Molecular modeling studies on binding of bFGF to heparin and its receptor FGFR1

Sugar induced protein-protein interactions play an important role in several biological processes. The carbohydrate moieties of proteoglycans, the glycosaminoglycans, bind to growth factors with a high degree of specificity and induce interactions with growth factor receptors, thereby regulate the growth factor activity. We have used molecular modeling method to study the modes of binding of heparin or heparan sulfate proteoglycans (HSPGs) to bFGF that leads to the dimerization of FGF receptor 1 (FGFR1) and activation of receptor tyrosine kinase. Homology model of FGFR1 Ig D(II)-D(III) domains was built to investigate the interactions between heparin, bFGF and FGFR1. The structural requirements to bridge the two monomeric bFGF molecules by heparin or HSPGs and to simulate the dimerization and activation of FGFR1 have been examined. A structural model of the biologically functional dimeric bFGF-heparin complex is proposed based on: (a) the stability of dimeric complex, (b) the favorable binding energies between heparin and bFGF molecules, and (c) its accessibility to FGFR1. The modeled complex between heparin, bFGF and FGFR1 has a stoichiometry of 1 heparin: 2 bFGF: 2 FGFR1. The structural properties of the proposed model of bFGF/heparin/FGFR1 complex are consistent with the binding mechanism of FGF to its receptor, the receptor dimerization, and the reported site-specific mutagenesis and biochemical cross-linking data. In the proposed model heparin bridges the two bFGF monomers in a specific orientation and the resulting complex induces FGF receptor dimerization, suggesting that in the oligosaccharide induced recognition process sugars orient the molecules in a way that brings about specific protein-protein or protein-carbohydrate interactions.