Sialyltransferases are
enzymes that transfer
sialic acid Sialic acids are a class of alpha-keto acid sugars with a nine-carbon backbone.
The term "sialic acid" (from the Greek for saliva, - ''sÃalon'') was first introduced by Swedish biochemist Gunnar Blix in 1952. The most common member of this gr ...
to nascent
oligosaccharide. Each sialyltransferase is specific for a particular
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...
substrate. Sialyltransferases add sialic acid to the terminal portions of the sialylated
glycolipid
Glycolipids are lipids with a carbohydrate attached by a glycosidic (covalent) bond. Their role is to maintain the stability of the cell membrane and to facilitate cellular recognition, which is crucial to the immune response and in the connec ...
s (
gangliosides) or to the N- or O-linked sugar chains of
glycoprotein
Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...
s.
The biosynthesis of disaccharides, oligosaccharides and polysaccharides involves the action of hundreds of different glycosyltransferases. These enzymes catalyse the transfer of sugar moieties from activated donor molecules to specific acceptor molecules, forming glycosidic bonds. A classification of glycosyltransferases using nucleotide diphospho-sugar, nucleotide monophospho-sugar and sugar phosphates () and related proteins into distinct sequence based families has been described.
This classification is available on the CAZy (CArbohydrate-Active EnZymes) web site.
The same three-dimensional fold is expected to occur within each of the families. Because 3-D structures are better conserved than sequences, several of the families defined on the basis of sequence similarities may have similar 3-D structures and therefore form 'clans'.
Sialyltransferases belong to glycosyltransferase family 29
CAZY GT_29 which comprises enzymes with a number of known activities; sialyltransferase (), beta-galactosamide alpha-2,6-sialyltransferase (), alpha-N-acetylgalactosaminide alpha-2,6-sialyltransferase (), beta-galactoside alpha-2,3-sialyltransferase (), N-acetyllactosaminide alpha-2,3-sialyltransferase (), alpha-N-acetyl-neuraminide alpha-2,8-sialyltransferase (); lactosylceramide alpha-2,3-sialyltransferase (). These enzymes use a nucleotide monophosphosugar as the donor (CMP-NeuA) instead of a nucleotide diphosphosugar.
Sialyltransferase may be responsible for the synthesis of the sequence NEUAC-Alpha-2,3-GAL-Beta-1,3-GALNAC-, found on sugar chains O-linked to thr or ser and also as a terminal sequence on certain gangliosides. These enzymes catalyse sialyltransfer reactions during glycosylation, and are type II membrane proteins.
There are about twenty different sialyltransferases which can be distinguished on the basis of the acceptor structure on which they act and on the type of sugar linkage they form. For example, a group of sialyltransferases adds sialic acid with an alpha-2,3 linkage to
galactose, while other sialyltransferases add sialic acid with an alpha-2,6 linkage to galactose or
''N''-acetylgalactosamine. A peculiar type of sialyltransferases add sialic acid to other sialic acid units with an alpha-2,8 linkage, forming structures referred to as
polysialic acid. As occurs for other
glycosyltransferases, the expression of sialyltransferases undergoes profound modifications during
cell differentiation
Cellular differentiation is the process in which a stem cell alters from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellula ...
and
neoplastic transformation; in some cases such changes induce
phenotypic alterations.
Human proteins containing this domain
SIAT4C;
SIAT9;
ST3GAL1;
ST3GAL2;
ST3GAL3
ST3 beta-galactoside alpha-2,3-sialyltransferase 3, also known as ST3GAL3, is a protein which in humans is encoded by the ''ST3GAL3'' gene.
Function
The protein encoded by this gene is a type II membrane protein that catalyzes the transfer of ...
;
ST3GAL4
is an enzyme that in humans is encoded by the ''ST3GAL4'' gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'' ...
;
ST3GAL5;
ST3GAL6;
ST3GalIII;
ST6GAL1;
ST6GAL2;
ST6Gal;
ST8SIA1;
ST8SIA2;
ST8SIA3;
ST8SIA4;
ST8SIA5;
ST8SIA6;
ST8Sia;
References
Further reading
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External links
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Transferases
EC 2.4
{{2.4-enzyme-stub