Cohen S G, Torem B, Vaidya V, Ehret A
J Biol Chem. 1976 Aug 10;251(15):4722-8.
Effects of hydrogen-bonding interactions of amide groups on reactivity of esters to alpha-chymotrypsin were studied. Of the methyl esters studied, only that from acetyl-L-phenylalanine has k3 rate-limiting. In methyl beta-phenylpropionates an alpha-acetamido substituent increased k2 greater than 550 times, k3 approximately 5 times; an alpha-acetylclycyloxy substituent increased k2 approximately 2 times, k3 approximately 6 times, both in comparison with the alpha-acetoxy esters. Essentially all carboxamidomethyl esters studied have k3 rate-limiting; reactivity to hydroxide is only 4 times that of methyl esters. In alpha-substituted beta-phenylpropionates, carboxamido-methyl esters show k2 values greater than 110 times greater than 280 times, greater than 26 times, and 7 times the k2 values of the methyl esters for the alpha substituents, acetoxy, acetylglycyloxy, hydroxy, and hydrogen, respectively. In esters of alpha-acetamido acids, carboxamidomethyl esters show k2 values 44 times, greater than 110 times, greater than 12 times, and approximately 33 times the k2 values of the methyl esters of glycine, alanine, leucine, and phenylalanine, respectively. Cyanomethyl esters also had k3 rate-limiting. Hydrogen-bonding to the enzyme of either an alpha-acetamido group or a carboxamidomethyl group combined with bonding of the beta-aryl group, orients the hydrolyzing groups properly, increasing k2. Hydrogen-bonding of both alpha-acetamido and carboxamido-methyl groups is effective to a lesser degree. The amide group appears to have small effects on Ks as hydrogen bonding is balanced by desolvation. It is proposed that desolvation during bonding increases k2 and Ks.
研究了酰胺基团的氢键相互作用对酯与α-胰凝乳蛋白酶反应活性的影响。在所研究的甲酯中，只有乙酰基-L-苯丙氨酸甲酯的k3是限速的。在β-苯丙酸甲酯中，α-乙酰氨基取代基使k2增加超过550倍，k3增加约5倍；α-乙酰基甘氨酰氧基取代基使k2增加约2倍，k3增加约6倍，两者均与α-乙酰氧基酯相比。基本上所有研究的羧酰胺甲基酯都有k3限速；对氢氧根的反应活性仅为甲酯的4倍。在α-取代的β-苯丙酸酯中，羧酰胺甲基酯的k2值分别比甲酯的k2值大110倍以上、280倍以上、26倍以上和7倍，其α取代基分别为乙酰氧基、乙酰基甘氨酰氧基、羟基和氢。在α-乙酰氨基酸酸酯中，羧酰胺甲基酯的k2值分别是甘氨酸、丙氨酸、亮氨酸和苯丙氨酸甲酯k2值的44倍、110倍以上、12倍以上和约33倍。氰甲基酯也有k3限速。α-乙酰氨基基团或羧酰胺甲基基团与β-芳基基团结合后与酶形成氢键，使水解基团正确定向，从而增加k2。α-乙酰氨基基团和羧酰胺甲基基团两者形成的氢键效果较小。由于氢键作用被去溶剂化作用平衡，酰胺基团对Ks似乎影响较小。有人提出，结合过程中的去溶剂化作用增加了k2和Ks。
