SyrB2

SyrB2 is a non-heme iron(II)-dependent halogenase encoded by the syrB2 gene in the syringomycin biosynthetic gene cluster of Pseudomonas syringae. It functions together with the carrier protein SyrB1 to chlorinate a specific carbon on a threonine residue tethered to SyrB1 during the assembly of the antibiotic syringomycin E. The enzyme introduces a chlorine atom at the gamma carbon of the threonine side chain, producing 4-chloro-L-threonine as the substrate is delivered to downstream nonribosomal peptide synthetase enzymes.

Mechanism and substrate: SyrB2 belongs to the family of Fe(II)/α-ketoglutarate-dependent halogenases. It uses Fe(II), α-ketoglutarate, and chloride to perform a halogenation reaction on a substrate that is covalently linked to the carrier protein SyrB1. Activation by O2 leads to a high-valent iron-oxo species that abstracts a hydrogen from the target carbon; rebound of chloride yields the chlorinated product and regenerates the Fe(II) state. The tethered, site-specific substrate delivery is crucial for achieving selective halogenation.

Structure and classification: Structural studies place SyrB2 within the characteristic halogenase fold, featuring the 2-His-1-carboxylate facial triad that coordinates Fe(II). The active site and substrate-binding arrangement position the gamma carbon of the tethered threonine for selective C–H activation and chlorination, distinguishing SyrB2 from many hydroxylases.

Significance: SyrB2 provided key insights into enzyme-catalyzed C–H chlorination and the use of carrier-protein–tethered substrates to achieve site-specific halogenation. Its characterization has informed broader understanding of Fe(II)/α-ketoglutarate–dependent halogenases and their potential for biocatalytic applications in natural product biosynthesis.