The pyridoxal 5*-phosphate (PLP)-dependent enzyme
1-aminocyclopropane-1-carboxylate deaminase (ACCD)
catalyzes a reaction that involves a ring opening of cyclopropanoid
amino acid, yielding a-ketobutyrate and
ammonia. Unlike other PLP-dependent enzymes, this
enzyme has no a-hydrogen atom in the substrate. Thus,
a unique mechanism for the bond cleavage is expected.
The crystal structure of ACCD from Hansenula saturnus
has been determined at 2.0 Å resolution by the multiple
wavelength anomalous diffraction method using mercury
atoms as anomalous scatterers. The model was
built on the electron density map, which was obtained
by the density averaging of multiple crystal forms. The
final model was refined to an R-factor of 22.5% and an
Rfree-factor of 26.8%. The ACCD folds into two domains,
each of which has an open twisted a/b structure similar
to the b-subunit of tryptophan synthase. However, in
ACCD, unlike in other members of the b family of PLPdependent
enzymes, PLP is buried deep in the molecule.
The structure provides the first view of the catalytic
center of the cyclopropane ring opening.