Structure of an aliphatic amidase from Geobacillus pallidus RAPc8
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Date
2007
Journal Title
Journal ISSN
Volume Title
Publisher
International Union of Crystallography
Abstract
The amidase from Geobacillus pallidus RAPc8, a moderate
thermophile, is a member of the nitrilase superfamily and
catalyzes the conversion of amides to the corresponding
carboxylic acids and ammonia. It shows both amide-hydrolysis
and acyl-transfer activities and also exhibits stereoselectivity
for some enantiomeric substrates, thus making it a potentially
important industrial catalyst. The crystal structure of
G. pallidus RAPc8 amidase at a resolution of 1.9 A ˚ was
solved by molecular replacement from a crystal belonging to
the primitive cubic space group P4232. G. pallidus RAPc8
amidase is homohexameric in solution and its monomers have
the typical nitrilase-superfamily α-β-β-α fold. Association in
the hexamer preserves the eight-layered α-β-β-α:α-β-β-α
structure across an interface which is conserved in the known
members of the superfamily. The extended carboxy-terminal
tail contributes to this conserved interface by interlocking the
monomers. Analysis of the small active site of the G. pallidus
RAPc8 amidase suggests that access of a water molecule to the
catalytic triad (Cys, Glu, Lys) side chains would be impeded by
the formation of the acyl intermediate. It is proposed that
another active-site residue, Glu142, the position of which is
conserved in the homologues, acts as a general base to catalyse
the hydrolysis of this intermediate. The small size of the
substrate-binding pocket also explains the specificity of this
enzyme for short aliphatic amides and its asymmetry explains
its enantioselectivity.
Description
Keywords
Geobacillus pallidus, Amidase, Industrial catalyst, Crystal structure, Hydrolase family
Citation
Kimani, S.W., et al.(2007). The crystal structure of an aliphatic amidase from Geobacillus pallidus RAPc8: evidence for a fourth nitrilase catalytic residue. Acta Crystallographica. Section D: Biological Crystallography, 63:1048-1058.