Kinetic and thermodynamic studies on T. thermophilus laccase catalysis

Abstract

Multicopper oxidases (MCOs) are lignin degrading enzymes with four copper sites involved in their catalysis. The consensus mechanism suggests that substrate oxidn. occurs near a type 1 Cu center (CuT1), followed by long-range electron transfer to a trinuclear Cu center (TNC) where O₂ is reduced to H₂O. Although laccases are capable of aerobic oxidn. of lignin as their primary function, the potential of CuT1 (E°(Cu²⁺/⁺) ≈0.5 V vs. NHE, pH 5.5) is as much as 0.5 V lower than that expected for one-electron oxidn. of polyphenolic substrates. Investigations on redox-active residues crit. for enzyme catalysis as well as reactive intermediates are crucial for elucidating this potential discrepancy and the mechanistic details of substrate oxidn. Our study focused on the MCO from a thermophilic bacterium, Thermus thermophilus HB27 (T. thermophilus Laccase) in particular (Figure 1), which is stable and active even above 90°C. Thus, thermodn. parameters assocd. with the enzyme's thermostability and improved activity at elevated temps. are of special importance. Approaches to enhancing our understanding of how the enzyme has evolved to utilize O₂ to degrade lignin from recalcitrant lignocellulosic substrates will be useful for developing environmentally friendly biocatalysts.