The stability of Taq DNA polymerase results from a reduced entropic folding penalty; identification of other thermophilic proteins with similar folding thermodynamics
Abstract
The thermal stability of Taq DNA polymerase is well known, and is the basis for its use in PCR. A comparative thermodynamic characterization of the large fragment domains of Taq (Klentaq) and E. coli (Klenow) DNA polymerases has been performed by obtaining full Gibbs-Helmholtz stability curves of the free energy of folding (ΔG) versus temperature. This analysis provides the temperature dependencies of the folding enthalpy and entropy (ΔH and ΔS), and the heat capacity (ΔCp) of folding. If increased or enhanced non-covalent bonding in the native state is responsible for enhanced thermal stabilization of a protein, as is often proposed, then an enhanced favourable folding enthalpy should, in general, be observed for thermophilic proteins. However, for the Klenow-Klentaq homologous pair, the folding enthalpy (ΔHfold) of Klentaq is considerably less favorable than that of Klenow at all temperatures. In contrast, it is found that Klentaq's extreme free energy of folding (ΔGfold) originates from a significantly reduced entropic penalty of folding (ΔSfold). Furthermore, the heat capacity changes upon folding are similar for Klenow and Klentaq. Along with this new data, comparable extended analysis of available thermodynamic data for 17 other mesophilic-thermophilic protein pairs (where enough applicable thermodynamic data exists) shows a similar pattern in seven of the 18 total systems. When analyzed with this approach, the more familiar "reduced ΔCp mechanism" for protein thermal stabilization (observed in a different six of the 18 systems) frequently manifests as a temperature dependent shift from enthalpy driven stabilization to a reduced-entropic-penalty model. © 2013 Wiley Periodicals, Inc.