Interaction of heparin with a synthetic pentadecapeptide from the C-terminal heparin-binding domain of fibronectin

The synthetic pentadecapeptide FN-C/H II (KNNQKSEPLIGRKKT-NH2) has the sequence of the carboxy-terminal heparin-binding domain of module III14 of fibronectin. Interaction of FN-C/H II with bovine lung heparin has been studied by H-1 and Na-23 NMR spectroscopy and by heparin affinity chromatography. FN-C/H II binds to heparin from pD <2 up to pD similar to 10; at higher pD, the binding decreases as the lysine side-chain ammonium groups are titrated. Na+ counterions are displaced from the counterion condensation volume that surrounds sodium heparinate by FN-C/H II, which provides direct evidence that the binding involves electrostatic interactions. The pK(A) values for each of the five ammonium groups of FN-C/H II increase upon binding to heparin which, together with chemical shift data, indicates that the binding involves both delocalized and direct electrostatic interactions between ammonium groups of FN-C/H II and carboxylate and/or sulfate groups of heparin. NMR data also provide evidence for the direct interaction of the guanidinium group of the arginine side chain with anionic sites on heparin. The affinity of heparin for FN-C/H II and for 13 analogue peptides in which lysine and arginine residues were systematically substituted with alanine increases as the number of basic residues increases. The relative contribution of each lysine and arginine to the affinity of heparin for FN-C/H II is R-12 > K-13 > K-14 > K-1 > K-5. Nuclear Overhauser enhancement (NOE) data indicate that, while FN-C/H II is largely unstructured in aqueous solution, the bound peptide interconverts among overlapping, turn-like conformations over the L-9 - T-15 Segment of the peptide. NOE data for the interaction of FN-C/H II with a heparin-derived hexasaccharide, together with the number of Naf ions displaced from heparin by FN-C/H II was determined by Na-23 NMR, indicates that the peptide binds to a hexasaccharide segment of heparin. Identical NMR and heparin affinity chromatography results were obtained for the interaction of FN-C/H II and its D-amino acid analogue peptide with heparin, which is of interest for the potential use of peptides as therapeutic agents for diseases in which cell adhesion plays a critical role.