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Three‐dimensional structure of (1,4)‐β‐d‐mannan mannanohydrolase from tomato fruit

The three‐dimensional crystal structure of tomato (Lycopersicon esculentum) β‐mannanase 4a (LeMAN4a) has been determined to 1.5 Å resolution. The enzyme adopts the (β/α)8 fold common to the members of glycohydrolase family GH5. The structure is comparable with those of the homologous Trichoderma reesei and Thermomonospora fusca β‐mannanases: There is a conserved three‐stranded β‐sheet located near the N terminus that stacks against the central β‐barrel at the end opposite the active site. Three noncanonical β‐helices surround the active site. Similar helices are found in T. reesei but not T. fusca β‐mannanase. By analogy with other β‐mannanases, the catalytic acid/base residue is E204 and the nucleophile residue is E318. The active site cleft of L. esculentum β‐mannanase most closely resembles that of the T. reesei isozyme. A model of substrate binding in LeMAN4a is proposed in which the mannosyl residue occupying the −1 subsite of the enzyme adopts the 1S5 skew‐boat conformation.