File:Warren GlycoNet.jpg

2020-05-27T23:38:01Z

Warren Wakarchuk:

User:Warren Wakarchuk

2020-05-27T23:35:56Z

Warren Wakarchuk:

Glycosyltransferase Family 42

2010-04-15T15:04:02Z

Warren Wakarchuk:

{{UnderConstruction}}
* [[Author]]: ^^^Warren Wakarchuk^^^
* [[Responsible Curator]]: ^^^Warren Wakarchuk^^^
----


<div style="float:right">
{| {{Prettytable}}
|-
|{{Hl2}} colspan="2" align="center" |'''Glycosyltransferase Family GT42'''
|-
|'''Clan'''
|GT-x
|-
|'''Mechanism'''
|retaining/inverting
|-
|'''Active site residues'''
|His188 is the catalytic base in the Campylobacter jejuni GT-42 known as Cst-II
|-
|{{Hl2}} colspan="2" align="center" |'''CAZy DB link'''
|-
| colspan="2" |http://www.cazy.org/fam/GT42.html
|}
</div>


== Substrate specificities ==
The enzymes in GT-42 were originally examined from isolates of ''C. jejuni'' that express a number of ganglioside mimics, some of which have an a2,8-a2,3 linked di-sialic acid moiety <cite>Gilbert2008</cite>. The CjGT-42 known as Cst-II was found as part of the LOS biosynthesis operon <cite>Gilbert2000</cite>, which facilitated the correlation of gene content to LOS structure. Based on the LOS structures which contained both a-2,3 and a-2,8 linked sialic acid it was predicted that there should be two sialyltransferases, but the surprising finding was that a single CjGT-42 enzyme, Cst-II, was making both linkages <cite>Gilbert2002</cite>. This bi-functional enzyme has been shown to be a determinant in the development of post-infectious neuropathies (Guillain-Barré syndrome), due to an anti-ganglioside antibody response in some patients who were infected with ''C. jejuni'' <cite>Koga2005</cite> <cite>vanBelkum2001</cite>. Some species of ''Campylobacter'' express a second GT-42 enzyme known as Cst-I, which was in fact cloned first, but it has been shown not to be involved in LOS biosynthesis on the basis of gene knockouts in strains with both cst genes (Michel Gilbert, personal communication). Cst-I contains an extended C-terminal domain of unknown function, and the target acceptor of this second CjGT-42 is not known at present. The GT-42 family also contains members from ''H. influenzae'' and ''P. multocida'', in which the GT-42 enzymes are one of two or three resident sialyltransferases. In ''H. influenzae'' the first GT-42 enzyme to be described was the a-2,3-sialyltransferase Lic3A (Hood et al. 2001), followed by a second a-2,3/2,8-bi-functional version known as Lic3B <cite>Fox2006</cite>. In ''H. influenzae'' the ''in vivo'' acceptor for Lic3A/B is the simple disaccharide lactose in contrast to the more complex ganglioside mimics seen in ''C. jejuni'' <cite>Schweda2007</cite>.

== Kinetics and Mechanism ==
Detailed kinetic studies have revealed that this enzyme follows an unusual steady state iso ordered bi bi kinetic mechanism for carrying out sialyl transfer with inversion <cite>Chan2009</cite>. His188 appears to be the catalytic base, which is somewhat unusual since this role is generally performed by Glu or Asp residues. Direct measurement of its pKa by NMR showed the pKa agrees with that deduced from the pH dependence of this reaction. This required the generation of an active monomeric form of the tetrameric enzyme <cite>Chan2009</cite>. Chemical rescue of the H188A mutant was performed with nucleophilic anions adding weight to the assignment of the function, and further supporting the SN2-like inverting mechanism.

== Catalytic Residues ==
His188 in CjGT-42 Cst-II from strain OH4383 appears to be the catalytic base.

== Three-dimensional structures ==
The CjGT-42 enzymes Cst-I and Cst-II were the first sialyltransferases whose structure was determined by X-ray crystallography <cite>chiu2004</cite><cite>chiu2007</cite>, and they share the same basic structure. GT-42 enzymes have a modified GT-A type fold characterized by a single a/b Rossmann fold nucleotide binding domain, and a flexible lid domain composed of a coil and two helices (Fig. 10). Unlike the typical GT-A fold, CjGT-42 enzymes lack the conserved DXD motif and are metal-ion independent. The enzyme structure revealed a tetramer (Fig. 8) in which each monomer carries an independent active site, and examination of the active site residues suggested a conserved histidine, His188 in Cst-II, as the catalytic base (Fig. 9)

== Family Firsts ==

;First general acid/base residue identification:
Cite some reference here, with a ''short'' (1-2 sentence) explanation
;First 3-D structure:
The CjGT-42 enzymes Cst-I and Cst-II were the first sialyltransferases whose structure was determined by X-ray crystallography (Chiu et al. 2004), (Chiu et al. 2007), and they share the same basic structure. GT-42 enzymes have a modified GT-A type fold characterized by a single a/b Rossmann fold nucleotide binding domain, and a flexible lid domain composed of a coil and two helices (Fig. 10).

<gallery widths=320px heights=240px perrow=2 caption="PDB ID 1RO8 from GH58 (click images for large versions)">
File:Cst II tetramer with ligand cmyk.png|top view
File:|side view
</gallery>
Cst II tetramer with ligand cmyk.png

== References ==
<biblio>
# Gilbert2008 isbn=978-1-55581-437-3
# Gilbert2002 pmid=11689567
# Gilbert2000 pmid=10660542
# Koga2005 pmid=16162859
# Vanbelkum2001 pmid=11433317
# hood2001 pmid=11136455
# fox2006 pmid=17071616
# scweda2007 pmid=17452015
# chan2009 pmid=19824695
# chiu2004 pmid=14730352
# chiu2007 pmid=17518445

</biblio>

[[Category:Glycosyltransferase Families|GH042]]

Glycosyltransferase Family 42

2010-04-15T15:01:11Z

Warren Wakarchuk:

File:Cst II tetramer with ligand cmyk.png

2010-04-15T14:13:23Z

Warren Wakarchuk:

Glycosyltransferase Family 42

2010-04-15T14:11:17Z

Warren Wakarchuk: