科研之友
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摘要
Plant biomass is central to the carbon cycle and to environmentally sustainable industries exemplified by the biofuel sector. Plant cell wall degrading enzymes generally contain noncatalytic carbohydrate binding modules (CBMs) that fulfil a targeting function, which enhances catalysis. CBMs that bind beta-glucan chains often display broad specificity recognizing beta 1,4-glucans (cellulose), beta 1,3-beta 1,4-mixed linked glucans and xyloglucan, a beta 1,4-glucan decorated with beta 1,6-xylose residues, by targeting structures common to the three polysaccharides. Thus, CBMs that recognize xyloglucan target the beta 1,4-glucan backbone and only accommodate the xylose decorations. Here we show that two closely related CBMs, CBM65A and CBM65B, derived from EcCel5A, a Eubacterium cellulosolvens endoglucanase, bind to a range of beta-glucans but, uniquely, display significant preference for xyloglucan. The structures of the two CBMs reveal a beta-sandwich fold. The ligand binding site comprises the beta-sheet that forms the concave surface of the proteins. Binding to the backbone chains of beta-glucans is mediated primarily by five aromatic residues that also make hydrophobic interactions with the xylose side chains of xyloglucan, conferring the distinctive specificity of the CBMs for the decorated polysaccharide. Significantly, and in contrast to other CBMs that recognize beta-glucans, CBM65A utilizes different polar residues to bind cellulose and mixed linked glucans. Thus, Gln(106) is central to cellulose recognition, but is not required for binding to mixed linked glucans. This report reveals the mechanism by which beta-glucan-specific CBMs can distinguish between linear and mixed linked glucans, and show how these CBMs can exploit an extensive hydrophobic platform to target the side chains of decorated beta-glucans.
- 出版日期2013-2-15
