Thermodynamics of charged oligopeptide-heparin interactions

To better understand the electrostatic component of the interaction between proteins and the polyanion heparin, we have investigated the thermodynamics of heparin binding to positively charged oligopeptides containing lysine or arginine and tryptophan (KWK-CO2 and RWR-CO2). The binding of these peptides to heparin is accompanied by an enhancement of the peptide tryptophan fluorescence, and we have used this to determine equilibrium binding constants. The extent of fluorescence enhancement is similar for both peptides, suggesting that the tryptophan interaction is similar for both. Titrations of these peptides with a series of simple salts suggest that this fluorescence enhancement is due to the interaction of tryptophan with sulfate moieties on the heparin. Equilibrium association constants, Kobs (M-1), for each peptide binding to heparin were measured as a function of temperature and monovalent salt concentration in the limit of low peptide binding density. At pH 6.0, 25 degrees C, 20 mM KCH3CO2, Kobs = 3.2 (+/- 0.3) x 10(3) M-1 for KWK-CO2 binding, whereas Kobs = 4.5 (+/- 0.5) x 10(3) M-1 for RWR-CO2. However, the dependence of Kobs on KCH3CO2 concentration is the same for both oligopeptides, each of which possesses a net charge of +2 at pH 6.0. The logarithm of Kobs is a linear function of the logarithm of [KCH3CO2] over the range from 12 mM < or = KCH3CO2 < or = 30 mM (pH 6.0, 25 degrees C), with (delta log Kobs/delta log [KCH3CO2]) = -2.0 +/- 0.3, indicating that approximately 2 ions are released per bound peptide upon formation of the complex.(ABSTRACT TRUNCATED AT 250 WORDS)