Oligosaccharide binding site of b1,4-galactosyltransferases (Gal-Ts)
defined by docking various glycan substrates in the binding site.    
Qasba, Pradman K.; Ramakrishnan, B.; Balaji, P. V..    LECB, CCR,  NCI,
NIH,  Frederick,  MD,  USA.    Abstracts of Papers, 223rd ACS National
Meeting, Orlando, FL, United States, April 7-11, 2002  (2002),    
COMP-217.  Publisher: American Chemical Society,  Washington, D. C  CODEN:
69CKQP  Conference; Meeting Abstract  written in English.    AN 2002:188618
   CAPLUS   (Copyright 2004 ACS on SciFinder (R))  

Abstract

b 1,4-galactosyltransferase (Gal-T) family members (Gal-T1- Gal-T5)
transfer galactose from UDP-a-Gal to N-acetylglucosamine (GlcNAc) residue
of an oligosaccharide of glycoprotein or glycolipid thereby linking Gal to
GlcNAc residue by b 1,4-linkage.  Crystal structures of bovine Gal-T1,
either in complex with UDP-a-Gal and Mn2+, or with GlcNAc and a-Lactalbumin
(LA), or crystal structure of human Gal-T1 and LA complex, reveal that
Gal-T1 undergoes a conformational change upon ligand binding which creates
an oligosaccharide binding site, metal binding site and an interacting site
for LA.  The crystal structures reveal that the oligosaccharide binding
site is an "open canal" shaped extended site with an av. width and length
of 10 .ANG. and 16 .ANG., resp., that lay behind the GlcNAc binding site. 
In order to probe the size and nature of the oligosaccharide binding site a
modeling study of the docking of various disaccharides and N-glycan ligands
in the binding site were carried out.  Each ligand conformation, inter-mol.
interaction energy between Gal-T1 and the saccharide was calcd. using CVFF
force field and Discover module of InsightII.  Only protein residues within
9 .ANG. from any of the ligand atoms were considered for energy calcns. 
The total energy, comprising the intermol. protein - ligand interaction
energy and intermol. ligand energy, was used as a guide in detg. the
possible allowed conformations of the ligand in the binding site.  These
modeling studies show that GlcNAc with an a-linked substitution such as
a-benzyl-GlcNAc cannot bind to Gal-T1 because of severe steric contacts
with the highly conserved Tyr286 residue, whereas GlcNAc with a b-linked
substitution such as b-benzyl-GlcNAc can bind without any steric hindrance.
 Docking of a biantennary N-glycan with GlcNAc at its reducing ends in the
extended sugar binding site reveals that the acceptor binding site in
Gal-T1 can accommodate a linear pentasaccharide all the way from the GlcNAc
moiety to the asparagine-linked GlcNAc.  The binding site can also
accommodate either the a-1-3 arm (GlcNAcb1-2Mana1-3Manb1-4GlcNAc b
1-4GlcNAc-N) or a-1-6 arm (GlcNAcb1-2Mana1-6Man b 1-4GlcNAc b 1-4GlcNAc-N)
of the N-glycan without any steric hindrance.