Arrhenius temperature dependence biochemical reactions

The Arrhenius relation will not be observed above the temperature at which the decomposition or, as may occur for enzymes, inactivation of one or more of the reactants occurs (r ax)- Indeed, adherence to this relation at temperatures well above T ax for niost microorganisms has been used as evidence for an abiotic, rather than a biologically mediated mechanism of transformation (Wolfe and Macalady, 1992). For biotransformations, the Arrhenius equation also fails to describe the temperature dependence of reaction rates below the temperature at which biological functions are inhibited (T in), and above the temperature of maximum transformation rate (Topt). An empirical equation introduced by O Neill (1968) may be used to estimate the rates of biotransformation as a function of ambient temperature, min opt max (in this case, the lethal temperature), and the maximum biotransformation rate (p-max)- Because of the complexity of biochemical systems and the myriad of different structures encompassed by pesticide compounds, Tmiii, Topt, Tmax and p max are all likely to vary among different compounds, microbial species and geochemical settings (e.g., Gan et al., 1999, 2000). However, Vink et al. (1994) demonstrated the successful application of the O Neill function to describe the temperature dependence of biotransformation for 1,3-dichloro-propene and 2,4-D in soils (Figure 13). 

Enzymatic reactions frequently undergo a phenomenon referred to as substrate inhibition. Here, the reaction rate reaches a maximum and subsequently falls as shown in Eigure 11-lb. Enzymatic reactions can also exhibit substrate activation as depicted by the sigmoidal type rate dependence in Eigure 11-lc. Biochemical reactions are limited by mass transfer where a substrate has to cross cell walls. Enzymatic reactions that depend on temperature are modeled with the Arrhenius equation. Most enzymes deactivate rapidly at temperatures of 50°C-100°C, and deactivation is an irreversible process. 

The rates of chemical and biochemical reactions usually increase with temperature. The dependence of the reaction rate on temperature can usually be represented by the following Arrhenius-type equation over a wide temperature range ... 

AT any biochemical processes involve very rapid reactions and transient intermediates. Frequently the rapidity of the reaction causes major technical difficulties in ascertaining the details of the events occurring in the process. One approach to overcome this inherent problem is to utilize the fact that most chemical reactions are temperature dependent. This relationship is quantitatively described by the Arrhenius equation, k = Ae E /RT, where k represents the rate constant, A is a constant (the frequency factor), and Ea is the energy of activation. Consequently, by initiating the reaction at a sufficiently low temperature, interconversion of the intermediates may be effectively stopped and they may be accumulated and stabilized individually. Although the focus of this article is on the application of this low-temperature approach to the study of enzyme catalysis, that is, cryoenzymology, the technique is potentially of much wider biological application (1, 2,3). 

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