Krieger M, Koeppe R E, Stroud R M
Biochemistry. 1976 Aug 10;15(16):3458-64. doi: 10.1021/bi00661a010.
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.
在pH值为8.9和37摄氏度的条件下，胰蛋白酶中组氨酸C-2质子与氚的交换半衰期，对于His-57为73天，对于His-40和His-91则大于1000天。这些半衰期比游离组氨酸C-2质子的交换半衰期（在pD 8.2时为2.8天）长得多，也比之前报道的任何pH大于8时的交换半衰期都长。结合胰蛋白酶的精细结构对这些极低的交换速率进行了讨论。His-57的氚交换取决于表观pKa为6.6。这个pKa可能代表酶的无活性构象中His-57咪唑的pKa。
