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Glycoside Hydrolase Family 76
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|Glycoside Hydrolase Family GH76|
|Active site residues||not known|
|CAZy DB link|
Glycoside hydrolases of family GH76 are endo-acting α-mannanases. GH76 genes are found within bacteria and fungi. Bacterial GH76 enzymes cleave α-1,6-mannans, such as those found within the α-1,6-linked backbone of fungal mannoproteins and mycobacterial cell wall lipomannan and lipoarabinomannan. This family was originally created from the cloning and characterization of Aman6 from Bacillus circulans TN-31 , which appears to be the same enzyme as that characterized much earlier by Ballou and co-workers . Aman6 degrades α-1,6-mannan to a mixture of the mannobiose and mannotriose ; mannotriose is the minimum substrate for the enzyme . A possible GH76 enzyme has been detected within Mycobacterium smegmatis, which has the capacity to degrade α-1,6-mannooligosaccharides .
Additional characterized GH76 enzymes include several from Bacteroides thetaiotaomicron . B. thetaiotaomicron expresses numerous GH76 enzymes. Several of these are found within polysaccharide utilization loci that are specifically up-regulated upon exposure to yeast α-mannan. These enzymes have the capacity to utilize unadorned linear α-1,6-mannan, but have little activity on highly branched wildtype α-mannan. Certain B. thetaiotaomicron GH76 enzymes are lipoenzymes that are associated with the cell surface, where they appear to act on large yeast mannan molecules that have undergone partial trimming to expose sections of the core α-1,6-mannan. Other periplasmic located GH76 enzymes have activity on shorter α-1,6-mannan fragments, which are obtained by importation of partially-digested fragments arising from the action of cell surface enzymes.
Fungal GH76 enzymes have been speculated to be involved in cross-linking of GPI-anchored proteins into the cell wall, where they are proposed to act as transglycosylases . No biochemical data has been obtained for any fungal GH76 enzyme in support of the so-called anchorage hypothesis.
Kinetics and Mechanism
Family GH76 endo-α-mannosidases are retaining enzymes, as first shown by 1H NMR analysis of the hydrolysis of 4-nitrophenyl α-mannosyl-1,6-α-mannopyranoside by Bacteroides thetaiotaomicron . GH76 enzymes are believed to proceed through a classical Koshland double-displacement mechanism.
A pair of conserved aspartic acid residues (D258 and D259) have been proposed as acid/base and nucleophile based on analysis of the X-ray structures of GH76 members .
Three-dimensional structures are available for several bacterial members of GH76, including Bacillus circulans, Bacteroides thetaiotaomicron and Listeria innocua Clip11262 (see the GH76 structure page in the CAZy Database). They have an (α/α)6 fold. A ligand bound structure with α-1,6-mannobiose is available for the B. circulans enzyme. This structure appears to involve binding within the active site cleft although it does not appear to involve ligand binding at the catalytic site.
- First stereochemistry determination
- Bacteroides thetaiotaomicron α-1,6-mannanase by 1H NMR 
- First catalytic nucleophile identification
- no experimental evidence
- First general acid/base residue identification
- no experimental evidence
- First 3-D structure of a GH76 enzyme
- Listeria innocua Lin0763 (unpublished, PDB ID 3k7x)
- Maruyama Y and Nakajima T. The aman6 gene encoding a yeast mannan backbone degrading 1,6-alpha-D-mannanase in Bacillus circulans: cloning, sequence analysis, and expression. Biosci Biotechnol Biochem. 2000 Sep;64(9):2018-20. DOI:10.1271/bbb.64.2018 |
- Nakajima T, Maitra SK, and Ballou CE. An endo-alpha1 leads to 6-D-mannanase from a soil bacterium. Purification, properties, and mode of action. J Biol Chem. 1976 Jan 10;251(1):174-81.
- Yokoyama K and Ballou CE. Synthesis of alpha 1----6-mannooligosaccharides in Mycobacterium smegmatis. Function of beta-mannosylphosphoryldecaprenol as the mannosyl donor. J Biol Chem. 1989 Dec 25;264(36):21621-8.
- Cuskin F, Lowe EC, Temple MJ, Zhu Y, Cameron EA, Pudlo NA, Porter NT, Urs K, Thompson AJ, Cartmell A, Rogowski A, Hamilton BS, Chen R, Tolbert TJ, Piens K, Bracke D, Vervecken W, Hakki Z, Speciale G, Munōz-Munōz JL, Day A, Peña MJ, McLean R, Suits MD, Boraston AB, Atherly T, Ziemer CJ, Williams SJ, Davies GJ, Abbott DW, Martens EC, and Gilbert HJ. Human gut Bacteroidetes can utilize yeast mannan through a selfish mechanism. Nature. 2015 Jan 8;517(7533):165-9. DOI:10.1038/nature13995 |
- Kitagaki H, Wu H, Shimoi H, and Ito K. Two homologous genes, DCW1 (YKL046c) and DFG5, are essential for cell growth and encode glycosylphosphatidylinositol (GPI)-anchored membrane proteins required for cell wall biogenesis in Saccharomyces cerevisiae. Mol Microbiol. 2002 Nov;46(4):1011-22.