Dioxygen reactivity of laccase: dependence on laccase source, pH, and anion inhibition

A study was carried out on the dioxygen reactivity of the laccases from Trametes villosa, Rhizoctonia solani, Myceliophthora thermophila, Scytalidium thermophilum, and Coprinus cinereus. At pH 5.5, these laccases showed an apparently constant Km (about 20-50 microM) for O2 with either 2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) or methyl syringate as the reducing substrate, in contrast to the kcat, which varied up to 100-fold. O2 reactivity of the Trametes and Myceliophthora laccases was also studied at various pH and NaF concentrations. The apparent Km of Trametes and Myceliophthora laccases varied only slightly when pH changed from 3.0 to 8.0 or when the laccases were inhibited by F- at pH 5.5, although the apparent kcat were more significantly affected by both factors. The dependence of the apparent Km on the source of laccase, pH, and F- inhibition suggested that the fungal laccases might have a conserved O2 pocket and that the F- or OH- inhibition might affect the O2 reduction step (kcat) more than the O2 binding step (Km) under steady-state conditions.