Temperature optima of enzymes: sifting fact from fiction

The classic approach to the description of the temperature-dependence of enzyme activity lumps both reversible and irreversible inactivations into one step. This generates an erroneous dependence of the enzymatic rate on temperature and the duration of assay. A two-step ‘equilibrium model’ has been proposed recently to correct these weaknesses. However, this model too has some drawbacks. At short assay times, there are two peaks on the rate–temperature–time surface. This difficulty with fast assays is avoided by slow assays; however, the latter carry a greater risk of irreversible denaturation. This problem disappears on increasing the order of the irreversible step from one to two, suggesting that a second-order equilibrium model may be appropriate for some thermal inactivations.