Comparative thermal denaturation of Thermus aquaticus and Escherichia coli type 1 DNA polymerases

Thermal denaturations of the type 1 DNA polymerases from Thermus aquaticus (Taq polymerase) and Escherichia coli (Pol 1) have been examined using differential scanning calorimetry and CD spectroscopy. The full-length proteins are single-polypeptide chains comprising a polymerase domain, a proofreading domain (inactive in Taq) and a 5' nuclease domain. Removal of the 5' nuclease domains produces the 'large fragment' domains of Pol 1 and Taq, termed Klenow and Klentaq respectively. Although the high temperature stability of Taq polymerase is well known, its thermal denaturation has never been directly examined previously. Thermal denaturations of both species of polymerase are irreversible, precluding rigorous thermodynamic analysis. However, the comparative melting behaviour of the polymerases yields information regarding domain structure, domain interactions and also the similarities and differences in the stabilizing forces for the two species of polymerase. In differential scanning calorimetry, Klenow and Klentaq denature as single peaks, with a melting temperature T(m) of 37 and 100 degrees C respectively at pH 9.5. Both full-length polymerases are found to be comprised of two thermodynamic unfolding domains with the 5' nuclease domains of each melting separately. The 5' nuclease domain of Taq denatures as a separate peak, 10 degrees C before the Klentaq domain. Melting of the 5' nuclease domain of Pol 1 overlaps with the Klenow fragment. Presence of the 5' nuclease domain stabilizes the large fragment in Pol 1, but destabilizes it in Taq. Both Klentaq and Klenow denaturations have a very similar dependence on pH and methanol, indicating similarities in the hydrophobic forces and protonation effects stabilizing the proteins. Melting monitored by CD yields slightly lower T(m) values, but almost identical van't Hoff enthalpy Delta H values, consistent with two-state unfolding followed by an irreversible kinetic step. Analysis of the denaturation scan rate dependences with Arrhenius formalism estimates a kinetic barrier to irreversible denaturation for Klentaq that is significantly higher than that for Klenow.