Glycoside Hydrolase Family 23 - Revision history

Harry Brumer: Text replacement - "\^\^\^(.*)\^\^\^" to "$1"

2021-12-18T21:14:00Z

Text replacement - "\^\^\^(.*)\^\^\^" to "$1"

Harry Brumer at 19:49, 26 August 2013

2013-08-26T19:49:36Z

Harry Brumer: increased figure size

2013-08-26T19:46:30Z

increased figure size

Spencer Williams: /* Kinetics and Mechanism */

2013-06-27T05:48:57Z

Kinetics and Mechanism

Harry Brumer: updated CAZyDBlink

2012-04-30T16:16:20Z

updated CAZyDBlink

Spencer Williams at 00:57, 20 June 2010

2010-06-20T00:57:44Z

Spencer Williams at 23:42, 19 June 2010

2010-06-19T23:42:14Z

Spencer Williams: /* Catalytic Residues */

2010-04-20T12:16:45Z

Catalytic Residues

Spencer Williams: /* Catalytic Residues */

2010-04-20T12:08:12Z

Catalytic Residues

Spencer Williams at 12:02, 20 April 2010

2010-04-20T12:02:33Z

@@ Line 1: / Line 1: @@
 <!-- CURATORS: Please replace the {{UnderConstruction}} tag below with {{CuratorApproved}} when the page is ready for wider public consumption -->
 {{CuratorApproved}}
-* [[Author]]: ^^^Anthony Clarke^^^
+* [[Author]]: [[User:Anthony Clarke|Anthony Clarke]]
-* [[Responsible Curator]]:  ^^^Anthony Clarke^^^
+* [[Responsible Curator]]:  [[User:Anthony Clarke|Anthony Clarke]]
 ----

@@ Line 34: / Line 34: @@
 == Kinetics and Mechanism ==
-[[Image:LT-GEWL.jpg|thumb|400px|right|'''Figure 1.''' Lytic and hydrolytic pathways of GH23 enzymes.  GEWL, g-type lysozymes; LT, lytic transglycosylase.  (''click to enlarge'').]]The enzymes of this family cleave the β-1,4-linkage between ''N''-acetylmuramyl and ''N''-acetylglucosaminyl residues in peptidoglycan (Figure 1).  Only the lysozymes of this family are capable of releasing ''N''-acetyl-{{Smallcaps|d}}-glucosamine residues from chitodextrins, and neither catalyze transglycosylation reactions.  The stereochemistry of the reaction catalysed by the family G lysozymes has been determined experimentally as [[inverting]] <cite>10</cite>.  On the other hand, the lytic transglycosidases, strictly speaking, are [[retaining]] enzymes.  However, unlike lysozyme, they are not hydrolases but rather catalyse an intramolecular transglycosylation to the C-6 hydroxyl group of the muramyl residue leading to the generation of a terminal 1,6-anhydromuramic acid product that is an acetal, and not a hemiacetal <cite>4</cite>.  The lytic transglycosylases require the peptide side chains in peptidoglycan for activity, accounting for their inactivity against chitin or chitooligosaccharides <cite>5</cite>.  No detailed analyses involving both steady-state and pre-steady state kinetic studies have been reported.
+[[Image:LT-GEWL.jpg|thumb|400px|right|'''Figure 1.''' Lytic and hydrolytic pathways of GH23 enzymes.  GEWL, g-type lysozymes; LT, lytic transglycosylase.]]The enzymes of this family cleave the β-1,4-linkage between ''N''-acetylmuramyl and ''N''-acetylglucosaminyl residues in peptidoglycan (Figure 1).  Only the lysozymes of this family are capable of releasing ''N''-acetyl-{{Smallcaps|d}}-glucosamine residues from chitodextrins, and neither catalyze transglycosylation reactions.  The stereochemistry of the reaction catalysed by the family G lysozymes has been determined experimentally as [[inverting]] <cite>10</cite>.  On the other hand, the lytic transglycosidases, strictly speaking, are [[retaining]] enzymes.  However, unlike lysozyme, they are not hydrolases but rather catalyse an intramolecular transglycosylation to the C-6 hydroxyl group of the muramyl residue leading to the generation of a terminal 1,6-anhydromuramic acid product that is an acetal, and not a hemiacetal <cite>4</cite>.  The lytic transglycosylases require the peptide side chains in peptidoglycan for activity, accounting for their inactivity against chitin or chitooligosaccharides <cite>5</cite>.  No detailed analyses involving both steady-state and pre-steady state kinetic studies have been reported.
 == Catalytic Residues ==
-[[Image:LTmechanism.jpg|thumb|400px|right|'''Figure 2.''' Detailed catalytic mechanism of GH23 lytic transglycosylases (''click to enlarge''). ]]Until recently, the family GH23 enzymes were thought to have only a single catalytic residue at their active centre. The identity of the catalytic [[general acid/base]] residue of the lysozymes was first inferred by X-ray crystallography of goose egg-white lysozyme (GEWL) as Glu73 <cite>6 7</cite>.  Likewise, analysis of the crystal structure of ''E. coli'' Slt70 identified Glu as the lone catalytic residue<cite>8</cite>.  Replacement of each respective residue results in loss of catalytic activity <cite>9 10</cite>.  The mechanism of action of the family GH23 enzymes has yet to be proven experimentally, but examination of crystal structures and theoretical considerations have led to separate proposals for the two classes of enzymes.
+[[Image:LTmechanism.jpg|thumb|400px|right|'''Figure 2.''' Detailed catalytic mechanism of GH23 lytic transglycosylases.]]Until recently, the family GH23 enzymes were thought to have only a single catalytic residue at their active centre. The identity of the catalytic [[general acid/base]] residue of the lysozymes was first inferred by X-ray crystallography of goose egg-white lysozyme (GEWL) as Glu73 <cite>6 7</cite>.  Likewise, analysis of the crystal structure of ''E. coli'' Slt70 identified Glu as the lone catalytic residue<cite>8</cite>.  Replacement of each respective residue results in loss of catalytic activity <cite>9 10</cite>.  The mechanism of action of the family GH23 enzymes has yet to be proven experimentally, but examination of crystal structures and theoretical considerations have led to separate proposals for the two classes of enzymes.
 With the lytic transglycosylases, there is still no evidence for a second catalytic residue at their active sites.  Hence, based on the complexes formed with 1,6-anhydromuropeptide <cite>12</cite> or bulgecin <cite>11</cite>, a two-step  mechanism that uses [[neighboring group participation]] (also termed substrate assisted catalysis), analogous to the family [[GH18]] chitinases and chitobiases and family [[GH20]] ''N''-acetyl-β-hexosaminidases, has been invoked.  Thus, in the first step the Glu73 residue is proposed to serve initially as a [[general acid]] to donate a proton to the glycosidic oxygen of the linkage (Figure 2). At the same time anchimeric assistance by the MurNAc 2-acetamido group results in the formation an [[oxazolinium ion]] [[intermediate]]. In the second step Glu73 acts as a [[general base]] to abstract the proton from the C-6 hydroxyl leading to its nucleophilic attack on the anomeric centre and the formation of 1,6-anhydromuramic acid product. The [[transition state]]s leading to and from the [[intermediate]] possess [[oxocarbenium ion]] character (Figure 2).

@@ Line 34: / Line 34: @@
 == Kinetics and Mechanism ==
-[[Image:LT-GEWL.jpg|thumb|right|'''Figure 1.''' Lytic and hydrolytic pathways of GH23 enzymes.  GEWL, g-type lysozymes; LT, lytic transglycosylase.  (''click to enlarge'').]]The enzymes of this family cleave the β-1,4-linkage between ''N''-acetylmuramyl and ''N''-acetylglucosaminyl residues in peptidoglycan (Figure 1).  Only the lysozymes of this family are capable of releasing ''N''-acetyl-{{Smallcaps|d}}-glucosamine residues from chitodextrins, and neither catalyze transglycosylation reactions.  The stereochemistry of the reaction catalysed by the family G lysozymes has been determined experimentally as [[inverting]] <cite>10</cite>.  On the other hand, the lytic transglycosidases, strictly speaking, are [[retaining]] enzymes.  However, unlike lysozyme, they are not hydrolases but rather catalyse an intramolecular transglycosylation to the C-6 hydroxyl group of the muramyl residue leading to the generation of a terminal 1,6-anhydromuramic acid product that is an acetal, and not a hemiacetal <cite>4</cite>.  The lytic transglycosylases require the peptide side chains in peptidoglycan for activity, accounting for their inactivity against chitin or chitooligosaccharides <cite>5</cite>.  No detailed analyses involving both steady-state and pre-steady state kinetic studies have been reported.
+[[Image:LT-GEWL.jpg|thumb|400px|right|'''Figure 1.''' Lytic and hydrolytic pathways of GH23 enzymes.  GEWL, g-type lysozymes; LT, lytic transglycosylase.  (''click to enlarge'').]]The enzymes of this family cleave the β-1,4-linkage between ''N''-acetylmuramyl and ''N''-acetylglucosaminyl residues in peptidoglycan (Figure 1).  Only the lysozymes of this family are capable of releasing ''N''-acetyl-{{Smallcaps|d}}-glucosamine residues from chitodextrins, and neither catalyze transglycosylation reactions.  The stereochemistry of the reaction catalysed by the family G lysozymes has been determined experimentally as [[inverting]] <cite>10</cite>.  On the other hand, the lytic transglycosidases, strictly speaking, are [[retaining]] enzymes.  However, unlike lysozyme, they are not hydrolases but rather catalyse an intramolecular transglycosylation to the C-6 hydroxyl group of the muramyl residue leading to the generation of a terminal 1,6-anhydromuramic acid product that is an acetal, and not a hemiacetal <cite>4</cite>.  The lytic transglycosylases require the peptide side chains in peptidoglycan for activity, accounting for their inactivity against chitin or chitooligosaccharides <cite>5</cite>.  No detailed analyses involving both steady-state and pre-steady state kinetic studies have been reported.
 == Catalytic Residues ==
-[[Image:LTmechanism.jpg|thumb|right|'''Figure 2.''' Detailed catalytic mechanism of GH23 lytic transglycosylases (''click to enlarge''). ]]Until recently, the family GH23 enzymes were thought to have only a single catalytic residue at their active centre. The identity of the catalytic [[general acid/base]] residue of the lysozymes was first inferred by X-ray crystallography of goose egg-white lysozyme (GEWL) as Glu73 <cite>6 7</cite>.  Likewise, analysis of the crystal structure of ''E. coli'' Slt70 identified Glu as the lone catalytic residue<cite>8</cite>.  Replacement of each respective residue results in loss of catalytic activity <cite>9 10</cite>.  The mechanism of action of the family GH23 enzymes has yet to be proven experimentally, but examination of crystal structures and theoretical considerations have led to separate proposals for the two classes of enzymes.
+[[Image:LTmechanism.jpg|thumb|400px|right|'''Figure 2.''' Detailed catalytic mechanism of GH23 lytic transglycosylases (''click to enlarge''). ]]Until recently, the family GH23 enzymes were thought to have only a single catalytic residue at their active centre. The identity of the catalytic [[general acid/base]] residue of the lysozymes was first inferred by X-ray crystallography of goose egg-white lysozyme (GEWL) as Glu73 <cite>6 7</cite>.  Likewise, analysis of the crystal structure of ''E. coli'' Slt70 identified Glu as the lone catalytic residue<cite>8</cite>.  Replacement of each respective residue results in loss of catalytic activity <cite>9 10</cite>.  The mechanism of action of the family GH23 enzymes has yet to be proven experimentally, but examination of crystal structures and theoretical considerations have led to separate proposals for the two classes of enzymes.
 With the lytic transglycosylases, there is still no evidence for a second catalytic residue at their active sites.  Hence, based on the complexes formed with 1,6-anhydromuropeptide <cite>12</cite> or bulgecin <cite>11</cite>, a two-step  mechanism that uses [[neighboring group participation]] (also termed substrate assisted catalysis), analogous to the family [[GH18]] chitinases and chitobiases and family [[GH20]] ''N''-acetyl-β-hexosaminidases, has been invoked.  Thus, in the first step the Glu73 residue is proposed to serve initially as a [[general acid]] to donate a proton to the glycosidic oxygen of the linkage (Figure 2). At the same time anchimeric assistance by the MurNAc 2-acetamido group results in the formation an [[oxazolinium ion]] [[intermediate]]. In the second step Glu73 acts as a [[general base]] to abstract the proton from the C-6 hydroxyl leading to its nucleophilic attack on the anomeric centre and the formation of 1,6-anhydromuramic acid product. The [[transition state]]s leading to and from the [[intermediate]] possess [[oxocarbenium ion]] character (Figure 2).

@@ Line 34: / Line 34: @@
 == Kinetics and Mechanism ==
-[[Image:LT-GEWL.jpg|thumb|right|'''Figure 1.''' Lytic and hydrolytic pathways of GH23 enzymes.  GEWL, g-type lysozymes; LT, lytic transglycosylase.  (''click to enlarge'').]]The enzymes of this family cleave the β-1,4-linkage between ''N''-acetylmuramyl and ''N''-acetylglucosaminyl residues in peptidoglycan (Figure 1).  Only the lysozymes of this family are capable of releasing ''N''-acetyl-{{Smallcaps|d}}-glucosamine residues from chitodextrins, and neither catalyze [[transglycosylation]] reactions.  The stereochemistry of the reaction catalysed by the family G lysozymes has been determined experimentally as [[inverting]] <cite>10</cite>.  On the other hand, the lytic transglycosidases, strictly speaking, are [[retaining]] enzymes.  However, unlike lysozyme, they are not hydrolases but rather catalyse an intramolecular transglycosylation to the C-6 hydroxyl group of the muramyl residue leading to the generation of a terminal 1,6-anhydromuramic acid product that is an acetal, and not a hemiacetal <cite>4</cite>.  The lytic transglycosylases require the peptide side chains in peptidoglycan for activity, accounting for their inactivity against chitin or chitooligosaccharides <cite>5</cite>.  No detailed analyses involving both steady-state and pre-steady state kinetic studies have been reported.
+[[Image:LT-GEWL.jpg|thumb|right|'''Figure 1.''' Lytic and hydrolytic pathways of GH23 enzymes.  GEWL, g-type lysozymes; LT, lytic transglycosylase.  (''click to enlarge'').]]The enzymes of this family cleave the β-1,4-linkage between ''N''-acetylmuramyl and ''N''-acetylglucosaminyl residues in peptidoglycan (Figure 1).  Only the lysozymes of this family are capable of releasing ''N''-acetyl-{{Smallcaps|d}}-glucosamine residues from chitodextrins, and neither catalyze transglycosylation reactions.  The stereochemistry of the reaction catalysed by the family G lysozymes has been determined experimentally as [[inverting]] <cite>10</cite>.  On the other hand, the lytic transglycosidases, strictly speaking, are [[retaining]] enzymes.  However, unlike lysozyme, they are not hydrolases but rather catalyse an intramolecular transglycosylation to the C-6 hydroxyl group of the muramyl residue leading to the generation of a terminal 1,6-anhydromuramic acid product that is an acetal, and not a hemiacetal <cite>4</cite>.  The lytic transglycosylases require the peptide side chains in peptidoglycan for activity, accounting for their inactivity against chitin or chitooligosaccharides <cite>5</cite>.  No detailed analyses involving both steady-state and pre-steady state kinetic studies have been reported.
 == Catalytic Residues ==

@@ Line 22: / Line 22: @@
 |{{Hl2}} colspan="2" align="center" |'''CAZy DB link'''
 |-
-| colspan="2" |http://www.cazy.org/fam/GH23.html
+| colspan="2" |{{CAZyDBlink}}GH23.html
 |}
 </div>

@@ Line 39: / Line 39: @@
 [[Image:LTmechanism.jpg|thumb|right|'''Figure 2.''' Detailed catalytic mechanism of GH23 lytic transglycosylases (''click to enlarge''). ]]Until recently, the family GH23 enzymes were thought to have only a single catalytic residue at their active centre. The identity of the catalytic [[general acid/base]] residue of the lysozymes was first inferred by X-ray crystallography of goose egg-white lysozyme (GEWL) as Glu73 <cite>6 7</cite>.  Likewise, analysis of the crystal structure of ''E. coli'' Slt70 identified Glu as the lone catalytic residue<cite>8</cite>.  Replacement of each respective residue results in loss of catalytic activity <cite>9 10</cite>.  The mechanism of action of the family GH23 enzymes has yet to be proven experimentally, but examination of crystal structures and theoretical considerations have led to separate proposals for the two classes of enzymes.
-With the lytic transglycosylases, there is still no evidence for a second catalytic residue at their active sites.  Hence, based on the complexes formed with 1,6-anhydromuropeptide <cite>12</cite> or bulgecin <cite>11</cite>, a two-step  mechanism that uses [[neighboring group participation]] (also termed substrate assisted catalysis), analogous to the family [[GH18]] chitinases and chitobiases and family [[GH20]] ''N''-acetyl-β-hexosaminidases, has been invoked.  Thus, in the first step the Glu73 residue is proposed to serve initially as a [[general acid]] to donate a proton to the glycosidic oxygen of the linkage (Figure 2). At the same time anchimeric assistance by the MurNAc 2-acetamido group results in the formation an oxazolinium ion intermediate. In the second step Glu73 acts as a [[general base]] to abstract the proton from the C-6 hydroxyl leading to its nucleophilic attack on the anomeric centre and the formation of 1,6-anhydromuramic acid product. The [[transition state]]s leading to and from the [[intermediate]] possess [[oxocarbenium ion]] character (Figure 2).
+With the lytic transglycosylases, there is still no evidence for a second catalytic residue at their active sites.  Hence, based on the complexes formed with 1,6-anhydromuropeptide <cite>12</cite> or bulgecin <cite>11</cite>, a two-step  mechanism that uses [[neighboring group participation]] (also termed substrate assisted catalysis), analogous to the family [[GH18]] chitinases and chitobiases and family [[GH20]] ''N''-acetyl-β-hexosaminidases, has been invoked.  Thus, in the first step the Glu73 residue is proposed to serve initially as a [[general acid]] to donate a proton to the glycosidic oxygen of the linkage (Figure 2). At the same time anchimeric assistance by the MurNAc 2-acetamido group results in the formation an [[oxazolinium ion]] [[intermediate]]. In the second step Glu73 acts as a [[general base]] to abstract the proton from the C-6 hydroxyl leading to its nucleophilic attack on the anomeric centre and the formation of 1,6-anhydromuramic acid product. The [[transition state]]s leading to and from the [[intermediate]] possess [[oxocarbenium ion]] character (Figure 2).
 For the g-type lysozymes, recent studies support a typical [[inverting]] mechanism of action <cite>2 3</cite> involving [[general acid]] and [[general base]] residues.  It was proposed that two catalytic [[general base]] residues, a primary and a secondary residue, position a catalytic water molecule and abstract a proton to effect substrate hydrolysis by the single displacement mechanism.  With GEWL, Asp97 and Asp86, respectively, are proposed to serve this function while these would be represented by the conserved Asp101 and Asp90 residues in the g-type lysozyme from Atlantic cod fish.  Indeed, the double replacement of Asp101 and Asp90 in the cod lysozyme results in over a 300-fold decrease in activity <cite>2</cite>.

@@ Line 39: / Line 39: @@
 [[Image:LTmechanism.jpg|thumb|right|'''Figure 2.''' Detailed catalytic mechanism of GH23 lytic transglycosylases (''click to enlarge''). ]]Until recently, the family GH23 enzymes were thought to have only a single catalytic residue at their catalytic centre. The identity of the catalytic [[general acid/base]] residue of the lysozymes was first inferred by X-ray crystallography of goose egg-white lysozyme (GEWL) as Glu73 <cite>6 7</cite>.   Likewise, analysis of the crystal structure of ''E. coli'' Slt70 identified Glu as the lone catalytic residue<cite>8</cite>.   Replacement of each respective residue results in loss of catalytic activity <cite>9 10</cite>.  The mechanism of action of the family GH23 enzymes has yet to be proven experimentally, but examination of crystal structures and theoretical considerations have led to separate proposals for the two classes of enzymes.
-With the lytic transglycosylases, there is still no evidence for a second catalytic residue at their active sites.  Hence, based on the complexes formed with 1,6-anhydromuropeptide <cite>12</cite> or bulgecin <cite>11</cite>, a two-step  mechanism that invokes [[neighboring group participation]] or substrate assisted catalysis, analogous to the family [[GH18]] chitinases and chitobiases and family [[GH20]] ''N''-acetyl-β-hexosaminidases, has been invoked.  Thus, in the first step the Glu73 residue is proposed to serve initially as a [[general acid]] to donate a proton to the glycosidic oxygen of the linkage (Figure 2). At the same time anchimeric assistance by the MurNAc 2-acetamido group results in the formation an oxazolinium ion intermediate. In the second step Glu73 acts as a [[general base]] to abstract the proton from teh C-6 hydroxyl leading to its nucleophilic attack on the anomeric centre and the formation of 1,6-anhydromuramic acid product. The transition state leading to and from the intermediate possess oxocarbenium ion character (Figure 2).
+With the lytic transglycosylases, there is still no evidence for a second catalytic residue at their active sites.  Hence, based on the complexes formed with 1,6-anhydromuropeptide <cite>12</cite> or bulgecin <cite>11</cite>, a two-step  mechanism that invokes [[neighboring group participation]] or substrate assisted catalysis, analogous to the family [[GH18]] chitinases and chitobiases and family [[GH20]] ''N''-acetyl-β-hexosaminidases, has been invoked.  Thus, in the first step the Glu73 residue is proposed to serve initially as a [[general acid]] to donate a proton to the glycosidic oxygen of the linkage (Figure 2). At the same time anchimeric assistance by the MurNAc 2-acetamido group results in the formation an oxazolinium ion intermediate. In the second step Glu73 acts as a [[general base]] to abstract the proton from the C-6 hydroxyl leading to its nucleophilic attack on the anomeric centre and the formation of 1,6-anhydromuramic acid product. The transition state leading to and from the intermediate possess oxocarbenium ion character (Figure 2).
 For the g-type lysozymes, recent studies support a typical inverting mechanism of action <cite>2 3</cite> involving catalytic acid and base residues.  It was proposed that two catalytic [[general base]] residues, a primary and a secondary residue, position a catalytic water molecule and abstract a proton to effect substrate hydrolysis by the single displacement mechanism.  With GEWL, Asp97 and Asp86, respectively, are proposed to serve this function while these would be represented by the conserved Asp101 and Asp90 residues in the g-type lysozyme from Atlantic cod fish.  Indeed, the double replacement of Asp101 and Asp90 in the cod lysozyme results in over a 300-fold decrease in activity <cite>2</cite>.

@@ Line 29: / Line 29: @@
 == Substrate specificities ==
-The glycoside hydrolases of this family are lytic transglycosylases (also referred to as peptidoglycan lyases) of both bacterial and bacteriophage origin, and family G lysozymes (EC [{{EClink}}3.2.1.17 3.2.1.17];  muramidase, peptidoglycan ''N''-acetylmuramoylhydrolase, 1,4-β-''N''-acetylmuramidase, ''N''-acetylmuramoylhydrolase) of eukaryotic origin.  Both of these enzymes are active on peptidoglycan, but only the lysozymes are active on chitin and chitooligosaccharides.  No other activities have been observed.
+The [[glycoside hydrolases]] of this family are lytic transglycosylases (also referred to as peptidoglycan lyases) of both bacterial and bacteriophage origin, and family G lysozymes (EC [{{EClink}}3.2.1.17 3.2.1.17];  muramidase, peptidoglycan ''N''-acetylmuramoylhydrolase, 1,4-β-''N''-acetylmuramidase, ''N''-acetylmuramoylhydrolase) of eukaryotic origin.  Both of these enzymes are active on peptidoglycan, but only the lysozymes are active on chitin and chitooligosaccharides.  No other activities have been observed.
 The lytic transglycosylases of GH23 constitute Family 1 of the organizational scheme of Blackburn and Clarke <cite>1</cite>.  This family has been subdivided into five subfamilies (1A-1E) with ''Escherichia coli'' soluble lytic transglycosylase 70 (Slt70), membrane-bound lytic transglycosylase C (MltC), MltE, MltD, and MltF (formerly, YfhD) serving as the prototypes for families 1A, 1B, 1C, 1D, and 1E, respectively.
 == Kinetics and Mechanism ==
-[[Image:LT-GEWL.jpg|thumb|right|'''Figure 1.''' Lytic and hydrolytic pathways of GH23 enzymes.  GEWL, g-type lysozymes; LT, lytic transglycosylase.  (''click to enlarge'').]]The enzymes of this family cleave the β-1,4 linkage between ''N''-acetylmuramoyl and ''N''-acetylglucosaminyl residues in peptidoglycan (Figure 1) .  Only the lysozymes of this family are capable of releasing ''N''-acetyl-{{Smallcaps|d}}-glucosamine residues from chitodextrins, and neither catalyze (inter) transglycosylation reactions.  The stereochemistry of the reaction catalysed by the family G lysozymes has been determined experimentally as inverting <cite>10</cite>.  On the other hand, the lytic transglycosidases, strictly speaking, are retaining enzymes.  However, unlike lysozyme, 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>4</cite>.   The lytic transglycosylases require the peptide side chains in peptidoglycan for activity, accounting for their inactivity against chitin or chitooligosaccharides <cite>5</cite>.  No detailed analyses involving both steady state and pre-steady state kinetic studies have been reported.
+[[Image:LT-GEWL.jpg|thumb|right|'''Figure 1.''' Lytic and hydrolytic pathways of GH23 enzymes.  GEWL, g-type lysozymes; LT, lytic transglycosylase.  (''click to enlarge'').]]The enzymes of this family cleave the β-1,4 linkage between ''N''-acetylmuramoyl and ''N''-acetylglucosaminyl residues in peptidoglycan (Figure 1).  Only the lysozymes of this family are capable of releasing ''N''-acetyl-{{Smallcaps|d}}-glucosamine residues from chitodextrins, and neither catalyze (inter) transglycosylation reactions.  The stereochemistry of the reaction catalysed by the family G lysozymes has been determined experimentally as [[inverting]] <cite>10</cite>.  On the other hand, the lytic transglycosidases, strictly speaking, are [[retaining]] enzymes.  However, unlike lysozyme, they are not hydrolases but rather catalyse an intramolecular glycosyl transfer reaction to the C-6 hydroxyl group of the muramoyl residue leading to the generation of a terminal 1,6-anhydromuramoyl product thus lacking a reducing end <cite>4</cite>.   The lytic transglycosylases require the peptide side chains in peptidoglycan for activity, accounting for their inactivity against chitin or chitooligosaccharides <cite>5</cite>.  No detailed analyses involving both steady state and pre-steady state kinetic studies have been reported.
 == Catalytic Residues ==
-[[Image:LTmechanism.jpg|thumb|right|'''Figure 2.''' Detailed catalytic mechanism of GH23 lytic transglycosylases (''click to enlarge''). ]]Until recently, the family GH23 enzymes were thought to have only a single catalytic residue at their catalytic centre. The identity of the catalytic acid/base residue of the lysozymes was first inferred by X-ray crystallography of goose egg-white lysozyme (GEWL) as Glu 73 <cite>6 7</cite>.   Likewise, analysis of the crystal structure of ''E. coli'' Slt70 identified Glu as the lone catalytic residue<cite>8</cite>.   Replacement of each respective residue results in loss of catalytic activity <cite>9 10</cite>.  The mechanism of action of the family GH23 enzymes has yet to be proven experimentally, but examination of crystal structures and theoretical considerations have led to separate proposals for the two classes of enzymes.
+[[Image:LTmechanism.jpg|thumb|right|'''Figure 2.''' Detailed catalytic mechanism of GH23 lytic transglycosylases (''click to enlarge''). ]]Until recently, the family GH23 enzymes were thought to have only a single catalytic residue at their catalytic centre. The identity of the catalytic [[general acid/base]] residue of the lysozymes was first inferred by X-ray crystallography of goose egg-white lysozyme (GEWL) as Glu73 <cite>6 7</cite>.   Likewise, analysis of the crystal structure of ''E. coli'' Slt70 identified Glu as the lone catalytic residue<cite>8</cite>.   Replacement of each respective residue results in loss of catalytic activity <cite>9 10</cite>.  The mechanism of action of the family GH23 enzymes has yet to be proven experimentally, but examination of crystal structures and theoretical considerations have led to separate proposals for the two classes of enzymes.
-With the lytic transglycosylases, there is still no evidence for a second catalytic residue at their active sites.  Hence, based on the complexes formed with 1,6-anhydromuropeptide <cite>12</cite> or bulgecin <cite>11</cite>, a  substrate-assisted mechanism, analogous to the family [[GH18]] chitinases and chitobiases and family [[GH20]] ''N''-acetyl-β-hexosaminidases, has been invoked.  Thus, the catalytic Glu73 is proposed to serve initially as an acid catalyst to donate a proton to the glycosidic oxygen of the linkage to be cleaved leading to the formation of an intermediate with oxocarbenium ion character (Figure 2).   In the absence of an anion/nucleophile in close proximity to stabilize this oxocarbenium intermediate,  the lytic transglycosylases  would employ anchimeric assistance of the MurNAc 2-acetamido group resulting in the formation an oxazolinium ion intermediate.  This would be followed by abstraction of the C-6 hydroxyl proton of the oxazolinium species involving Glu73 which now serves as the base catalyst leading to nucleophilic attack and the formation of 1,6-anhydromuramic acid product.
+With the lytic transglycosylases, there is still no evidence for a second catalytic residue at their active sites.  Hence, based on the complexes formed with 1,6-anhydromuropeptide <cite>12</cite> or bulgecin <cite>11</cite>, a  mechanism that invokes [[neighboring group participation]] or substrate assisted catalysis, analogous to the family [[GH18]] chitinases and chitobiases and family [[GH20]] ''N''-acetyl-β-hexosaminidases, has been invoked.  Thus, the catalytic Glu73 is proposed to serve initially as a [[general acid]] to donate a proton to the glycosidic oxygen of the linkage to be cleaved leading to the formation of an intermediate with oxocarbenium ion character (Figure 2). In the absence of an anion/nucleophile in close proximity to stabilize this oxocarbenium intermediate,  the lytic transglycosylases  would employ anchimeric assistance of the MurNAc 2-acetamido group resulting in the formation an oxazolinium ion intermediate.  This would be followed by abstraction of the C-6 hydroxyl proton of the oxazolinium species involving Glu73 which now serves as the base catalyst leading to nucleophilic attack and the formation of 1,6-anhydromuramic acid product.
-For the g-type lysozymes, recent studies support a typical inverting mechanism of action <cite>2 3</cite> involving catalytic acid and base residues.  In fact, it is possible that two catalytic general base residues, a primary and a secondary residue, position a catalytic water molecule and abstract a proton to affect substrate hydrolysis by the single displacement mechanism.  With GEWL, Asp97 and Asp86, respectively, are proposed to serve this function while these would be represented by the conserved Asp101 and Asp90 residues in the g-type lysozyme from Atlantic cod fish.  Indeed, the double replacement of Asp101 and Asp90 in the cod lysozyme results in over a 300-fold decrease in activity <cite>2</cite>.
+For the g-type lysozymes, recent studies support a typical inverting mechanism of action <cite>2 3</cite> involving catalytic acid and base residues.  In fact, it is possible that two catalytic [[general base]] residues, a primary and a secondary residue, position a catalytic water molecule and abstract a proton to affect substrate hydrolysis by the single displacement mechanism.  With GEWL, Asp97 and Asp86, respectively, are proposed to serve this function while these would be represented by the conserved Asp101 and Asp90 residues in the g-type lysozyme from Atlantic cod fish.  Indeed, the double replacement of Asp101 and Asp90 in the cod lysozyme results in over a 300-fold decrease in activity <cite>2</cite>.
 == Three-dimensional structures ==
@@ Line 48: / Line 48: @@
 == Family Firsts ==
 ;First identification of lytic transglycosylase: Soluble lytic transglycosylase 70 <cite>4</cite>.
-;First catalytic nucleophile identification: For g-type lysozymes <cite>3</cite>; Not applicable for lytic transglycosylases.
+;First [[catalytic nucleophile]] identification: For g-type lysozymes <cite>3</cite>; Not applicable for lytic transglycosylases.
-;First general acid/base residue identification: Inferred by X-ray crystallography of goose egg-white lysozyme <cite>7</cite>.
+;First [[general acid/base]] residue identification: Inferred by X-ray crystallography of goose egg-white lysozyme <cite>7</cite>.
 ;First 3-D structure: Goose egg-white lysozyme <cite>13</cite>.