Steady-state kinetic mechanism of LodA, a novel cysteine tryptophylquinone-dependent oxidase

LodA is a novel lysine-ε-oxidase which possesses a cysteine tryptophylquinone cofactor. It is the first tryptophylquinone enzyme known to function as an oxidase. A steady-state kinetic analysis shows that LodA obeys a ping-pong kinetic mechanism with values of kcat of 0.22 ± 0.04 s−1, Klysine of 3.2 ± 0.5 μM and KO2 of 37.2 ± 6.1 μM. The kcat exhibited a pH optimum at 7.5 while kcat/Klysine peaked at 7.0 and remained constant to pH 8.5. Alternative electron acceptors could not effectively substitute for O2 in the reaction. A mechanism for the reductive half reaction of LodA is proposed that is consistent with the ping-pong kinetics.