Pioneering Studies on the Structure-Function Relationships of the Enzymes of .ALPHA.-Amylase Family

The crystal structure study of Taka-amylase (TAA) was started in early 1970th in the Institute for Protein Research of Osaka University. TAA has been extensively studied centered on this Institute by the group of Prof. Akabori. The elucidation of the three-dimensional structure of this enzyme has been the final goal of these studies. The first structure of TAA was published in 1979 at low resolution. The backbone model was published in 1980, and the complete molecular model at 3 Å resolution in 1984, with reference to the amino acid sequence determined by Prof. Narita’s group in 1982. Since then, the author and his group have determined the structures of six different enzymes of the α-amylase family: Taka-amylase, cyclodextrin glucosyltransferase, maltotetraose-forming amylase, isoamylase, trehalose synthase and neopullulanase. The three-dimensional structures of these α-amylase family enzymes are briefly reviewed focusing on their substrate specificities. Through these studies, it was shown that a conserved network structure composed of amino acids and a water exists at the active site of the enzymes of this family. The roles of the three essential catalytic residues are discussed based on the substrate-complexed structures of mutant enzymes. The role of Asp 297 (in TAA), particularly, which has not been clarified before, is discussed and shown to work crucially for evoking a distortion on the glucose ring at subsite -1, which leads to the cleavage of glucosidic bond. A reaction scheme involving the three essential catalytic residues is presented. Furthermore, the characteristic binding mode of the substrate amylose with respect to the (α/β)8-barrel in the family enzymes is addressed.