Alpha-amylase structure and activity.

Two computerized methods of predicting protein secondary structure from amino acid sequences are evaluated by using them on the alpha-amylase of Aspergillus oryzae, for which the three-dimensional structure has been determined. The methods are then used, with amino acid alignments, to predict the structures of other alpha-amylases. It is found that all alpha-amylases of known amino acid sequence have the same basic structure, a barrel of eight parallel stretches of extended chain surrounded by eight helices. Strong similarities are found in those areas of the proteins believed to bind an essential calcium ion and at that part of the active site that catalyzes bond hydrolysis in the substrates. The active site, as a whole, is formed mainly of amino acids situated on loops joining extended chain to the adjacent helix. Variations in the length and amino acid sequence of these loops, from one alpha-amylase to another, provide the differences in binding the substrates believed to account for the known variations in action pattern of alpha-amylases of different biological origins.