A model of the platelet factor 4 complex with heparin

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Modeling

Background:

• Bovine platelet factor 4 (BPF4) is a protein involved in blood clotting and inflammation.
• Heparin is a glycosaminoglycan anticoagulant that interacts with various proteins, including PF4.
• Understanding the structural basis of heparin-BPF4 interaction is crucial for drug development and biological studies.

Purpose of the Study:

• To develop a detailed structural model of heparin bound to bovine platelet factor 4 (BPF4).
• To elucidate the specific binding interactions and electrostatic complementarity between heparin and BPF4.
• To investigate the role of key residues, such as lysines and His-38, in heparin binding.

Main Methods:

• Graphical modeling of a heparin molecule using established chemical representations.
• Utilizing crystallographic coordinates of the native bovine platelet factor 4 tetramer (2.6 Å resolution, R=0.237).
• Performing electrostatic calculations on the BPF4 tetramer to identify charged regions.

Main Results:

• A model was generated showing heparin (≥8 disaccharide units, highly sulfated) bound to the BPF4 tetramer.
• BPF4 monomers feature an alpha-helix with solvent-accessible lysines, forming a positive charge ring across the tetramer.
• The model positions heparin perpendicularly to the alpha-helices, interacting electrostatically with the positive charge ring and salt-linking to lysines.

Conclusions:

• The study provides a detailed structural model for heparin-BPF4 complex formation.
• Heparin binding is mediated by electrostatic interactions along a positively charged ring on the BPF4 tetramer.
• Key lysine residues on the alpha-helix and His-38 are critical for high-affinity heparin binding.