New Roles for Oxygen-Activating Diiron Enzymes in Antibiotic Biosynthesis
Anna Jo Komora*, Thomas M. Makrisb, Brent Rivardb, Lawrence Que, Jr.a, John D. Lipscombb
a
b
Department of Chemistry, University of Minnesota, Minneapolis, MN, 55455
Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota,
Minneapolis, MN, 55455
The emerging roles of oxygen-activating diiron enzymes in antibiotic biosynthesis are
exemplified by two enzymes involved in the biosynthesis of the broad-spectrum antibiotic
chloramphenicol: the β-hydroxylase CmlA that works in concert with the nonribosomal peptide
synthetase (NRPS) CmlP,1 and the NH2-oxygenase CmlI that catalyzes the conversion of an
arylamine precursor to the final arylnitro group-containing choloramphenicol.2 The diiron active
site of CmlA is bound in a metallo-β-lactamase protein fold unusual to diiron enzymes and
ligated by three histidine and three carboxylate residues, as revealed by a recent 2.17 Å
resolution crystal structure.3 Although no crystal structure is available for CmlI, comparison to
an enzyme with a homologous sequence suggests a three-histidine four-carboxylate protein
architecture.4 These ligation strategies are in contrast to the two-histidine four-carboxylate
protein architecture common to canonical diiron enzymes such as methane monooxygenase and
ribonucleotide reductase. In addition to structural novelty, both CmlA and CmlI have unusual
reactivities. CmlA is the first known diiron catalyst involved in amino acid β-hydroxylation on
NRPS biosynthetic pathways, a reaction typically associated with α-ketoglutarate-dependent
nonheme monoiron enzymes or cytochrome P450 enzymes. CmlI is one of only three known
enzymes that performs aromatic n-oxygenation, and thus is of interest to both biochemists and
synthetic chemists. Kinetic and spectroscopic studies that address both structure and mechanism
of these enzymes will be discussed.
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