An oligosaccharide (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek
ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a
saccharide polymer
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
containing a small number (typically two to ten) of
monosaccharides (simple sugars). Oligosaccharides can have many functions including
cell recognition and
cell adhesion.
They are normally present as
glycans: oligosaccharide chains are linked to
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...
s or to compatible
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
side chains in
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s, by ''N''- or ''O''-
glycosidic bonds
A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.
A glycosidic bond is formed between the hemiacetal or hemiketal gr ...
. ''N''-Linked oligosaccharides are always pentasaccharides attached to
asparagine
Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...
via a beta linkage to the amine nitrogen of the side chain.
[.] Alternately,
''O''-linked oligosaccharides are generally attached to
threonine or
serine on the alcohol group of the side chain. Not all natural oligosaccharides occur as components of glycoproteins or glycolipids. Some, such as the
raffinose
Raffinose is a trisaccharide composed of galactose, glucose, and fructose. It can be found in beans, cabbage, brussels sprouts, broccoli, asparagus, other vegetables, and whole grains. Raffinose can be hydrolyzed to D-galactose and sucrose ...
series, occur as storage or transport
carbohydrates in plants. Others, such as
maltodextrins or
cellodextrins
Cellodextrins are glucose polymers (polysaccharides) of varying length (two or more glucose monomers) resulting from cellulolysis, the breakdown of cellulose.
Classification
A cellodextrin is classified by its degree of polymerization (DP) which ...
, result from the microbial breakdown of larger
polysaccharides such as
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human die ...
or
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
.
Glycosylation
In biology,
glycosylation is the process by which a carbohydrate is covalently attached to an organic molecule, creating structures such as glycoproteins and glycolipids.
''N''-Linked oligosaccharides
''N''-Linked glycosylation involves oligosaccharide attachment to
asparagine
Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...
via a beta linkage to the amine nitrogen of the side chain.
The process of ''N''-linked glycosylation occurs cotranslationally, or concurrently while the proteins is being translated. Since it is added cotranslationally, it is believed that ''N''-linked glycosylation helps determine the folding of polypeptides due to the hydrophilic nature of sugars. All ''N''-linked oligosaccharides are pentasaccharides: five monosaccharides long.
In
''N''-glycosylation for eukaryotes, the oligosaccharide substrate is assembled right at the membrane of the
endoplasmatic reticulum. For
prokaryote
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Con ...
s, this process occurs at the
plasma membrane. In both cases, the acceptor substrate is an
asparagine
Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...
residue. The asparagine residue linked to an ''N''-linked oligosaccharide usually occurs in the sequence Asn-X-Ser/Thr,
where X can be any amino acid except for
proline, although it is rare to see Asp, Glu, Leu, or Trp in this position.
''O''-Linked oligosaccharides
Oligosaccharides that participate in ''O''-linked glycosylation are attached to
threonine or
serine on the
hydroxyl group of the side chain.
''O''-linked glycosylation occurs in the
Golgi apparatus
The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles i ...
, where monosaccharide units are added to a complete polypeptide chain. Cell surface proteins and extracellular proteins are ''O''-glycosylated.
Glycosylation sites in ''O''-linked oligosaccharides are determined by the
secondary and
tertiary structures of the polypeptide, which dictate where
glycosyltransferases will add sugars.
Glycosylated biomolecules
Glycoproteins and glycolipids are by definition
covalently bonded to carbohydrates. They are very abundant on the surface of the cell, and their interactions contribute to the overall stability of the cell.
Glycoproteins
Glycoproteins have distinct Oligosaccharide structures which have significant effects on many of their properties, affecting critical functions such as
antigenicity,
solubility
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
, and resistance to
protease
A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
s. Glycoproteins are relevant as
cell-surface receptors
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (th ...
, cell-adhesion molecules,
immunoglobulins, and tumor antigens.
Glycolipids
Glycolipids are important for cell recognition, and are important for modulating the function of membrane proteins that act as receptors.
Glycolipids are lipid molecules bound to oligosaccharides, generally present in the
lipid bilayer. Additionally, they can serve as receptors for cellular recognition and cell signaling.
The head of the oligosaccharide serves as a binding partner in
receptor
Receptor may refer to:
*Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse
*Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...
activity. The binding mechanisms of receptors to the oligosaccharides depends on the composition of the oligosaccharides that are exposed or presented above the surface of the membrane. There is great diversity in the binding mechanisms of glycolipids, which is what makes them such an important target for pathogens as a site for interaction and entrance. For example, the
chaperone activity of glycolipids has been studied for its relevance to HIV infection.
Functions
Cell recognition
All cells are coated in either glycoproteins or glycolipids, both of which help determine cell types.
Lectins, or proteins that bind carbohydrates, can recognize specific oligosaccharides and provide useful information for cell recognition based on oligosaccharide binding.
An important example of oligosaccharide cell recognition is the role of glycolipids in determining
blood types. The various blood types are distinguished by the glycan modification present on the surface of blood cells. These can be visualized using mass spectrometry. The oligosaccharides found on the A, B, and H
antigen
In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respon ...
occur on the
non-reducing ends of the oligosaccharide. The H antigen (which indicates an O blood type) serves as a precursor for the A and B antigen.
Therefore, a person with A blood type will have the A antigen and H antigen present on the glycolipids of the red blood cell plasma membrane. A person with B blood type will have the B and H antigen present. A person with AB blood type will have A, B, and H antigens present. And finally, a person with O blood type will only have the H antigen present. This means all blood types have the H antigen, which explains why the O blood type is known as the "universal donor".
How do transport vesicles know the final destination of the protein that they are transporting?
Vesicles are directed by many ways, but the two main ways are:
# The sorting signals encoded in the amino acid sequence of the proteins.
# The Oligosaccharide attached to the protein.
The sorting signals are recognised by specific receptors that reside in the membranes or surface coats of budding vesicles, ensuring that the protein is transported to the appropriate destination.
Cell adhesion
Many cells produce specific carbohydrate-binding proteins known as lectins, which mediate cell adhesion with oligosaccharides.
Selectins, a family of lectins, mediate certain cell–cell adhesion processes, including those of leukocytes to endothelial cells.
In an immune response, endothelial cells can express certain selectins transiently in response to damage or injury to the cells. In response, a reciprocal selectin–oligosaccharide interaction will occur between the two molecules which allows the white blood cell to help eliminate the infection or damage. Protein-Carbohydrate bonding is often mediated by
hydrogen bonding and
van der Waals force
In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and ...
s.
Dietary oligosaccharides
Fructo-oligosaccharides (FOS), which are found in many vegetables, are short chains of
fructose molecules. They differ from
fructan
A fructan is a polymer of fructose molecules. Fructans with a short chain length are known as fructooligosaccharides. Fructans can be found in over 12% of the angiosperms including both monocots and dicots such as agave, artichokes, asparagus, l ...
s such as
inulin, which as polysaccharides have a much higher
degree of polymerization
The degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule.
For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymerization is given by ...
than FOS and other oligosaccharides, but like inulin and other fructans, they are considered soluble dietary fibre. Using fructo-oligosaccharides (FOS) as fiber supplementations is shown to have an effect on glucose homeostasis quite similar to insulin. These (FOS) supplementations which can be considered Prebiotics which produce something called a short-chain fructo-oligosaccharides (scFOS). Galacto-oligosaccharides (GOS) in particular are used to create a prebiotic effect for infants that are not being breastfed.
Galactooligosaccharides
Galactooligosaccharides (GOS), also known as oligogalactosyllactose, oligogalactose, oligolactose or transgalactooligosaccharides (TOS), belong to the group of prebiotics. Prebiotics are defined as non-digestible food ingredients that beneficiall ...
(GOS), which also occur naturally, consist of short chains of
galactose molecules.
Human milk is an example of this and contains oligosaccharides, known as
human milk oligosaccharide
Human milk oligosaccharides (HMOs), also known as human milk glycans, are short polymers of simple sugars that can be found in high concentrations in human breast milk. Human milk oligosaccharides promote the development of the immune system, ca ...
s (HMOs), which are derived from
lactose.
These oligosaccharides have
biological function in the development of the
gut flora of
infants. Examples include
lacto-N-tetraose, lacto-N-neotetraose, and lacto-N-fucopentaose.
These compounds cannot be
digested in the human
small intestine, and instead pass through to the
large intestine, where they promote the growth of ''
Bifidobacteria
''Bifidobacterium'' is a genus of gram-positive, nonmotile, often branched anaerobic bacteria. They are ubiquitous inhabitants of the gastrointestinal tract though strains have been isolated from the vagina and mouth ('' B. dentium'') of ma ...
'', which are beneficial to gut health.
HMOs can also protect infants by acting as decoy receptors against viral infection. HMOs accomplish this by mimicking viral receptors which draws the virus particles away from host cells.
Experimentation has been done to determine how glycan-binding occurs between HMOs and many viruses such as influenza, rotavirus,
human immunodeficiency virus (HIV), and respiratory syncytial virus (RSV). The strategy HMOs employ could be used to create new antiviral drugs.
Mannan oligosaccharides (MOS) are widely used in
animal feed to improve gastrointestinal health. They are normally obtained from the yeast cell walls of ''
Saccharomyces cerevisiae
''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have b ...
''. Mannan oligosaccharides differ from other oligosaccharides in that they are not fermentable and their primary mode of action includes agglutination of type-1 fimbria pathogens and immunomodulation.
Sources
Oligosaccharides are a component of
fibre from plant tissue. FOS and inulin are present in
Jerusalem artichoke,
burdock,
chicory,
leeks,
onion
An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the on ...
s, and
asparagus. Inulin is a significant part of the daily diet of most of the world’s population. FOS can also be synthesized by enzymes of the fungus ''
Aspergillus niger'' acting on
sucrose. GOS is naturally found in
soybeans and can be synthesized from
lactose. FOS, GOS, and inulin are also sold as nutritional supplements.
See also
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References
External links
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Nutrition
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Carbohydrate chemistry