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Glycoside Hydrolase Family 49

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Glycoside Hydrolase Family GHnn
Clan GH-N
Mechanism inverting
Active site residues known
CAZy DB link
http://www.cazy.org/GH49.html


Substrate specificities

Glycoside hydrolases of family 49 cleave α-1,6-glucosidic linkages or α-1,4-glucosidic linkages of polysaccharides containing α-1,6-glucosidic linkages, dextran and pullulan. The major activities reported for this family of glycoside hydrolases are dextranase (EC 3.2.1.11), and a dextranase from Penicillium minioluteum, Dex49A, is currently the most characterised enzyme. Dextran 1,6-α-isomaltotriosidase (EC 3.2.1.95) [1] and isopullulanase (EC 3.2.1.57) activities have also been described.

Kinetics and Mechanism

Family GH49 α-glycosidases are inverting enzymes, as first shown by NMR on a dextranase Dex49A from Penicillium minioluteum [2] .

Catalytic Residues

Three Asp residues (Asp376, Asp395, and Asp396 in Dex49A) are conserved in the catalytic centre of members of clan GH-N, GH49 and GH28 enzymes [2, 3], and all three of the Asp mutants of a GH49 enzyme, isopullulanase, lost their activities [4]. The general acid was first identified in Dex49A from Penicillium minioluteum as Asp395 following the three-dimensional structure determination. To date, it is unclear whether either (or both) of the Asp residues (Asp376 and Asp396 in Dex49A) acts as a general base in the reaction of GH49 and GH28 enzymes [2, 5, 6].

Three-dimensional structures

Two structures of GH49 enzymes are available so far [2, 3], and they display a two domain structure. The N-terminal domain is a β-sandwich and the C-terminal domain adopts a right-handed parallel β-helix. The similarity of the β-helix fold between GH49 and GH28 enzymes has been described, although almost none of the amino acid residues other than the three catalytic Asp residues is conserved between the two families [2, 3]. Each coil forming the cylindrical β-helix fold is composed of three β-sheets, which are named PB1, PB2, and PB3, following the original definition for a PL1 enzyme, pectate lyase C [7].

Family Firsts

First gene cloning
Dextranase from Arthrobacter sp. CB-8 [8].
First sterochemistry determination
Dextranase (Dex49A) from Penicillium minioluteum [2].
First general acid residue identification
Dextranase (Dex49A) from Penicillium minioluteum [2].
First 3-D structure
Dextranase (Dex49A) from Penicillium minioluteum by X-ray crystallography (PDB ID 1ogm) [2].

References

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  1. Error fetching PMID 10540747: [Mizuno1999]
  2. Error fetching PMID 12962629: [Larsson2003]
  3. Error fetching PMID 18155243: [Mizuno2008]
  4. Error fetching PMID 15560783: [Akeboshi2004]
  5. Error fetching PMID 10521427: [vanSanten1999]
  6. Error fetching PMID 12022868: [Shimizu2002]
  7. Error fetching PMID 8502994: [Yoder1993]
  8. Error fetching PMID 1859672: [Okushima1991]
All Medline abstracts: PubMed | HubMed