|
The crystal structure of beta-amylase from Bacillus cereus var. mycoides was
determined by the multiple isomorphous replacement method. The structure was
refined to a final R-factor of 0.186 for 102,807 independent reflections with
F/sigma(F) > or = 2.0 at 2.2 A resolution with root-mean-square deviations
from ideality in bond lengths, and bond angles of 0.014 A and 3.00 degrees,
respectively. The asymmetric unit comprises four molecules exhibiting a
dimer-of-dimers structure. The enzyme, however, acts as a monomer in solution.
The beta-amylase molecule folds into three domains; the first one is the
N-terminal catalytic domain with a (beta/alpha)8 barrel, the second one is the
excursion part from the first one, and the third one is the C-terminal domain
with two almost anti-parallel beta-sheets. The active site cleft, including two
putative catalytic residues (Glu172 and Glu367), is located on the carboxyl side
of the central beta-sheet in the (beta/alpha)8 barrel, as in most amylases. The
active site structure of the enzyme resembles that of soybean beta-amylase with
slight differences. One calcium ion is bound per molecule far from the active
site. The C-terminal domain has a fold similar to the raw starch binding domains
of cyclodextrin glycosyltransferase and glucoamylase.
|
