Molecular modelling of platelet endothelial cell adhesion molecule 1 and its interaction with glycosaminoglycans
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The Platelet Endothelial Cell Adhesion Molecule 1 (PECAM-1) has many functions including its roles in leukocyte extravasation as part of the inflammatory response, and in the maintenance of vascular integrity through its contribution to endothelial cell-cell adhesion. Various heterophilic ligands of PECAM-1 have been proposed. The possible interaction of PECAM-1 with glycosaminoglycans (GAGs) is the focus of this thesis. The three dimensional structure of the extracellular immunoglobulin (Ig)-domains of PECAM-1 was constructed using homology modelling and threading methods. Potential heparin/heparan sulfate binding sites were predicted on the basis of their amino acid consensus sequences and a comparison with known structures of sulfate binding proteins. Heparin and other GAG fragments have been docked to investigate the structural determinants of their protein binding specificity and selectivity. It is predicted that two regions in PECAM-1 appear to bind heparin oligosaccharides. A high affinity binding region was located in Ig-domains 2 and 3 and a low affinity region was located in Ig-domains 5 and 6.These GAG binding regions are distinct from regions involved in PECAM-1 homophilic interactions. Docking of heparin fragments of different size revealed that fragments as small as a pentasaccharide appear to be able to bind to domains 2 and 3 with high affinity. Binding of longer heparin fragments suggests that key interactions can occur between six sulfates in a hexasaccharide with no further increase in binding affinity for longer fragments. Molecular dynamics simulations were also used to characterise and quantify the interactions of heparin fragments with PECAM-1. These simulations confirmed the existence of regions of high and low affinity for GAG binding and revealed that both electrostatic and van der Waals interactions determine the specificity and binding affinity of GAG fragments to PECAM-1. The simulations also suggested the existence of ‘open’ and ‘closed’ conformations of PECAM-1 around domains 2 and 3.
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