Structure and function of cereal and related higher plant (1 → 4)-β-xylan endohydrolases

(1-->4)-beta-Xylan endohydrolases (EC 3.2.1.8) are key enzymes in the metabolism of heteroxylans, which are particularly abundant polymers in the cell wall matrix of cereals. They hydrolyse the internal (1-->4)-beta-xylosidic linkages of the xylan backbone of arabinoxylan, and together with other endo- and exohydrolases contribute to the degradation and re-modelling of plant cell walls. Their catalytic mechanism and substrate binding properties are largely deduced from (1-->4)-beta-xylan endohydrolases of fungal and saprophytic bacterial origin, since relatively few of plant origin have been studied. Recent advances in our understanding of the structure-function properties of a barley (1-->4)-beta-xylan endohydrolase has prompted us to re-examine the sequence data of four related cereal enzymes and to assemble a comprehensive comparison of their primary structure and predicted functional domains. The common structural motifs in these cereal enzymes are further compared with those of I I related (1-->4)-beta-xylan endohydrolases, whose sequence was deduced from the Arabidopsis genome. Our structural analyses predict a role for several conserved residues and domains in either the carbohydrate-binding, catalytic activity, or sub-cellular localisation of these plant enzymes, whose future investigation could enhance our understanding of cell wall metabolism. (C) 2002 Elsevier Science Ltd. All rights reserved.