α-Amylases (α-1,4-glucan 4-glucanohydrolase, E.C.126.96.36.199.) catalyze the cleavage of α-1,4-glucosidic linkages of starch components, glycogen, and various oligosaccharides. Thermostable α-amylases from Bacillus species are of great industrial importance in the production of corn syrup or dextrose. Thermostable α-amylase from Bacillus licheniformis, a monomeric enzyme with molecular mass of 55,200 Da (483 amino acid residues), shows a remarkable heat stability: its optimum temperature is 90°C and only 10% of activity is lost after heat treatment at 90°C for 30 min. Thus this enzyme provides an attractive model for investigating the structural basis for thermostability of proteins. The three-dimensional structure of thermostable α-amylase from Bacillus licheniformis has been determined by the multiple isomorphous replacement method of X-ray crystallography. The structure has been refined to a crystallographic R-factor of 0.199 for 58,601 independent reflections with Fo>2σ(Fo) between 8.0 and 1.7resolution, with excellent stereochemistry. The final model consists of 468 amino acid residues and 294 water molecules. Missing from the model are the segment between Trp182 and Asn192 and both the N- and C-termini. The polypeptide chain fold into three distinct domains. The first domain (domain A), consisting of 291 residues (from residue 3 to 103 and 207 to 396), forms a (β/α)8-barrel structure. The second domain (domain B), consisting of residues 104 to 206, is inserted between the third β-strand and the third α-helix of domain A. The third C-terminal domain (domain C), consisting of residues 397 to 482, folds into an eight-stranded antiparallel β-barrel.
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