title: The modification of tryptophan in bovine thrombin.
abstract: 2-Hydroxy-5-nitrobenzyl bromide, at a 100-fold molar excess, was observed to react withthrombin at pH 4.0 to give a modified enzyme which possessed 20% of the fibrinogen clotting activity and 80% of the esterase activity compared to a control preparation. Spectrophotometric analysis of the modified protein indicated that this effect on catalytic activity was associated with the incorporation of 1 mol of reagent per mol of thrombin. Amino acid analysis showed no loss of amino acids other than tryptophan. The reaction of N-bromosuccinimide with thrombin at 2-fold molar excess resulted in the modification of one tryptophan per mol of enzyme with the loss of 80% of the fibrinogen clotting activity with, as above, a considerably smaller loss of esterase activity. Oxidation of thrombin with N-bromosuccinimide decreased the extent of subsequent tryptophan modification with 2-hydroxy-5-nitrobenzyl bromide. Thrombin modified with 2-hydroxy-5-nitrobenzyl bromide showed a 3-4 fold increase in Km and a decrease in V for the ester substrate. The reaction of thrombin with 2-acetoxy-5-nitrobenzyl bromide, a substrate analogue, also resulted in the inactivation of the enzyme. The data are interpreted to show the presence of a tryptophan residue at or near the enzyme's substrate binding site.
mesh_terms: D015077:2-Hydroxy-5-nitrobenzyl Bromide; D000596:Amino Acids / Q000032:analysis; D000818:Animals; D001665:Binding Sites; D002417:Cattle; D005340:Fibrinogen; D011485:Protein Binding; D013917:Thrombin / Q000502:physiology; D014364:Tryptophan / Q000032:analysis
chemical_list: D000596:Amino Acids; D015077:2-Hydroxy-5-nitrobenzyl Bromide; D014364:Tryptophan; D005340:Fibrinogen; D013917:Thrombin