Structural and Biochemical Characterization of SbnC as a Representative Type B Siderophore Synthetase

Staphyloferrin B
is a key siderophore secreted by Staphylococcus
aureus to acquire ferric ions from a host during infection,
and its biosynthetic pathway has been validated to develop efficient
antibacterial agents. Herein, we report the crystal structure of AMP-bound
SbnC from S. aureus (SaSbnC) as
the first representative structure of type B synthetases in the biosynthesis
of α-hydroxycarboxylate siderophores. While type B synthetases
specifically use α-ketoglutarate (α-KG) as their carboxylic
acid substrate, SaSbnC showed unique structural features
in the substrate pocket compared with the type A and C synthetases.
Screening of α-KG analogues suggested that the hydrogen-bonding
interaction between the α-carbonyl group of α-KG and residue
Lys552 is a key determinant for the substrate selectivity of type
B synthetases. Interestingly, citrate, the product of the tricarboxylic
acid cycle and the substrate of type A synthetases, was found to inhibit
the activity of SaSbnC with an IC50 value
of 83 μM by mimicking α-KG binding, suggesting a potential
regulatory role of the tricarboxylic acid cycle, whose activity is
under the control of the intracellular iron concentration, to SaSbnC and other type B synthetases. These results provide
critical new information to understand the structure, function, and
regulation of type B synthetases in the siderophore-based iron acquisition
system employed by a large number of pathogenic microbes.