Hemoglobin S Polymerization and Sickle Cell Disease: An Overview

Introduction

Polymerization of hemoglobin S is the root cause of the pathology of sickle cell disease. It has been 70 years since the legendary 20th-century genius of chemistry, Linus Pauling, discovered that aggregation of an abnormal hemoglobin into “rigid rods” is responsible for sickle cell disease.^1,2 Pauling left several important questions unanswered that have motivated an enormous amount of basic research since then. These include the following: What is the structure of his “rigid rods?”³ What are the thermodynamics, kinetics, and mechanisms of polymerization? How can the disease be treated? This overview will briefly describe research that has played a major role in answering these questions. A more extensive account of polymerization and its role in disease pathogenesis and therapy can be found in 4 previous reviews^4-7 and in Chapter 2 of this book.

Fiber Structure

The low-resolution structure of the sickle fiber was determined using transmission electron microscopy and sophisticated image reconstruction methods by Stuart Edelstein.^8,9 A few years earlier, Warner Love had solved the x-ray structure of deoxyhemoglobin S, but it was not clear at the time whether his structure, which showed the details of the β-globin 6 intermoloecular contact, had any relationship to the fibers that form in sickle cells.¹⁰ Edelstein recognized that he could construct a 14-stranded fiber structure that is consistent with both the fiber-diffraction data of Magdoff-Fairchild et al¹¹ and the orientation of the hemoglobin S molecules in the fiber determined by polarized optical absorption data on single sickled cells¹² by helically twisting 7 double strands of the x-ray structure.^8,9 Subsequent polymerization studies by Ronald Nagel, Ruth Benesch, Rheinhold Benesch, and their colleagues on mixtures of hemoglobin S with non-S hemoglobin variants were critically important for building the detailed molecular model, which is the current operative model today.^13,14

Thermodynamics

A gel of hemoglobin S is very much like a crystal solution equilibrium, that is, a mixture of 2 phases, 1 solid and 1 liquid, as theoretically predicted by Allen Minton¹⁵ and confirmed in experiments.^16-18 The thermodynamics become considerably more complex when considering the control of polymerization by oxygen and mixtures of hemoglobin S with non-S hemoglobins such as hemoglobins A and F. Understanding how oxygen controls polymerization is the result of careful solubility studies, the direct measurement of binding of oxygen to the fibers, and the application of the Monod, Wyman, and Changeux (MWC) model. Hofrichter¹⁹ performed the first solubility measurement using carbon monoxide as a surrogate for oxygen, and later this was done using oxygen by Sunshine et al.²⁰ Application of linear dichroism in these studies allowed the measurement of the polymer binding curve of a gel because molecules in the liquid phase are randomly oriented and therefore make no contribution to the spectral changes of the fiber ...