Heparin Decamer Bridges a Growth Factor and an Oligolysine by Different Charge-Driven Interactions

Full-length heparin is widely used in tissue engineering applications due its multiple protein-binding sites that allow it to retain growth factor affinity while associating with oligopeptide components of the tissue scaffold. However, the extent to which oligopeptide coupling interferes with cognate protein binding is difficult to predict. To investigate such simultaneous interactions, we examined a well-defined ternary system comprised of acidic fibroblast growth factor (FGF), tetralysine (K4), with a heparin decamer (dp10) acting as a noncovalent coupler. Electrospray ionization mass spectrometry was used to assess binding affinities and complex stoichiometries as a function of ionic strength for dp10·K4 and FGF·dp10. The ionic strength dependence of K4·dp10 formation is qualitatively consistent with binding driven by the release of condensed counterions previously suggested for native heparin with divalent oligopeptides (Mascotti, D. P.; Lohman, T. M. Biochemistry 1995, 34, 2908–2915). On the other hand, FGF binding displays more complex ionic strength dependence, with higher salt resistance. Remarkably, dp10 that can bind two FGF molecules can only bind one tetralysine. The limited binding of K4 to dp10 suggests that the tetralysine might not block growth factor binding, and the 1:1:1 ternary complex is indeed observed. The analysis of mass distribution of the bound dp10 chains in FGF·dp10, FGF2·dp10, and FGF·dp10·K4 complexes indicated that higher degrees of dp10 sulfation promote the formation of FGF2·dp10 and FGF·dp10·K4. Thus, the selectivity of appropriately chosen short heparin chains could be used to modulate growth factor sequestration and release in a way not feasible with heterogeneous native heparin. In support of this, human hepatocellular carcinoma cells (HEP3Bs) treated with FGF·dp10·K4 were found to exhibit biological activity similar to cells treated with FGF.