Mechanistic Picture for Monomeric Human Fibroblast Growth Factor 1 Stabilization by Heparin Binding

Human fibroblast growth factor (FGF) 1 or hFGF1 is a member of the FGF family that is involved in various vital processes such as cell proliferation, cell differentiation, angiogenesis, and wound healing. hFGF1, which is associated with low stability in vivo, is known to be stabilized by binding heparin sulfate, a glycosaminoglycan that aids the protein in the activation of its cell surface receptor. The poor thermal and proteolytic stability of hFGF1 and the stabilizing role of heparin have long been observed experimentally; however, the mechanistic details of these phenomena are not well understood. Here, we have used microsecond-level equilibrium molecular dynamics (MD) simulations to quantitatively characterize the structural dynamics of monomeric hFGF1 in the presence and absence of heparin hexasaccharide. We have observed a conformational change in the heparin-binding pocket of hFGF1 that occurs only in the absence of heparin. Several intramolecular interactions were also identified within the heparin-binding pocket that form only when hFGF1 interacts with heparin. The loss of both intermolecular and intramolecular interactions in the absence of heparin plausibly leads to the observed conformational change. This conformational transition results in increased flexibility of the heparin-binding pocket and provides an explanation for the susceptibility of apo hFGF1 to proteolytic degradation and thermal instability. This study provides a glimpse into mechanistic details of the heparin-mediated stabilization of hFGF1 and encourages the use of microsecond-level MD in studying the effect of binding on protein structure and dynamics. In addition, the observed differential behavior of hFGF1 in the absence and presence of heparin provides an example, where microsecond-level all-atom MD simulations are necessary to see functionally relevant biomolecular phenomena that otherwise will not be observed on sub-microsecond time scales.