Campylobacter jejuni i
s highly unu
sual among bacteria in forming
N-linked glycoprotein
s. Thehepta
saccharide produced by it
s pgl sy
stem i
s attached to protein A
sn through it
s terminal 2,4-diacetamido-2,4,6-trideoxy-
D-Glc (QuiNAc4NAc or
N,
N'-diacetylbacillo
samine) moiety. The crucial, la
st part of thi
s sugar'
ssynthe
si
s i
s the acetylation of UDP-2-acetamido-4-amino-2,4,6-trideoxy-
D-Glc by the enzyme PglD, with acetyl-CoA a
s a co
sub
strate. We have determined the cry
stal
structure
s of PglD in CoA-bound and unbound form
s,refined to 1.8 and 1.75 Å re
solution, re
spectively. PglD i
s a trimer of
subunit
s each compri
sed of two domain
s,an N-terminal
![](/image<font color=)
s/gifchar
s/alpha.gif" BORDER=0>/
![](/image<font color=)
s/gifchar
s/beta2.gif" BORDER=0 ALIGN="middle">-domain and a C-terminal left-handed
![](/image<font color=)
s/gifchar
s/beta2.gif" BORDER=0 ALIGN="middle">-helix. Few
structural difference
s accompany CoAbinding, except in the C-terminal region following the
![](/image<font color=)
s/gifchar
s/beta2.gif" BORDER=0 ALIGN="middle">-helix (re
sidue
s 189-195), which adopt
s an extended
structure in the unbound form and fold
s to extend the
![](/image<font color=)
s/gifchar
s/beta2.gif" BORDER=0 ALIGN="middle">-helix upon binding CoA. Computational moleculardocking
sugge
st
s a different mode of nucleotide-
sugar binding with re
spect to the acetyl-CoA donor, with themolecule
s arranged in an "L-
shape", compared with the "in-line" orientation in related enzyme
s. Modelingindicate
s that the oxyanion intermediate would be
stabilized by the NH group of Gly143', with Hi
s125' themo
st likely re
sidue to function a
s a general ba
se, removing H
+ from the amino group prior to nucleophilicattack at the carbonyl carbon of acetyl-CoA. Site-
specific mutation
s of active
site re
sidue
s confirmed theimportance of Hi
s125', Glu124', and A
sn118. We conclude that A
sn118 exert
s it
s function by
stabilizing theintricate hydrogen bonding network within the active
site and that Glu124' may function to increa
se the p
Kaof the putative general ba
se, Hi
s125'.