On the temperature dependence of enzyme-catalyzed rates

One of the critical variables that determine the rate of any reaction is temperature. For biological systems, the effects of temperature are convoluted with myriad (and often opposing) contributions from enzyme catalysis, protein stability and temperature-dependent regulation, for example. We have coined the phrase "macromolecular rate theory (MMRT)" to describe the temperature dependence of enzyme-catalysed rates independent of stability or regulatory processes. Central to MMRT is the observation that enzyme-catalysed reactions occur with significant values of ∆Cp(‡) that are in general negative. That is, the heat capacity (∆Cp) for the enzyme-substrate complex is generally larger than ∆Cp for the enzyme-transition state complex. Consistent with a classical description of enzyme catalysis (Pauling), a negative value for ∆Cp(‡) is the result of the enzyme binding weakly to the substrate and very tightly to the transition state. This observation of negative ∆Cp(‡) has important implications for the temperature dependence of enzyme-catalysed rates. Here, we lay out the fundamentals of MMRT. We present a number of hypotheses that arise directly from MMRT including a theoretical justification for the large size of enzymes and the basis for their optimum temperatures. We rationalise the behaviour of psychrophilic enzymes and describe a "psychrophilic trap" which places limits on the evolution of enzymes in low temperature environments. One of the defining characteristics of biology is catalysis of chemical reactions by enzymes and enzymes drive much of metabolism. Therefore we also expect to see characteristics of MMRT at the level of cells, whole organisms and even ecosystems.