Mycobacterium smegmatis G (MbsG) is a
flavin-dependent
monooxygenase that catalyzes the
NAD(P)H- and
oxygen-dependent hydroxylation of the terminal amino group on the side chain of
l-lysine in the biosynthetic pathway of the
siderophore mycobactin.
Mycobactins are essential for mycobacterium growth under
iron-limiting conditions encountered during
infection in mammals. Thus,
enzymes involved in the biosynthesis of
mycobactin represent potential
drug targets. MbsG was expressed in Escherichia coli and purified using
metal affinity and ionic exchange chromatographies. Recombinant MbsG represents the first member of this class of
enzymes isolated in the active form, with a tightly bound
FAD cofactor. The k(cat) value for formation of hydroxylated
l-lysine under steady-state conditions was 5.0 min(-1), and K(m) values of 0.21 mM for
l-lysine, 1.1 mM for
NADH, and 2.4 mM for
NADPH were calculated. The
enzyme functioned as an
oxidase when the activity of MbsG was measured by monitoring oxygen consumption in the absence of
l-lysine, oxidizing
NADH and
NADPH with k(cat) values of 59 and 49 min(-1), respectively. Under these conditions, MbsG produced both
hydrogen peroxide and
superoxide. In contrast, when
l-lysine was present, the reaction became more coupled, producing hydroxylated
l-lysine and decreasing the
oxidase activity. These results suggest that substrate binding modulates the function of MbsG from an
oxidase to a
monooxygenase.