Journal article
Enzymatic synthesis of Gb3 and iGb3 ceramides
Gb3 and iGb3 are physiologically important trihexosylceramides with a terminal alpha-d-Galp-(1-->4)-beta-d-Galp- and alpha-d-Galp-(1-->3)-beta-d-Galp sequence, respectively. In particular iGb3 is attracting considerable attention as it is believed to serve as a ligand for natural killer T cells. Whether or not iGb3 is present in humans and which enzyme might be responsible for its synthesis is at present a matter of lively debate.
In the current investigation we evaluated human blood group B galactosyltransferase (GTB) for its ability to catalyze the formation of iGb3 from lactosylceramide and UDP-Galp. GTB is a retaining glycosyltransferase that in vivo catalyzes the transfer of galactose from UDP-Galp donors to OH-3 of Galp on the H-antigen (alpha-l-Fucp-(1-->2)-beta-d-Galp) acceptor forming the blood group B antigen.
GTB tolerates modifications in donor and acceptor substrates and its ability to accept lactosides as acceptors makes it a possible candidate for iGb3 production in humans. For comparison iGb3 and Gb3 were also synthesized from the same acceptor using an alpha-(1-->3)- and alpha-(1-->4)-specific galactosyltransferase, respectively.
All the enzymes tested catalyzed the desired reactions. Product characterization by NMR analysis clearly differentiated between the alpha-Galp-(1-->3)-Galp and alpha-Galp-(1-->4)-Galp product, with the GTB product being identical to that of the alpha-(1-->3)-GalT-catalyzed reaction. The rate of transfer by GTB however was very low, only 0.001% of the rate obtained with a good substrate, H antigen disaccharide (octyl alpha-l-Fucp-(1-->2)-beta-d-Galp).
This is too low to account for the possible formation of the iGb3 structure in humans in vivo.
Language: | English |
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Publisher: | Elsevier BV |
Year: | 2010 |
Pages: | 1384-1388 |
ISSN: | 1873426x and 00086215 |
Types: | Journal article |
DOI: | 10.1016/j.carres.2010.02.006 |
Animals Bovine α-(1→3)-galactosyltransferase Carbohydrate Sequence Cattle Galactosyltransferases Gb3 Globosides Human blood group B galactosyltransferase Humans Lactosylceramides NKT cells Neisseria meningitidis Trihexosylceramides Uridine Diphosphate Galactose globotrihexosylceramide iGb3 isoglobotrihexosylceramide α-(1→4)-Galactosyltransferase