Publication Abstract from PubMed
The enzymatic degradation of plant cell walls is an important biological process of increasing environmental and industrial significance. Xylan, a major component of the plant cell wall, consists of a backbone of xylose (Xylp) units that are often decorated with arabinofuranose (Araf) side chains. A large penta-modular arabinoxylanase, CtXyl5A, was shown previously to specifically target arabinoxylans. However, the mechanism of substrate recognition displayed by the enzyme remains unclear. Here we report the crystal structure of the tetra-modular arabinoxylanase, and the enzyme in complex with ligands. The data showed that the multi-modular protein adopts a rigid structure, which stabilises the catalytic domain. The C-terminal non-catalytic carbohydrate binding module could not be observed in the crystal structure indicating positional flexibility. The structure of the enzyme in complex with Xylp-1,4-Xylp-1,4-Xylp-[1,3-Araf]-1,4-Xylp showed that the Araf decoration linked O3 to the xylose in the active site is located in the pocket (-2* subsite) that abuts onto the catalytic centre. The -2* subsite can also bind to Xylp and Arap, explaining why the enzyme can utilize xylose and arabinose as specificity determinants. Alanine substitution of Glu68, Tyr92 or Asn139, which interact with arabinose and xylose side chains at the -2* subsite abrogate catalytic activity. Distal to the active site the xylan backbone makes limited apolar contacts with the enzyme and the hydroxyls are solvent exposed. This explains why CtXyl5A is capable of hydrolysing xylans that are extensively decorated, and which are recalcitrant to classic endo-xylanase attack.
The mechanism by which arabinoxylanases can recognise highly decorated xylans.,Labourel A, Crouch LI, Bras JL, Jackson A, Rogowski A, Gray J, Yadav MP, Henrissat B, Fontes CM, Gilbert HJ, Najmudin S, Basle A, Cuskin F J Biol Chem. 2016 Aug 16. pii: jbc.M116.743948. PMID:27531750[1]
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
