Thermostability of native and pegylated Myceliophthora thermophila laccase in aqueous and mixed solvents

A commercial preparation of laccase (EC 1.10.3.2), cloned from Myceliophthora thermophila and expressed in Aspergillus oryzae (MtL), was purified and modified by conjugation with poly(ethylene glycol) (M-r = 5000) and is labeled PEG-MtL. Native enzyme was found to have a molecular mass of 80 kDa, as determined by gel filtration, and 110 kDa, by SDS-PAGE. The oxidative dimerization of 2,6-dimethoxyphenol (DMP) to produce the corresponding dibenzoquinone was catalyzed by MtL in a manner comparable to that for a diffusion-controlled reaction (k(cat)/K-M congruent to 10(8) M-1 s(-1) and E-a congruent to 18 kJ M-1). PEG-MtL was found, by TNBS titration, to have blocked 54% of lysine groups; its hydrodynamic and charge properties were different from those of MtL. Catalytic efficiency (k(cat)/K-M) of PEG-MtL was similar to that of MtL with DMP as substrate; however, k(cat)/K-M was 2-fold reduced for the reaction in which 2', 2-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) is oxidized to form a radical cation. E-a values were similar in both enzyme preparations when assayed in buffered solutions. Far-UV CD spectra were similar for MtL and PEG-MtL and consistent with a protein rich in beta-sheet structure with negligible content of alpha-helices. A blue shift of near-UV CD spectrum for PEG-MtL as compared to MtL was consistent with the decreased polarity of the tyrosyl side chains upon PEG conjugation. Also the blue band of the copper active site was shifted from lambda similar to 610 nm (MtL) to lambda similar to 575 nm (PEG-MtL). Scanning microcalorimetry showed small denaturation enthalpies (6.3 and 7.5 J g(-1) for MtL and PEG-MtL, respectively), indicating the high stability of the beta-sheet folding pattern of laccases. However, PEG-MtL proved to be more stable, its half-denaturation temperature being 2 degrees C higher than that of MtL. In 30% alcohol, pegylated laccase showed slower enzyme-activity decay rates than the unmodified enzyme; this behavior was caused by a decrease in the activation entropy of the denaturation reaction. Results can be explained by entropic stabilization by PEG conjugation because of the restricted motion of some surface amino acid side chains, which results in a more stable active site.