Tyrosine and tryptophan residues and amino groups in thrombin related to enzymic activities

Abstract

1. 1.|The reactivities and ionization characteristics of tyrosine and tryptophan residues and amino groups in thrombin were examined in relation to esterolytic, proteolytic and clotting activities. 2. 2.|All 9 tyrosine residues in the thrombin molecule may be classified into 3 types: (a) n = 4 with p K = 10.2 ( m = 1.0); (b) n = 2 with p K = 11.1 ( m = 3.0); and (c) n = 3 with higher p K values including slowly ionizing and non-ionizing residues. 3. 3.|Acetylimidazole reacted with the 4 tyrosine residues which are possibly identical with the first group of 4 residues having a p K = 10.2 ( m = 1.0). 4. 4.|2 of the 6 tryptophan residues in the thrombin molecule reacted with H 2O 2; its esterolytic, proteolytic or clotting activities decreased, respectively, to 60, 30 and 30% of the original level. 5. 5.|13 of the total 23 amino groups in the thrombin molecule reacted with β-naphthoquinone 4,6-disulfonic acid (NQDS-4,6), and some amino groups reactive at lower concentrations of NQDS-4,6 were closely associated with the proteolytic and clotting activities. 6. 6.|The results are discussed in comparison with the trypsin molecule which resembles thrombin in its substrate specificity.