N-terminal catalytic domain of Gh57 branching enzyme TK 1436 and similar proteins
The subfamily is represented by a novel branching-enzyme TK1436 of hyperthermophilic archaeon Thermococcus kodakaraensis KOD1. Branching enzymes (BEs, EC 2.4.1.18) play a key role in synthesis of alpha-glucans and they generally are classified into glycoside hydrolase family 13 (GH13). However, TK1436 belongs to the GH57 family. It functions as a monomer and possesses BE activity. TK1436 is composed of a distorted N-terminal (beta/alpha)7-barrel domain and a C-terminal five alpha-helical domain, both of which participate in the formation of the active-site cleft.
Structure:3N98; Thermococcus kodakaraensis branching-enzyme TK1436 binds two glucose and two glycerol molecules in the substrate binding site, contacts at 4A.
Comment:Key active site residues include two acidic catalytic residues, Glu and Asp, an N-terminal polarizer His, four aromatic Trp gate-keepers, and a key Tyr residue. This Tyr residue inserts its hydroxyl group in the crevice formed by the catalytic residues; and may stabilize the intermediate reaction product for subsequent transfer to a new alpha-1,6 position in amylopectin chains.
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