Abstract
Thermal unfolding of Pseudomonas cepacia lipase (PCL) was studied by differential scanning calorimetry (DSC) at pH 7. The peak temperature tp of the DSC trace increased with increasing concentration of the protein. The DSC traces could be successfully analyzed on the basis of the following mechanism, assuming the dissociation of a calcium ion upon denaturation; N Ca2+_??_D+Ca2+, where N and D represent native and denatured states of PCL, respectively. In the presence of 1-5% alcohols (methanol, ethanol, n-propanol, and n-butanol), tp decreased with increasing alcohol concentration and longer alkyl chain. In contrast to the case of tp, the denaturation enthalpy dh did not depend on the protein concentration or alcohol concentration used. The change in heat capacity on denaturation, Δcpd, evaluated directly from the DSC traces, was close to zero both in the absence or presence of alcohol, which could be due to the open conformation of the enzyme exposing a large hydrophobic surface to the solvent.
