This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Degradation/Utilization/Assimilation → Carbohydrates Degradation → Polysaccharides Degradation → Xyloglucan Degradation|
|Degradation/Utilization/Assimilation → Polymeric Compounds Degradation → Polysaccharides Degradation → Xyloglucan Degradation|
Expected Taxonomic Range: Fungi
Xyloglucan is the major component of the hemicellulose in dicotyledon type I cell walls. The cross-linking between xyloglucan chains is a major factor in the stability of the three-dimensional matrix between adjacent cellulose microfibrils [Whitney95].
Xyloglucan has a backbone of β-1,4-linked cellotetraose units, which are substituted in C-6 by α-D-xylosyl residues at two or three of the first four glucosyl residues (but not the residue closest to the reducing end) [York93]. Some of the xylosyl residues are subsequently further substituted by (1,2)-β-D-galactosyl residues at the second and/or third xylose residue and by (1,2)-α-L-fucose at the galactosyl unit of the third xylose residue [Madson03, Lerouxel06].
A common terminology uses G for basic non-substituted glucosyl units within a xylosylglycan, X for glucose residues substituted by α-D-xylose, L for glucose residues substituted by α-D-xylose and β-D-galactose, and F for glucose residues substituted by α-D-xylose, β-D-galactose and α-L-fucose [Fry93]. Using this terminology, most xylosylglucans consist of repeating heptasaccharide XXXG units as well further modified XXLG, XLXG, and XLLG units.
About This Pathway
The enzyme described in this pathway has been originally isolated from the fungus Geotrichum sp. M128. It is an exoglucanase that recognizes and attacks the reducing end of oligoxyloglucan, releasing segments composed of two glucosyl residues (with their xylose modifications) from the main xyloglucan chain (a modified cellobiohydrolase activity). For example, when the enzyme acts on the reducing end of the oligoxyloglucan heptasaccharide XXXG, it produces the tetrasaccharide XX and the trisaccharide XG.
A thorough study of the substrate specificity showed that the terminal residue (site +2) must be an unbranched glucose. The glycosylation of the glucose at site +1 is not important, thus the reducing end unit that is being cleaved may be GG, XG, or LG. It is also important that the third glucose residue from the reducing end (site -1) is branched with a unmodified xylose residue [Yaoi02].
Bauer05: Bauer S, Vasu P, Mort AJ, Somerville CR (2005). "Cloning, expression, and characterization of an oligoxyloglucan reducing end-specific xyloglucanobiohydrolase from Aspergillus nidulans." Carbohydr Res 340(17);2590-7. PMID: 16214120
Madson03: Madson M, Dunand C, Li X, Verma R, Vanzin GF, Caplan J, Shoue DA, Carpita NC, Reiter WD (2003). "The MUR3 gene of Arabidopsis encodes a xyloglucan galactosyltransferase that is evolutionarily related to animal exostosins." Plant Cell 15(7);1662-70. PMID: 12837954
Whitney95: Whitney, S. E. C., Brigham, J. E., Darke, A. H., Reid, J. S. G., Gidley, M. J. (1995). "In vitro assembly of cellulose/xyloglucan networks: ultrastructure and molecular aspects." Plant J 8, 491-504.
Yaoi02: Yaoi K, Mitsuishi Y (2002). "Purification, characterization, cloning, and expression of a novel xyloglucan-specific glycosidase, oligoxyloglucan reducing end-specific cellobiohydrolase." J Biol Chem 277(50);48276-81. PMID: 12374797
York93: York WS, Harvey LK, Guillen R, Albersheim P, Darvill AG (1993). "Structural analysis of tamarind seed xyloglucan oligosaccharides using beta-galactosidase digestion and spectroscopic methods." Carbohydr Res 248;285-301. PMID: 8252539
Grishutin04: Grishutin SG, Gusakov AV, Markov AV, Ustinov BB, Semenova MV, Sinitsyn AP (2004). "Specific xyloglucanases as a new class of polysaccharide-degrading enzymes." Biochim Biophys Acta 1674(3);268-81. PMID: 15541296
Yaoi04: Yaoi K, Kondo H, Noro N, Suzuki M, Tsuda S, Mitsuishi Y (2004). "Tandem repeat of a seven-bladed beta-propeller domain in oligoxyloglucan reducing-end-specific cellobiohydrolase." Structure 12(7);1209-17. PMID: 15242597
Yaoi07: Yaoi K, Kondo H, Hiyoshi A, Noro N, Sugimoto H, Tsuda S, Mitsuishi Y, Miyazaki K (2007). "The structural basis for the exo-mode of action in GH74 oligoxyloglucan reducing end-specific cellobiohydrolase." J Mol Biol 370(1);53-62. PMID: 17498741
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