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Glycoside hydrolases (GHs) are broadly classified into 2 classes, retaining and inverting enzymes, on the basis of changes in their anomeric configurations during the reactions. The detailed mechanism of action of these enzymes has been elucidated, and the active sites of both classes of enzyme contain 2 acidic residues (Figure 1). In retaining GH, 1 residue acts as a nucleophile and the other, as an acid/base catalyst. The hydrolysis reaction proceeds by a double-displacement mechanism in which a covalent glycosyl-enzyme intermediate is formed. Normal hydrolysis occurs when this intermediate reacts with water in a general acid/base catalyzed process. The active-site structure is arranged as (acid/base)–substrate– (nucleophile), and the 2 catalytic residues are located about 5.5 Å apart. The inverting reaction mechanism involves 2 catalytic residues, a general acid and a general base catalyst. The reaction occurs by the attack of nucleophilic water molecules activated by the general base, and this reaction is aided by the general acid. The active-site structure is arranged as (general acid) – substrate – nucleophilic water – (general base), and the 2 catalytic residues are located 6–11 Å apart. A vast number of GHs are classified into GH families on the basis of similarities in the amino acid sequences. Currently, 115 distinct GH families have been classified on the basis of their sequence (August, 2009). The classification, detailed substrate specificities, reaction mechanism, and historical background of each family are summarized and 立体構造から見えた反転型と保持型加水分解酵素の意外に近い関係

Unexpected Close Structural Relationships between Inverting and Retaining Glycoside Hydrolases