Human platelet factor 4 subunit association/dissociation thermodynamics and kinetics

Area of Science:

• Biochemistry
• Biophysics
• Protein Dynamics

Background:

• Platelet factor 4 (PF4) is a protein known to form aggregates.
• Understanding the thermodynamics and kinetics of these aggregations is crucial for comprehending its biological functions.

Purpose of the Study:

• To investigate the thermodynamic and kinetic parameters of Platelet factor 4 (PF4) subunit association and dissociation.
• To elucidate the molecular interactions governing PF4 aggregate formation.

Main Methods:

• Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy at 500 MHz was employed.
• Analysis of Y60 3,5 ring proton resonances to determine monomer, dimer, and tetramer populations.
• Temperature-dependent studies (10-40°C) to derive equilibrium thermodynamic parameters.
• Saturation-transfer and spin-lattice relaxation experiments to assess kinetic parameters.

Main Results:

• Thermodynamic parameters for dimer association: ΔG = -5.1 kcal/mol, ΔH = +2.5 ± 1 kcal/mol, ΔS = +26 ± 7 eu.
• Thermodynamic parameters for tetramer association: ΔG = -5.7 kcal/mol, ΔH = -7.5 ± 1 kcal/mol, ΔS = -7 ± 3 eu.
• Unimolecular dissociation rate constants at 30°C: 35 ± 10 s⁻¹ for dimer and 6 ± 2 s⁻¹ for tetramer.

Conclusions:

• Dimer formation is primarily driven by electrostatic/hydrophobic interactions in a low dielectric medium.
• Tetramer formation is governed by hydrogen bonding interactions.
• Kinetic data provides insights into the rates of PF4 aggregate dissociation.