The alkanesulfonate
monooxygenase system from Escherichia coli is involved in scavenging
sulfur from
alkanesulfonates under
sulfur starvation. An
FMN reductase (SsuE) catalyzes the reduction of
FMN by
NADPH, and the reduced
flavin is transferred to the
monooxygenase (SsuD). Rapid reaction kinetic analyses were performed to define the microscopic steps involved in SsuE catalyzed
flavin reduction. Results from single-wavelength analyses at 450 and 550 nm showed that reduction of
FMN occurs in three distinct phases. Following a possible rapid equilibrium binding of
FMN and
NADPH to SsuE (MC-1) that occurs before the first detectable step, an initial fast phase (241 s(-1)) corresponds to the interaction of
NADPH with
FMN (CT-1). The second phase is a slow conversion (11 s(-1)) to form a charge-transfer complex of reduced FMNH(2) with
NADP(+) (CT-2), and represents electron transfer from the
pyridine nucleotide to the
flavin. The third step (19 s(-1)) is the decay of the charge-transfer complex to SsuE with bound products (MC-2) or product release from the CT-2 complex. Results from
isotope studies with [(4R)-(2)H]
NADPH demonstrates a rate-limiting step in electron transfer from
NADPH to
FMN, and may imply a partial rate-limiting step from CT-2 to MC-2 or the direct release of products from CT-2. While the utilization of
flavin as a substrate by the alkanesulfonate
monooxygenase system is novel, the mechanism for
flavin reduction follows an analogous reaction path as standard
flavoproteins.