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Two different crystal forms of pig pancreatic alpha-amylase isoenzyme II
(PPAII), free and complexed to a carbohydrate inhibitor (acarbose), have been
compared together and to previously reported structures of PPAI. A crystal form
obtained at 4 degrees C, containing nearly 72% solvent, made it possible to
obtain a new complex with acarbose, different from a previous one obtained at 20
degrees C [Qian, M., Buisson, G., Duée, E., Haser, H. & Payan, F. (1994)
Biochemistry 33, 6284-6294]. In the present form, six contiguous subsites of the
enzyme active site are occupied by the carbohydrate ligand; the structural data
indicate that the binding site is capable of holding more than the five glucose
units of the scheme proposed through kinetic studies. A monosaccharide ring
bridging two protein molecules related by the crystal packing is located on the
surface, at a distance of 2.0 nm from the reducing end of the inhibitor ligand;
the symmetry-related glucose ring in the crystal lattice is found 1.5 nm away
from the non-reducing end of the inhibitor ligand.
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