Thrombin specificity

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Abstract
α-Thrombin cleavage of 30 polypeptide hormones and their derivatives were analysed by quantitative aminoterminal analysis. The polypeptides included secretin, vasoactive intestinal polypeptide, cholecystokinin fragment, dynobrphin A, somatostatins, gastrin-releasing peptide, calcitonins and human parathyroid hormone fragment. Most of them were selected mainly on the ground that they contain sequence structures homologous to the well known tripeptide substrates of α-thrombin. All selected polypeptides have one single major cleavage site and both Arg-Xaa and Lys-Xaa bonds were found to be selectively cleaved by α-thrombin. Under fixed conditions (1 nmol polypeptide/0.5 NIH unit α-thrombin in 20 μl of 50 mM ammonium bicarbonate at 25°C), the time required for 50% cleavage ranges from less than 1 min to longer than 24 h. Heparin invariably enhanced thrombin cleavage on all polypeptide analysed. The optimum cleavage site for α-thrombin has the structures of (a) P4-P3-Pro-Arg-P1′-P2′, where P3 and P4 are hydrophobic amino acid and P1′, P2′ are nonacidic amino acids and (b) P2-Arg-P1′, where P2 or P1′ are Gly. The requirement for hydrophobic P3 and P4 was further demonstrated by the drastic decrease of thrombin cleavage rates in both gastrin-releasing peptide and calcitonins after chemical removal of hydrophobic P3 and P4 residues. The requirement for nonacidic P1′ and P2′ residues was demonstrated by the drastic increase of thrombin cleavage rates in both calcitonin and parathyroid hormone fragments, after specific chemical modification of acidic P1′ and P2′ residues. These findings confirm the importance of hydrophobic P2–P4 residues for thrombin specificity and provide new evidence to indicate that apolar P1′ and P2′ residue are also crucial for thrombin specifity. It is concluded that specific cleavage of polypeptides by α-thrombin can be reasonably predicted and that chemical modification can be a useful tool in enhancing thrombin cleavage.