Sickle cell disease: A physicochemical analysis of hemoglobin S assembly suggests a new mechanism

Abstract

Time dependent physicochemical methods measure average solution properties. Electron microscopy (EM), however, reveals polydispersity in rod-like polymer systems through fiber lengths. The length of each fiber indicates when in time it was nucleated. The double nucleation model (Ferrone et al., 1985b) describes the mechanism of hemoglobin S fiber formation and growth. It encompasses all the kinetic observations for hemoglobin S gelation and predicts the nucleation rate for fiber formation over time. For all but short times and long rods, the length distribution is predicted to be a negative exponential, time independent and highly concentration dependent. Fiber length distributions by EM on hemoglobin S at 14mM heme in 0.1M potassium phosphate, pH 7.0, 20{dollar}\sp\circ{dollar}, show (1) a negative exponential above a minimum length, (2) time independence, (3) a marked deficit of short fibers, and (4) little or no concentration dependence between 13 and 14mM heme. (1) and (2) are consistent with the current model but (3) and (4) are not. We propose a new model based on the polymorphic breakage of newly formed segments at fiber ends that encompasses all the kinetic data represented in the double nucleation model and these new observations on polydispersity. Under this new model, the exponential length distribution depends only on a concentration independent breaking process and the short fiber deficit results from a minimum breaking length, L{dollar}\sb{lcub}\rm B{rcub}{dollar}.