Halling P J
Department of Bioscience and Biotechnology, University of Strathclyde, Glasgow, UK.
Enzyme Microb Technol. 1994 Mar;16(3):178-206. doi: 10.1016/0141-0229(94)90043-4.
This article discusses the application of thermodynamic and related analysis to reaction systems for enzymic or whole cell catalysis, in which there are high proportions of organic liquid, gas, or supercritical fluid. A variety of predictions may be made, especially based on the partitioning of components between the different phases normally present. In many cases, observed behavior can be explained without invoking any direct molecular effects on the biocatalyst. The predictable changes should always be allowed for before seeking explanations for the residual effects, which are often very different from the crude observations. A summary of the general thermodynamics of multiphase systems is presented, and then the main classes of component that distribute between the phases are discussed in turn. Thermodynamic water activity (aw) determines the mass action effects of water on hydrolytic equilibria. It also describes the distribution of water between the various phases that can compete in binding water. Because catalytic activity is very sensitive to the hydration of the enzyme molecules, aw often predicts an unchanging optimum as other aspects of the system are changed. Hence the aw should be measured and/or controlled in these systems, whether the primary aim is to study the effects of water or of other changes. The methods available for measurement and control of aw are discussed. Adverse effects of organic solvents or similar nonpolar species partly reflect their tendency to partition into the relatively polar phase around the biocatalyst, especially when this is dilute aqueous. The well-established log P parameter is a measure of this. But other mechanisms of inactivation can occur: directly through contact of the biocatalyst with the phase interface, or indirectly via hydration changes. In these cases the molecular property log P is probably not the best solvent parameter. In low-water systems the biocatalyst remains in a separate phase even when water-miscible solvents are used. Hence, the categorization of solvents in terms of miscibility becomes less relevant. This accounts for the "two peak" dependence of catalytic activity on water content in some miscible systems. Differential solvation of reactants and products, as the bulk phase is altered, causes changes in concentration-based equilibrium constants and yields. These changes in solvation may be monitored through partition coefficient or solubility measurements. Reactant solvation can also account for differences in biocatalyst kinetics, whether or not partitioning into a dilute aqueous phase is involved. These predictable effects should be allowed for when studying effects of solvent or similar changes on activity or specificity.(ABSTRACT TRUNCATED AT 400 WORDS)
本文讨论了热力学及相关分析在酶催化或全细胞催化反应体系中的应用，这些体系中含有高比例的有机液体、气体或超临界流体。可以做出多种预测，特别是基于不同相之间通常存在的组分分配。在许多情况下，无需考虑对生物催化剂的任何直接分子效应，就能解释观察到的行为。在寻求对残余效应的解释之前，应始终考虑可预测的变化，这些残余效应往往与粗略观察结果大不相同。本文首先介绍了多相系统的一般热力学，然后依次讨论了在各相之间分配的主要组分类型。热力学水活度（aw）决定了水对水解平衡的质量作用效应。它还描述了水在能够竞争结合水的各相之间的分布。由于催化活性对酶分子的水合作用非常敏感，当系统的其他方面发生变化时，aw通常能预测出不变的最佳值。因此，无论主要目的是研究水的影响还是其他变化的影响，都应在这些系统中测量和/或控制aw。本文讨论了可用于测量和控制aw的方法。有机溶剂或类似非极性物质的不利影响部分反映了它们倾向于分配到生物催化剂周围相对极性的相中，尤其是当生物催化剂处于稀水相时。公认的log P参数就是对此的一种度量。但也可能发生其他失活机制：直接通过生物催化剂与相界面的接触，或间接通过水合变化。在这些情况下，分子性质log P可能不是最佳的溶剂参数。在低水体系中，即使使用与水混溶溶剂，生物催化剂仍处于单独的相中。因此，根据混溶性对溶剂进行分类就变得不那么重要了。这就解释了在一些混溶体系中催化活性对水含量的“双峰”依赖性。随着主体相的改变，反应物和产物的差异溶剂化会导致基于浓度的平衡常数和产率发生变化。这些溶剂化变化可以通过分配系数或溶解度测量来监测。反应物溶剂化也可以解释生物催化剂动力学的差异，无论是否涉及分配到稀水相中。在研究溶剂或类似变化对活性或特异性的影响时，应考虑这些可预测的效应。（摘要截选至400字）
