CRYSTAL-STRUCTURE OF PIG PANCREATIC ALPHA-AMYLASE ISOENZYME-II, IN COMPLEX WITH THE CARBOHYDRATE INHIBITOR ACARBOSE

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% so lvent, made it possible to obtain a new complex with acarbose, differe nt from a previous one obtained at 20 degrees C [Qian, M., Buisson, G. , Duee, 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 glucos e units of the scheme proposed through kinetic studies. A monosacchari de 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 e nd of the inhibitor ligand; the symmetry-related glucose ring in the c rystal lattice is found 1.5 nm away from the non-reducing end of the i nhibitor ligand.