pH dependence of tritium exchange with the C-2 protons of the histidines in bovine trypsin.
At pH 8.9 and 37 degrees C the half-times for tritium exchange with the C-2 protons of the histidines of trypsin are 73 days for His-57, and greater than 1000 days for His-40 and His-91. These half-times are much longer than the half-life of exchange for the C-2 proton of free histidine (2.8 days at pD 8.2), and longer than any previously reported half-time of exchange at pH greater than 8. These very low rates of exchange are discussed with reference to the refined structure of trypsin. The tritium exchange of His-57 depends on an apparent pKa of 6.6. This pKa may represent the pKa of the imidazole of His-57 in an inactive conformation of the enzyme.
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D000818:Animals; D001665:Binding Sites; D002417:Cattle; D002918:Chymotrypsin; D006639:Histidine / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D007553:Isotope Labeling; D007700:Kinetics; D008433:Mathematics; D010446:Peptide Fragments / Q000032:analysis; D011485:Protein Binding; D014316:Tritium; D014357:Trypsin / Q000378:metabolism; D014360:Trypsin Inhibitor, Kunitz Soybean / Q000494:pharmacology
D000596:Amino Acids; D010446:Peptide Fragments; D014316:Tritium; D006639:Histidine; D014360:Trypsin Inhibitor, Kunitz Soybean; D002918:Chymotrypsin; D014357:Trypsin