Over 4000 natural products contain halide atoms such as chlorine, bromine, or
iodine.1 Halogenated natural products are
medically valuable and include antibiotics (chlorotetracycline and vancomycin),
antitumor agents (rebeccamycin and calichemycin), and human thyroid hormone
(thyroxine).2 Halogenation is essential to the
biological activity and chemical reactivity of such compounds, and often
generates versatile molecular building blocks for synthetic organic chemists.
Scientists have wondered about the mechanism by which proteins halogenate
natural products, since the analogous reaction by organic synthesis poses
multiple challenges. In many cases, the moiety to be halogenated is completely
nonreactive, requiring employment of a powerful catalyst.
Researchers at the Massachusetts Institute of Technology used the
macromolecular crystallography facilities at Stanford Synchrotron Radiation
Laboratory to solve the first crystal structure of an iron-dependent halogenase
(Fig 1A). This enzyme, SyrB2, from the plant pathogen Pseudomonas
syringae
catalyses the chlorination of threonine during biosynthesis of the anti-fungal
agent syringomycin E.3 Unlike typical non-heme
Fe(II)/a-ketoglutarate dependent enzymes that
catalyze hydroxylation reactions, SyrB2 is a member of a new subclass of
mononuclear iron enzymes that catalyze halogenation.
Figure 1
Crystal structure of the non-heme iron halogenase SyrB2. A. Overall structure
showing cupin fold. B. Active site iron coordination. Iron (brown) is
coordinated by two histidines, a-ketoglutarate (grey sticks), water (cyan), and
chloride (green).
The structural analysis revealed a novel coordination environment for the
catalytic iron and the presence of a naturally occurring iron-chloride bond.
Blasiak et al used x-ray diffraction data collected at SSRL beamline 9-2
to pinpoint the location of the iron and halogen (Fig 1B). The carboxylate
ligand that usually occupies one of the triangular sides of the octahedral iron
coordination geometry in the hydroxylation reaction is replaced by a chloride
iron. The active site architecture suggests a mechanism by which nature can
harness the catalytic prowess necessary to perform the most chemically
challenging of halogenation reactions. The typical mechanism of hydroxylation
by non-heme iron enzymes involves formation of a high-valent oxoiron species
that abstracts a hydrogen from the substrate.4 The
resulting substrate radical then recombines with OH to give the alcohol
product. The position of the chloride ligand in the SyrB2 active site
presumably allows for rerouting of the substrate radical for recombination with
chlorine radical, yielding the observed chlorinated product. This work
provides an important step forward in understanding the biological synthesis of
medically useful halogenated natural products.
This work was supported by NIH Grants GM 69857 (CLD), GM 49338 (CTW), a
T32-GM08334 Training Grant (LCB), a Merck-sponsored Fellowship of the Helen Hay
Whitney Foundation (FHV) and a Natural Sciences and Engineering Research
Council of Canada Postdoctoral Fellowship (FHV).
Primary Citation
References
Blasiak, L.C., Vaillancourt, F.H., Walsh, C.T., Drennan, C.L. Crystal
structure of the non-haem iron halogenase SyrB2 in syringomycin biosynthesis.
Nature v440 pp.368-371, 2006
SSRL is supported by the Department of Energy, Office of Basic Energy Sciences.
The SSRL Structural Molecular Biology Program is supported by the Department of
Energy, Office of Biological and Environmental Research, and by the National
Institutes of Health, National Center for Research Resources, Biomedical
Technology Program, and the National Institute of General Medical Sciences.
Last
Updated:
23 JUNE 2006
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Catherine L. Drennan, MIT
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