Comparative studies of the interaction of human and bovine platelet factor 4 with heparin using histidine NMR resonances as spectroscopic probes

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Abstract
ThepK a values of His-38 and His-50 of the heparin-binding protein, bovine platelet factor 4, are 5.6 and 6.5, respectively, as determined by1H NMR spectroscopy. The1H NMR resonance of His-38 of bovine platelet factor 4 which exhibits the lowerpK a value is perturbed upon heparin binding to a greater degree than the resonance of His-50. Human platelet factor 4 contains the homologous residues His-23 and His-35. ThepK a values of the two histidine residues of human platelet factor 4 are 5.3 and 6.4. The1H NMR resonance of the histidine of human platelet factor 4 exhibiting the lowerpK a value also is perturbed upon heparin binding to a greater degree than the histidine resonance exhibiting the higherpK a , thereby suggesting comparable heparin-protein interactions in bovine and human platelet factor 4.