Bio Effect of Temperature on Enzymes

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Effect of temperature and pressure

Rates of all reactions, including those catalysed by enzymes, rise with increase in temperature in accordance with the Arrhenius equation.

(1.21)

where k is the kinetic rate constant for the reaction, A is the Arrhenius constant, also known as the frequency factor, DG* is the standard free energy of activation (kJ M-1) which depends on entropic and enthalpic factors, R is the gas law constant and T is the absolute temperature. Typical standard free energies of activation (15 - 70 kJ M-1) give rise to increases in rate by factors between 1.2 and 2.5 for every 10°C rise in temperature. This factor for the increase in the rate of reaction for every 10°C rise in temperature is commonly denoted by the term Q10 (i.e. in this case, Q10 is within the range 1.2 - 2.5). All the rate constants contributing to the catalytic mechanism will vary independently, causing changes in both Km and Vmax. It follows that, in an exothermic reaction, the reverse reaction (having a higher activation energy) increases more rapidly with temperature than the forward reaction. This, not only alters the equilibrium constant (see equation 1.12), but also reduces the optimum temperature for maximum conversion as the reaction progresses. The reverse holds for endothermic reactions such as that of glucose isomerase (see reaction [1.5]) where the ratio of fructose to glucose, at equilibrium,