Atkins William M, Lu Weiya Doug, Cook Daniel L
Department of Medicinal Chemistry, University of Washington, Seattle, Washington 98195-7610, USA.
J Biol Chem. 2002 Sep 6;277(36):33258-66. doi: 10.1074/jbc.M204425200. Epub 2002 Jun 24.
The cytochrome P450s (CYPs) are the major enzymatic detoxification and drug metabolism system. Recently, it has become clear that several CYP isoforms exhibit positive and negative homotropic cooperativity. However, the toxicological implications of allosteric kinetics have not been considered, nor understood. The allosteric kinetics are particularly enigmatic in several respects. In many cases, CYPs bioactivate substrates to more toxic products, thus making it difficult to rationalize a functional advantage for positive cooperativity. Also, CYPs exhibit cooperativity with many structurally diverse ligands, in marked contrast to the specificity observed with other allosteric systems. Here, kinetic simulations are used to compare the probabilistic time- and concentration-dependent integrated toxicity function during conversion of substrate to product for CYP models exhibiting Michaelis-Menten (non-cooperative) kinetics, positive cooperativity, or negative cooperativity. The results demonstrate that, at low substrate concentrations, the slower substrate turnover afforded by cooperative CYPs compared with Michaelis-Menten enzymes can be a significant toxicological advantage, when toxic thresholds exist. When present, the advantage results from enhanced "distribution" of toxin in two pools, substrate and product, for an extended period, thus minimizing the chance that either exceeds its toxic threshold. At intermediate concentrations, the allosteric kinetics can be a modest advantage or modest disadvantage, depending on the kinetic parameters. However, at high substrate concentrations associated with a high probability of toxicity, fast turnover is desirable, and this advantage is provided also by the cooperative enzymes. For the positive homotropic cooperativity, the allosteric kinetics minimize the probability of toxicity over the widest range of system parameters. Furthermore, this apparent functional cooperativity is achieved without specific molecular recognition that is the hallmark of "traditional" allostery.
细胞色素P450（CYPs）是主要的酶促解毒和药物代谢系统。最近，已经明确几种CYP同工型表现出正协同性和负协同性。然而，变构动力学的毒理学意义尚未得到考虑，也未被理解。变构动力学在几个方面尤其难以捉摸。在许多情况下，CYPs将底物生物激活为毒性更强的产物，因此难以合理化正协同性的功能优势。此外，CYPs与许多结构多样的配体表现出协同性，这与其他变构系统所观察到的特异性形成鲜明对比。在这里，动力学模拟用于比较表现出米氏（非协同）动力学、正协同性或负协同性的CYP模型在底物转化为产物过程中概率性的时间和浓度依赖性综合毒性函数。结果表明，在低底物浓度下，当存在毒性阈值时，与米氏酶相比，协同性CYP提供的较慢底物周转可能具有显著的毒理学优势。当存在这种优势时，是由于毒素在底物和产物两个池中长时间增强的“分布”，从而最小化了任何一个超过其毒性阈值的可能性。在中等浓度下，变构动力学可能是适度的优势或适度的劣势，这取决于动力学参数。然而，在与高毒性概率相关的高底物浓度下，快速周转是可取的，而协同酶也能提供这种优势。对于正同促协同性，变构动力学在最广泛的系统参数范围内最小化了毒性概率。此外，这种明显的功能协同性是在没有“传统”变构标志的特异性分子识别情况下实现的。
