Structural and functional properties of low molecular weight endo-1,4-beta-xylanases

There are currently four crystal structures of low molecular weight endo-1,4-beta-xylanases (E.C.3.2.1.8), i.e. family G/11 xylanases, available at the Brookhaven Data Bank: 2 xylanases from Trichoderma reesei (Törrönen et al., 1994; Törrönen and Rouvinen, 1995) and one from Bacillus circulans and another from Trichoderma harzianum (Campbell et al., 1993). They consist of two beta-sheets and one alpha-helix and have been described to resemble a partly-closed right hand. The catalytic residues are two conserved glutamate residues, which are located opposite to each other in an open active site cleft. The catalytic mechanism is thought to resemble that of the widely-studied enzyme lysozyme. The role of one glutamate is to act as an acid/base catalyst whereas the other is a nucleophile and stabilizes the reaction intermediate. Complex structures of partly-bound xylotetraose in mutated XYN from Bacillus circulans (Wakarchuck et al., 1994a) and three recently-obtained structures of XYNII from Trichoderma reesei with epoxyalkyl-xylose derivatives (Havukainen et al., 1996) have provided important information on substrate binding. Family G/11 xylanases show clear amino acid homology and thus have a common fold. However, variations in their functional properties, such as catalytic activity, substrate cleaving patterns, pH optima and thermostabilities, exist.