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Difference between revisions of "Glycoside Hydrolase Family 18"

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== Kinetics and Mechanism ==
 
== Kinetics and Mechanism ==
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GH18 enzymes belong to a growing group of enzymes (now including GH18, 20, 25, 56, 84 and 85) that perform a double-displacement reaction but instead of the more common enzyme-derived nucleophile they utlize the N-acetamido carbonyl oxygen in what is termed "neighbouring group participation" / "substrate participation" or "anchimeric assistance". Figures showing such a mechanism date back to Koshland's 1953 review, indeed they frequent the chemical literature of participating groups long before that, but it is primarily through the work on GH18 and simultaneously GH20 that such a mechanism became well established. In such a mechanism, which occurs with (net) retention of anomeric configuration, the enzyme provides a catalytic acid function to protonate the leaving group to facilitate its departure with the substrate carbonyl oxygen playing the role of nucleophile to generate a bicyclic "oxazoline" intermediate (which subsequently breaks down following the microscopic reverse ''via'' hydrolysis or occasionally transglycosylation).
  
  
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#AVTA2 pmid=7495789  
 
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#Armand1994 pmid=8168626
 
#Armand1994 pmid=8168626
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#Koshland1953
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#Armand
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#Tews2006
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[[Category:Glycoside Hydrolase Families|GH018]]
 
[[Category:Glycoside Hydrolase Families|GH018]]

Revision as of 08:23, 6 October 2010

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Glycoside Hydrolase Family GH18
Clan GH-K
Mechanism retaining
Active site residues known (acid/neighbouring group)
CAZy DB link
http://www.cazy.org/fam/GH18.html


Substrate specificities

GH18 is unusual in having both catalytically active chitinase (EC 3.2.1.14) and endo-β-N-acetylglucosaminidases (EC 3.2.1.96) but there are also sub-families of non-hydrolytic proteins that function as carbohydrate binding modules / "lectins" or as xylanase inhibitors.


Kinetics and Mechanism

GH18 enzymes belong to a growing group of enzymes (now including GH18, 20, 25, 56, 84 and 85) that perform a double-displacement reaction but instead of the more common enzyme-derived nucleophile they utlize the N-acetamido carbonyl oxygen in what is termed "neighbouring group participation" / "substrate participation" or "anchimeric assistance". Figures showing such a mechanism date back to Koshland's 1953 review, indeed they frequent the chemical literature of participating groups long before that, but it is primarily through the work on GH18 and simultaneously GH20 that such a mechanism became well established. In such a mechanism, which occurs with (net) retention of anomeric configuration, the enzyme provides a catalytic acid function to protonate the leaving group to facilitate its departure with the substrate carbonyl oxygen playing the role of nucleophile to generate a bicyclic "oxazoline" intermediate (which subsequently breaks down following the microscopic reverse via hydrolysis or occasionally transglycosylation).


Catalytic Residues

The catalytically active GH18 enzymes use a double displacement reaction mechanism with "neighbouring group participation".


Three-dimensional structures

Content is to be added here.


Family Firsts

First sterochemistry determination
Sometimes incorrectly reported as inverting, this family performs catalysis with retention of anomeric configuration as first shown on the Bacillus ciculans enzyme [1].
First catalytic nucleophile identification
This family is one of many that uses neighbouring group participation for catalysis with the N-acetyl carbonyl group acting as the nucleophile; first proposed (I believe) for this family in [2].
First general acid/base residue identification
On the basis of 3-D structure [3].
First 3-D structure
The first two 3-D structures for GH18 members were the Serratia marcescens chitinase A and the plant defence protein hevamine published "back-to-back" in Structure in 1994 [3, 4].

References

  1. Armand S, Tomita H, Heyraud A, Gey C, Watanabe T, and Henrissat B. (1994). Stereochemical course of the hydrolysis reaction catalyzed by chitinases A1 and D from Bacillus circulans WL-12. FEBS Lett. 1994;343(2):177-80. DOI:10.1016/0014-5793(94)80314-5 | PubMed ID:8168626 [Armand1994]
  2. Terwisscha van Scheltinga AC, Armand S, Kalk KH, Isogai A, Henrissat B, and Dijkstra BW. (1995). Stereochemistry of chitin hydrolysis by a plant chitinase/lysozyme and X-ray structure of a complex with allosamidin: evidence for substrate assisted catalysis. Biochemistry. 1995;34(48):15619-23. DOI:10.1021/bi00048a003 | PubMed ID:7495789 [AVTA2]
  3. Perrakis A, Tews I, Dauter Z, Oppenheim AB, Chet I, Wilson KS, and Vorgias CE. (1994). Crystal structure of a bacterial chitinase at 2.3 A resolution. Structure. 1994;2(12):1169-80. DOI:10.1016/s0969-2126(94)00119-7 | PubMed ID:7704527 [Perrakis]
  4. Terwisscha van Scheltinga AC, Kalk KH, Beintema JJ, and Dijkstra BW. (1994). Crystal structures of hevamine, a plant defence protein with chitinase and lysozyme activity, and its complex with an inhibitor. Structure. 1994;2(12):1181-9. DOI:10.1016/s0969-2126(94)00120-0 | PubMed ID:7704528 [ATVA1]
  5. [Koshland1953]
  6. [Armand]
  7. [Tews2006]
  8. [Daan]

All Medline abstracts: PubMed