Electrostatic attraction governs the dimer assembly of human hemoglobin

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Dynamics

Background:

• Hemoglobin assembly into alpha beta dimers is crucial for oxygen transport.
• Understanding the kinetics of this process is vital for comprehending hemoglobinopathies.
• Surface charge variations in hemoglobin subunits can influence protein interactions.

Purpose of the Study:

• To investigate the role of surface charge on the assembly rate of human hemoglobin A alpha beta dimers.
• To compare the assembly kinetics of normal beta A subunits with charge variants.
• To elucidate the mechanism of alpha beta dimer formation.

Main Methods:

• Utilized subunit competition experiments with 3H-labeled alpha A subunits and various beta subunits (beta A, beta N, beta J, beta S, beta C).
• Analyzed reconstituted hemoglobins using ion-exchange HPLC, gel electrofocusing, and fluorography.
• Measured the ratio of variant Hb X to Hb A formation to determine relative monomer combination rates (kXa/kAa).

Main Results:

• The rate of alpha beta dimer assembly decreased monotonically with increasing negative surface charge on the beta subunit.
• Surface charge did not significantly affect the rate of alpha beta dimer dissociation (kd).
• At pH 8.0, where alpha chains are neutral, assembly rates with beta A and beta C were similar.

Conclusions:

• Electrostatic interactions between oppositely charged alpha and beta subunits facilitate the initial formation of an encounter complex.
• The subsequent rearrangement to form the stable alpha beta dimer is independent of surface charge.
• A two-step mechanism involving an electrostatically driven encounter complex followed by a charge-independent rearrangement explains hemoglobin assembly.