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The orf6 gene from the clavulanic acid biosynthesis gene cluster encodes an OAT
(ornithine acetyltransferase). Similar to other OATs the enzyme has been shown
to catalyse the reversible transfer of an acetyl group from N-acetylornithine to
glutamate. OATs are Ntn (N-terminal nucleophile) enzymes, but are distinct from
the better-characterized Ntn hydrolase enzymes as they catalyse acetyl transfer
rather than a hydrolysis reaction. In the present study, we describe the X-ray
crystal structure of the OAT, corresponding to the orf6 gene product, to 2.8 A
(1 A=0.1 nm) resolution. The larger domain of the structure consists of an
alphabetabetaalpha sandwich as in the structures of Ntn hydrolase enzymes.
However, differences in the connectivity reveal that OATs belong to a structural
family different from that of other structurally characterized Ntn enzymes, with
one exception: unexpectedly, the alphabetabetaalpha sandwich of ORF6 (where ORF
stands for open reading frame) displays the same fold as an DmpA
(L-aminopeptidase D-ala-esterase/amidase from Ochrobactrum anthropi), and so the
OATs and DmpA form a new structural subfamily of Ntn enzymes. The structure
reveals an alpha2beta2-heterotetrameric oligomerization state in which the
intermolecular interface partly defines the active site. Models of the
enzyme-substrate complexes suggest a probable oxyanion stabilization mechanism
as well as providing insight into how the enzyme binds its two differently
charged substrates.
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