Difference between revisions of "Glycoside Hydrolase Family 102"

Revision as of 14:32, 19 February 2010

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Author: ^^^Anthony Clarke^^^
Responsible Curator: ^^^Anthony Clarke^^^

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

Substrate specificities

Figure 1. Reaction catalyzed by family GH102 enzymes. LT, lytic transglycosylase. (click to enlarge).

The glycoside hydrolases of this family are lytic transglyosylases (also referred to as peptidoglycan lyases) of bacterial origin and they constitute family 2 of the classification scheme of Blackburn and Clarke [1]. The prototype for this family is membrane-bound lytic transglycosylase A (MltA) from Escherichia coli. These enzymes cleave the β-1,4 linkage between N-acetylmuramoyl and N-acetylglucosaminyl residues in peptidoglycan (Figure 1), but unlike the lytic transglycosylases of other families, they are active on peptidoglycan fragments lacking their stem peptides [2]. No other activities have been observed.

Kinetics and Mechanism

The lytic transglycosidases, strictly speaking, are retaining enzymes. However, they are not hydrolases but rather catalyse an intramolecular glycosyl transferase reaction onto the C-6 hydroxyl group of the muramoyl residue leading to the generation of a terminal 1,6-anhdyromuramoyl product (Figure 1) thus lacking a reducing end [3]. No detailed analyses involving both steady state and pre-steady state kinetic studies have been reported, but the Michaelis Menten (K_M and V_max) parameters have been estimated for E. coli MltA acting on insoluble peptidoglycan sacculi [2]

Catalytic Residues

As with other lytic transglycosylases (families GH23, GH103, and GH104), the GH102 enzymes are thought to possess a single catalytic acid/base residue. This residue in E. coli MltA has been identified as Asp308 and, indeed, its replacement with Ala abolishes catalytic activity [4].

The mechanism of action of the family GH102 enzymes has yet to be proven experimentally, but examination of crystal structures of E. coli MltA complexed with chitohexoase has led to the proposal of a novel mechanism of action involving the stabilization of the transition state oxocarbenium ion by by an alpha-helix dipole [5].

Three-dimensional structures

Three-dimensional structures are available for several Family GH102 enzymes, the first solved being that of E. coli MltA [4]. Unlike the other lytic transglycosylases of families GH23, GH103, and GH104 which possesses the well characterized α+β “lysozyme fold,” these enzymes have a unique structure consisting of two domains. One has a double-ψ β-barrel fold similar to the catalytic domain of the family GH45 endoglucanase V from Humicola insolens [6]. The second and smaller domain has a β-barrel fold topology. The large groove between the two domains serves as the active site cleft.

Family Firsts

First identification of lytic transglycosylase: MltA from E. coli [2].
First catalytic nucleophile identification: Not applicable for lytic transglycosylases.
First general acid/base residue identification: Inferred by X-ray crystallography of E. coli MltA [4].
First 3-D structure: E. coli MltA [4].
First identification as a lipoprotein: E. coli MltA [7].
First identification of localization to outer membrane: E. coli MltA [7].
Frist demonstration of molecular interactions between GH102 enzymes and penicillin-binding proteins: E. coli MltA [8].

References

All Medline abstracts: PubMed

[[Category:Glycoside Hydrolase Families|GH102]

@@ Line 30: / Line 30: @@
 == Substrate specificities ==
 [[Image:LTreaction.jpg|thumb|right|'''Figure 1.''' Reaction catalyzed by family GH102 enzymes. LT, lytic transglycosylase.  (''click to enlarge'').]]The glycoside hydrolases of this family are lytic transglyosylases (also referred to as peptidoglycan lyases) of bacterial origin and they constitute family 2 of the classification scheme of Blackburn and Clarke <cite>1</cite>.  The prototype for this family is membrane-bound lytic transglycosylase A (MltA) from ''Escherichia coli.''  These enzymes cleave the β-1,4 linkage between ''N''-acetylmuramoyl and ''N''-acetylglucosaminyl residues in peptidoglycan (Figure 1), but unlike the lytic transglycosylases of other families, they are active on peptidoglycan fragments lacking their stem peptides <cite>2</cite>.  No other activities have been observed.
 == Kinetics and Mechanism ==
-The lytic transglycosidases, strictly speaking, are retaining enzymes.  However, they are not hydrolases but rather catalyse an intramolecular glycosyl transferase reaction onto the C-6 hydroxyl group of the muramoyl residue leading to the generation of a terminal 1,6-anhdyromuramoyl product thus lacking a reducing end <cite>3</cite>.   No detailed analyses involving both steady state and pre-steady state kinetic studies have been reported, but the Michaelis Menten (''K''<sub>M</sub> and ''V''<sub>max</sub>) parameters have been estimated for ''E. coli'' MltA acting on insoluble peptidoglycan sacculi <cite>2</cite>
+The lytic transglycosidases, strictly speaking, are retaining enzymes.  However, they are not hydrolases but rather catalyse an intramolecular glycosyl transferase reaction onto the C-6 hydroxyl group of the muramoyl residue leading to the generation of a terminal 1,6-anhdyromuramoyl product (Figure 1) thus lacking a reducing end <cite>3</cite>.   No detailed analyses involving both steady state and pre-steady state kinetic studies have been reported, but the Michaelis Menten (''K''<sub>M</sub> and ''V''<sub>max</sub>) parameters have been estimated for ''E. coli'' MltA acting on insoluble peptidoglycan sacculi <cite>2</cite>