Xylans main structural features

Some of the main structural features of the xylans isolated from the Gramineae are outlined in Table I. 

The detailed molecular structure of the hemicelluloses varies from species to species and even from part to part of the same plant, but a common feature is that the main chain is decorated with side groups, some of which can contain a carboxylic acid group. The decoration stops the hemicellulose molecules aggregating undecorated xylan or mannan are nearly as insoluble as cellulose. 

CE Family 1 is very large and contains members which do not act on carbohydrate-derived substrates. The crystal structure of a CE 1 domain of XynlOB modular enzyme from Clostridium thermocellum has been solved. " The CE 1 domain is a feruloyl esterase which hydrolyses the feruloyl groups attached to some arabinofuranosyl 05 groups in native xylan. (The Xyn lOB protein as a whole consists of two CBM 22 domains, a dockerin domain, and a GH 20 xylanase domain, and forms part of a cellulosome - see Section 5.10.) The enzyme has the common a/p hydrolase fold. Studies of ferulic acid complexes of the inactive alanine mutant of the active site serine revealed the classic catalytic triad, and two main-chain peptide NH bonds are in place to form an oxyanion hole . A remarkable feature is that the enzyme as repeatedly isolated was esterilied on the active site serine by phosphate or sulfate. 

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