Structural Determinants of Flavin Dynamics in a Class B Monooxygenase

Area of Science:

• Biochemistry
• Enzymology
• Structural Biology

Background:

• SidA, an ornithine hydroxylase from Aspergillus fumigatus, is crucial for synthesizing hydroxamate siderophores.
• Class B flavin-dependent monooxygenases exhibit significant conformational changes in their flavin adenine dinucleotide (FAD) cofactor during catalysis.
• Residue Met101 in SidA interacts with the FAD cofactor in its 'in' state.

Purpose of the Study:

• To investigate the role of Met101 in the conformational dynamics of SidA.
• To understand the functional significance of FAD motion in class B flavin monooxygenases.
• To elucidate the catalytic mechanism and identify rate-limiting steps affected by Met101 mutation.

Main Methods:

• Site-directed mutagenesis (M101A variant creation).
• Steady-state and pre-steady-state enzyme kinetics.
• pH profiles and solvent kinetic isotope effect studies.
• X-ray crystallography (resting enzyme and NADP+ complex structures).

Main Results:

• The M101A mutant exhibited a 25-fold decrease in turnover number compared to wild-type SidA.
• Kinetic analyses indicated a bottleneck in the final catalytic step, involving flavin dehydration and product release.
• Crystal structures revealed that the M101A mutant can adopt an 'out' FAD conformation even when bound to NADP+, suggesting a stalled state.

Conclusions:

• Met101 plays a critical role in facilitating the release of NADP+ and hydroxy-l-ornithine.
• The conformational change of FAD from 'in' to 'out' is essential for efficient cofactor ejection and enzyme turnover.
• This study supports the hypothesis that FAD motion in class B flavin monooxygenases aids in NADP+ release for subsequent catalytic cycles.