The effect of temperature on enzyme activity: new insights and their implications

Thermal sensitivity of mitochondrial functions in permeabilized muscle fibers from two populations of Drosophila simulans with divergent mitotypes

In ectotherms, the external temperature is experienced by the mitochondria, and the mitochondrial respiration of different genotypes is likely to change as a result. Using high-resolution respirometry with permeabilized fibers (an in situ approach), we tried to identify differences in mitochondrial performance and thermal sensitivity of two Drosophila simulans populations with two different mitochondrial types (siII and siIII) and geographical distributions. Maximal state 3 respiration rates obtained with electrons converging at the Q junction of the electron transport system (ETS) differed between the mitotypes at 24°C. Catalytic capacities were higher in flies harboring siII than in those harboring siIII mitochondrial DNA (2,129 vs. 1,390 pmol O(2)·s(-1)·mg protein(-1)). The cytochrome c oxidase activity was also higher in siII than siIII flies (3,712 vs. 2,688 pmol O(2)·s(-1)·mg protein(-1)). The higher catalytic capacity detected in the siII mitotype could provide an advantage in terms of intensity of aerobic activity, endurance, or both, if the intensity of exercise that can be aerobically performed is partly dictated by the aerobic capacity of the tissue. Moreover, thermal sensitivity results showed that even if temperature affects the catalytic capacity of the different enzymes of the ETS, both mitotypes revealed high tolerance to temperature variation. Previous in vitro study failed to detect any consistent functional mitochondrial differences between the same mitotypes. We conclude that the in situ approach is more sensitive and that the ETS is a robust system in terms of functional and regulatory properties across a wide range of temperatures.

Protein dynamics and enzyme catalysis: insights from simulations

The role of protein dynamics in enzyme catalysis is one of the most active and controversial areas in enzymology today. Some researchers claim that protein dynamics are at the heart of enzyme catalytic efficiency, while others state that dynamics make no significant contribution to catalysis. What is the biochemist - or student - to make of the ferocious arguments in this area? Protein dynamics are complex and fascinating, as molecular dynamics simulations and experiments have shown. The essential question is: do these complex motions have functional significance? In particular, how do they affect or relate to chemical reactions within enzymes, and how are chemical and conformational changes coupled together? Biomolecular simulations can analyse enzyme reactions and dynamics in atomic detail, beyond that achievable in experiments: accurate atomistic modelling has an essential part to play in clarifying these issues. This article is part of a Special Issue entitled: Protein Dynamics: Experimental and Computational Approaches.

The molecular basis of the effect of temperature on enzyme activity

Experimental data show that the effect of temperature on enzymes cannot be adequately explained in terms of a two-state model based on increases in activity and denaturation. The Equilibrium Model provides a quantitative explanation of enzyme thermal behaviour under reaction conditions by introducing an inactive (but not denatured) intermediate in rapid equilibrium with the active form. The temperature midpoint (Teq) of the rapid equilibration between the two forms is related to the growth temperature of the organism, and the enthalpy of the equilibrium (ΔHeq) to its ability to function over various temperature ranges. In the present study, we show that the difference between the active and inactive forms is at the enzyme active site. The results reveal an apparently universal mechanism, independent of enzyme reaction or structure, based at or near the active site, by which enzymes lose activity as temperature rises, as opposed to denaturation which is global. Results show that activity losses below Teq may lead to significant errors in the determination of ΔG*cat made on the basis of the two-state (‘Classical’) model, and the measured kcat will then not be a true indication of an enzyme's catalytic power. Overall, the results provide a molecular rationale for observations that the active site tends to be more flexible than the enzyme as a whole, and that activity losses precede denaturation, and provide a general explanation in molecular terms for the effect of temperature on enzyme activity.

Deactivation of TEM-1 β-Lactamase Investigated by Isothermal Batch and Non-Isothermal Continuous Enzyme Membrane Reactor Methods

The thermal deactivation of TEM-1 β-lactamase was examined using two experimental techniques: a series of isothermal batch assays and a single, continuous, non-isothermal assay in an enzyme membrane reactor (EMR). The isothermal batch-mode technique was coupled with the three-state "Equilibrium Model" of enzyme deactivation, while the results of the EMR experiment were fitted to a four-state "molten globule model". The two methods both led to the conclusions that the thermal deactivation of TEM-1 β-lactamase does not follow the Lumry-Eyring model and that the T(eq) of the enzyme (the point at which active and inactive states are present in equal amounts due to thermodynamic equilibrium) is at least 10 °C from the T(m) (melting temperature), contrary to the idea that the true temperature optimum of a biocatalyst is necessarily close to the melting temperature.

Improving low-temperature activity of Sulfolobus acidocaldarius 2-keto-3-deoxygluconate aldolase

Sulfolobus acidocaldarius 2-keto-3-deoxygluconate aldolase (SacKdgA) displays optimal activity at 95 degrees C and is studied as a model enzyme for aldol condensation reactions. For application of SacKdgA at lower temperatures, a library of randomly generated mutants was screened for improved synthesis of 2-keto-3-deoxygluconate from pyruvate and glyceraldehyde at the suboptimal temperature of 50 degrees C. The single mutant SacKdgA-V193A displayed a threefold increase in activity compared with wild type SacKdgA. The increased specific activity at 40-60 degrees C of this mutant was observed, not only for the condensation of pyruvate with glyceraldehyde, but also for several unnatural acceptor aldehydes. The optimal temperature for activity of SacKdgA-V193A was lower than for the wild type enzyme, but enzymatic stability of the mutant was similar to that of the wild type, indicating that activity and stability were uncoupled. Valine193 has Van der Waals interactions with Lysine153, which covalently binds the substrate during catalysis. The mutation V193A introduced space close to this essential residue, and the increased activity of the mutant presumably resulted from increased flexibility of Lysine153. The increased activity of SacKdgA-V193A with unaffected stability demonstrates the potential for optimizing extremely thermostable aldolases for synthesis reactions at moderate temperatures.