Effects of Temperature on Enzyme Reactions

Abstract

The thermodynamic treatment for the temperature dependence of simple chemical reactions, discussed in Chapter 2 (Section 2.6), applies equally well to enzyme-catalyzed reactions, but in practice several complications arise that must be properly understood if any useful information is to be obtained from temperature-dependence studies of enzyme reactions. First, almost all enzymes become denatured if they are heated much above physiological temperatures and the conformation of the enzyme is altered, often irreversibly, with loss of catalytic activity. Exceptions are the thermophylic microorganisms , which are capable of working within a much broader range of temperature. The loss of catalytic activity if often due to denaturation; denaturation is chemically a very complex process, considering a large molecular size of proteins and the complexity of their three-dimensional structure. The second influence of temperature is a usual influence of temperature on the rate of chemical reactions. The major effect of temperature is often not only on rates of chemical reaction, but also on the possible conformation changes before and after the chemical step, since these may have larger standard Gibbs free energies of activation than the chemical steps. Because of the overlapping influence on the structure of the protein molecule, straightforward analysis of the influence of temperature on the rates of enzyme-catalyzed reactions can be often obtained only within a fairly small range of temperature, usually between and A special branch of enzymology, cryoenzymology, deals with enzyme reactions in mixed solvents at subzero temperatures (Douzou, 1977; Fink & Greeves, 1979). 15.1 FREE ENERGY PROFILES The energetics of an enzyme-catalyzed reaction is usually discussed in terms of a free energy profile; this is a diagram showing the relative free energy levels of all enzyme-reactant complexes and the transition states for conversion between them, at some chosen set of standard conditions (Lumry, 1959,1995). Figure 1 shows such a profile for an uncatalyzed and enzyme-catalyzed uni-molecular chemical reaction. Free energy profiles are commonly shown as points representing the relative free energy changes, connected by a smooth curve. However, it should be pointed out that experimental observations typically can only provide information for the maxima and minima of these profiles and not on points in between