Crystal structure of the pig pancreatic alpha-amylase complexed with malto-oligosaccharides.

The structural X-ray map of a pig pancreatic alpha-amylase crystal soaked (and flash-frozen) with a maltopentaose substrate showed a pattern of electron density corresponding to the binding of oligosaccharides at the active site and at three surface binding sites. The electron density region observed at the active site, filling subsites-3 through-1, was interpreted in terms of the process of enzyme-catalyzed hydrolysis undergone by maltopentaose. Because the expected conformational changes in the "flexible loop" that constitutes the surface edge of the active site were not observed, the movement of the loop may depend on aglycone site being filled. The crystal structure was refined at 2.01 A resolution to an R factor of 17.0% ( R(free) factor of 19.8%). The final model consists of 3910 protein atoms, one calcium ion, two chloride ions, 103 oligosaccharide atoms, 761 atoms of water molecules, and 23 ethylene glycol atoms.