aggregation (below solubility), as a function of temperature, concentration, primary and quaternary structure. HbS in the T structure shows much higher overall aggregation, but lower enthalphy than R-structure HbS as well as R and T structure HbA, the latter showing aggregation properties very similar to one another. We conclude that at room temperature the axial contacts are significantly weaker than the lateral ones. The enthalpy for the reaction, however, is much greater for the axial contacts than lateral, and axial and lateral strengths will be much more commensurate at physiological temperatures. Unexpectedly, the data require the presence of substantial fractions of dimers in polymerization, or alternatively, of locally stable intermediates, which have stability that is greater than either their predecessors or successors in the reaction pathway.