Backes W L, Cawley G, Eyer C S, Means M, Causey K M, Canady W J
Department of Pharmacology, Louisiana State University Medical Center, New Orleans 70112.
Arch Biochem Biophys. 1993 Jul;304(1):27-37. doi: 10.1006/abbi.1993.1317.
The subject of hydrocarbon inhibition of cytochrome P450-dependent reactions as well as data on other enzyme-catalyzed reactions from the literature was examined to determine the relationship between the "hydrophobicity" of the hydrocarbons and their ability to act as inhibitors. The compounds used in these studies (benzene, toluene, ethylbenzene, n-propylbenzene, and n-butylbenzene) behave as competitive inhibitors, with the affinity increasing as the size of the inhibiting hydrocarbon increases. A similarity was seen in the size dependence for both hydrocarbon inhibition of cytochrome P450-dependent activities (-0.6 to -0.7 kcal/mol/methylene group) and transfer of these compounds between aqueous and organic phases (-0.68 kcal/mol/methylene group), suggesting that the active site of cytochrome P450, in some ways, is comparable to an organic solvent in its ability to accommodate hydrophobic compounds. A more detailed examination of this process was initiated to separate the "hydrophobic effect" into its two component processes: (i) hydration of the hydrocarbon ligand and (ii) transfer of the unhydrated hydrocarbon onto the enzyme active site. In other words, do larger hydrocarbons bind more avidly to the active site because they are drawn more effectively into that site (pull), or is the size-dependent increase in hydrocarbon binding the result of the larger compounds being more efficiently expelled from the aqueous medium (push)? The results indicate that the predominant force involved in binding is the ability of the active site of cytochrome P450 and an impressive number of other enzymes to draw the hydrocarbon from the aqueous medium. The hydration of the hydrocarbon is much less dependent on the size of the hydrocarbon, indicating that dehydration or partial dehydration of the hydrocarbon molecule (upon leaving the solution and combining with the enzyme) contributes to the overall binding process to a much lesser extent; hydrophobic binding in the most widely used sense (entropy driven) is not the primary driving force that is responsible for the observed size dependence effects. It is pointed out that not all types of binding would be expected to follow the law which describes the size dependence for simple hydrocarbons because of heat-entropy relationships. The different temperature dependence of these heat-entropy relationships further complicates the analogy between enzyme-ligand binding and ligand partitioning between aqueous and organic phases. The maximum contribution that can be attributed to entropy driven hydrophobic binding (in the most widely used sense) is -0.1 to -0.2 kcal/mol/methylene group for the aromatic hydrocarbons examined here.
研究了文献中关于烃类对细胞色素P450依赖性反应的抑制作用以及其他酶催化反应的数据，以确定烃类的“疏水性”与其作为抑制剂的能力之间的关系。这些研究中使用的化合物（苯、甲苯、乙苯、正丙苯和正丁苯）表现为竞争性抑制剂，随着抑制性烃类尺寸的增加，其亲和力也增加。在烃类对细胞色素P450依赖性活性的抑制作用（-0.6至-0.7千卡/摩尔/亚甲基）以及这些化合物在水相和有机相之间的转移（-0.68千卡/摩尔/亚甲基）的尺寸依赖性方面观察到相似性，这表明细胞色素P450的活性位点在某种程度上在容纳疏水性化合物的能力方面与有机溶剂相当。对这一过程进行了更详细的研究，以将“疏水效应”分为其两个组成过程：（i）烃类配体的水合作用和（ii）未水合烃类转移到酶活性位点上。换句话说，较大的烃类是否因为它们更有效地被吸引到该位点（拉力）而更强烈地结合到活性位点上，或者烃类结合中尺寸依赖性的增加是较大化合物更有效地从水相中排出（推力）的结果？结果表明，结合中涉及的主要力量是细胞色素P450活性位点以及大量其他酶从水相中吸引烃类的能力。烃类的水合作用对烃类尺寸的依赖性要小得多，这表明烃类分子（离开溶液并与酶结合时）的脱水或部分脱水对整体结合过程的贡献要小得多；最广泛意义上的疏水结合（熵驱动）不是导致观察到的尺寸依赖性效应的主要驱动力。需要指出的是，由于热熵关系，并非所有类型的结合都预计会遵循描述简单烃类尺寸依赖性的规律。这些热熵关系不同的温度依赖性进一步使酶-配体结合与配体在水相和有机相之间的分配之间的类比变得复杂。对于此处研究的芳烃，可归因于熵驱动疏水结合（最广泛意义上）的最大贡献为-0.1至-0.2千卡/摩尔/亚甲基。
