The Growth of Sickle Hemoglobin Polymers

Abstract

The measurement of polymer growth is an essential element in characterization of assembly. We have developed a precise method of measuring the growth of sickle hemoglobin polymers by observing the time required for polymers to traverse a photolytically produced channel between a region in which polymers are created and a detection region. The presence of the polymer is functionally detected by observing its ability to create new polymers through the well-established process of heterogeneous nucleation. Using this method, we have determined the rate constants for monomer addition to and release from polymer ends, as well as their temperature dependences. At 25°C we find k(+) = 84 ± 2 mM⁻¹ s⁻¹ and k(-) = 790 ± 80 molecules/s from each end. These numbers are in accord with differential interference contrast measurements, and their ratio gives a solubility measured on individual fibers. The single-fiber solubility agrees with that measured in sedimentation experiments. The concentration dependence of the monomer addition rate is consistent with monomer addition, but not oligomer addition, to growing polymers. The concentration dependence suggests the presence of an activation enthalpy barrier, and the rate of monomer addition is not diffusion-limited. Analysis of the temperature dependence of the monomer addition rate reveals an apparent activation energy of 9.1 ± 0.6 kcal/mol.

Modeling sickle hemoglobin fibers as one chain of coarse-grained particles

“…The present model could be combined with a hybrid method of dynamic coarse-graining, to simulate HbS polymerization. Since the HbS polymerization process is governed by monomer addition as opposed to oligomer addition (Aprelev et al, 2011), the polymerization has to be studied at the atomistic level. Therefore, the tip of a HbS fiber, where polymerization occurs, could be simulated at the atomistic level while the already polymerized HbS fiber could be simulated by the proposed coarse-grained model to reduce the overall computational cost.…”

Multifunctional magnetic rotator for micro and nanorheological studies

“…This polymer is very rigid, its elastic modulus is measured in megapascals. 41 Therefore, the widely used biophysical techniques based on laser tweezers are not applicable here. The AFM based approaches are also questionable because the polymers are metastable: HbS is prone to a spontaneous polymerization when the concentration of deoxyHbS exceeds a well-defined level.…”

Predicting the morphology of sickle red blood cells using coarse-grained models of intracellular aligned hemoglobin polymers

“…The implicit assumption here is that the rate-limiting process for the aligned hemoglobin polymer domain development is the stall force on the sickle hemoglobin polymers instead of sickle hemoglobin monomer diffusion as discussed in ref. 32. Incorporation of anisotropic fiber growth rates may result in further heterogeneous morphological states.…”

“…32. This process is relatively short as compared to the total time of the “sickling” procedure ( O (10)to O (100) s).…”

“…This result is also validated by experimental measurements. 32 On the other hand, k on and k off represent the growth and dissociation rates in terms of the CG polymer beads and should be adjusted according to the choice of the length scale of the CG model, as discussed in the following section. For each time step Δ t , a single DPD particle is added to the polymer end according to the probability P t = 1 − e − k t Δ t .…”