Interaction of heparin with internally quenched fluorogenic peptides derived from heparin-binding consensus sequences, kallistatin and anti-thrombin III

Internally quenched fluorogenic (IQF) peptides bearing the fluorescence donor/acceptor pair o-aminobenzoic acid (Abz)/N-(2,4-dinitrophenyl)ethylenediamine (EDDnp) at N- and C-terminal ends were synthesized containing heparin-binding sites from the human serpins kallistatin and antithrombin, as well as consensus heparin-binding sequences (Cardin clusters). The dissociation constant (K_d), as well as the stoichiometry for the heparin–peptide complexes, was determined directly by measuring the decrease in fluorescence of the peptide solution. Experimental procedures were as sensitive as those used to follow the fluorescence change of tryptophan in heparin-binding proteins. The conformation of the peptides and the heparin–peptide complexes were obtained from measurements of time-resolved fluorescence decay and CD spectra. Kallistatin (Arg³⁰⁰–Pro³¹⁹)-derived peptide (HC2) and one derived from antithrombin III helix D [(AT3D), corresponding to Ser¹¹²–Lys¹³⁹], which are the heparin-binding sites in these serpins, showed significant affinity for 4500Da heparin, for which K_d values were 17nM and 100nM respectively. The CD spectra of the heparin–HC2 peptide complex did not show any significant α-helix content, different from the situation with peptide AT3D, for which complex-formation with heparin resulted in 24% α-helix content. The end-to-end distance distribution and the time-resolved fluorescence-decay measurements agree with the CD spectra and K_d values. The synthetic α-methyl glycoside pentasaccharide AGA∗IA_M (where A represents N,6-O-sulphated α-d-glucosamine; G, β-d-glucuronic acid; A∗, N,3,6-O-sulphated α-d-glucosamine; I, 2-O-sulphated α-l-iduronic acid; and A_M, α-methyl glycoside of A) also binds to AT3D and other consensus heparin-binding sequences, although with lower affinity. The interaction of IQF peptides with 4500Da heparin was displaced by protamine. In conclusion, IQF peptides containing Abz/EDDnp as the donor/acceptor fluorescence pair are very promising tools for structure–activity relationship studies on heparin–peptide complexes, as well as for the development of new peptides as heparin reversal-effect compounds.