Laccase-catalyzed oxidation of syringates in presence of albumins

Laccases-catalyzed oxidation of methyl syringate (MS) was investigated at pH 6.0 and 25 °C. The laccases used were recombinant Polyporus pinsitus laccase (rPpL) and recombinant Myceliophthora thermophila laccase (rMtL). The apparent Michaelis–Menten constant was 1.1 mM for rPpL and 3.6 mM for rMtL in air-saturated buffer solution. The bimolecular constants calculated as kox=kcat/Km were 10.6 and 6.1 (mM s)−1, respectively. During MS oxidation an inactivation of rMtL was indicated showing limited turnover capacity (TC) of the laccase. Human serum albumin (HSA) and bovine serum albumin (BSA) prevent laccase inactivation. Spectral measurements revealed MS complexation with albumins with dissociation constant 15.2 μM and 18.7 μM for BSA and HSA, respectively. Docking calculations showed that MS might complex within hydrophobic binding sites of albumins. The radical of MS formed during oxidation also binds within the same sites. The hypothesis was made that albumin is capable of trapping the radical preventing laccase inactivation.