Difference between revisions of "Glycoside Hydrolase Family 11"

Revision as of 13:27, 22 October 2009

Glycoside Hydrolase Family GH11
Clan	GH-C
Mechanism	retaining
Active site residues	known
CAZy DB link
http://www.cazy.org/fam/GH11.html

Substrate specificities

The glycoside hydrolases of this family are endo-β-1,4-xylanases. No other activities have been observed.

Kinetics and Mechanism

Family GH11 xylanases are retaining enzymes, as first shown by NMR [1] and follow a classical Koshland double-displacement mechanism. No detailed analyses involving both steady state and pre-steady state kinetic studies have been reported. However, recent studies of the contributions of each substrate hydroxyl to transition state binding, and thus to catalysis, have been reported [2].

xylanase reaction

Catalytic Residues

The catalytic nucleophile was first identified in the Bacillus circulans endo-xylanase as Glu78 in the sequence ITELD through trapping of the 2-deoxy-2-fluoroxylobiosyl-enzyme intermediate and subsequent peptide mapping via LC-MS/MS technologies [3]. The importance of the precise positioning of the nucleophile was probed by detailed kinetic analysis of mutants modified at that position [4]. The general acid/base catalyst was first identified as Glu127 in this same enzyme through detailed mechanistic analysis of mutants at that position, which included azide rescue experiments [5]. Indeed detailed analyses of the role of the general acid/base catalyst have been described. NMR measurements of changes in pK_a values of enzymic residues during the catalytic cycle reveal pK_a cycling [6], while the changes that occur upon repositioning or removing the acid/base residue reveal its contributions [7].

Three-dimensional structures

Three-dimensional structures are available for several Family GH11 enzymes, the first solved being that of the Bacillus circulans xylanase [8]. As members of Clan GH-C they have a jellyroll fold. NMR assignment has allowed insights into both dynamics and electrostatics within the protein [8, 9, 10, 11].

Family Firsts

First sterochemistry determination: Bacillus circulans endo-xylanase Bcx by NMR [1]
First catalytic nucleophile identification: Bacillus circulans endo-xylanase Bcx by 2-fluoroglucose labeling [3]
First general acid/base residue identification: Bacillus circulans endo-xylanase Bcx by rescue kinetics with mutants [5]
First 3-D structure: Bacillus circulans endo-xylanase Bcx [8]

References

All Medline abstracts: PubMed

@@ Line 30: / Line 30: @@
 Family GH11 xylanases are [[retaining]] enzymes, as first shown by NMR <cite>1</cite> and follow a classical [[Koshland double-displacement mechanism]]. No detailed analyses involving both steady state and pre-steady state kinetic studies have been reported. However, recent studies of the contributions of each substrate hydroxyl to transition state binding, and thus to catalysis, have been reported <cite>2</cite>.
-[[Image:XYLANASE REACTION.gif|thumb|xylanase reaction]]
+[[Image:xylanase.png|thumb|xylanase reaction]]
 == Catalytic Residues ==