Download Glycoconjugates

Carbohydrates
Carbohydrates
•Basic molecular formula (CH2O)n or
(C·H2O)n
•Photosynthesis
Glycoconjugates
•Important components of cell walls and
extracellular structures in plants,
animals , and bacteria
•Glycolipids
--- lipid molecules
--- biological membranes
•Glycoprotein
--- protein
Three groups of carbohydrates
•Monosaccharides
--- simple sugars
--- (CH2O)n
--- cannot be broken down into smaller sugars
•Oligosccharides
--- “
few”
--- 2~10 simple sugar molecules
--- disaccharides are common in nature, and
trisaccharides also occur frequently
--- 4~6-sugar-unit oligosaccharides are
usually bound covalently to other molecules,
including glycoprotein
Chemical features of
carbohydrates
• The existence of at least one and often two or more
asymmetric centers (不對稱中心)
• The ability to exist either in linear or ring structures
• The capacity to form polymeric structures via
glycosidic bonds
• The potential to form multiple hydrogen bonds with
water or other molecules in their environment
Three groups of carbohydrates
•Polysaccharides
--- polymers of the simple sugars and their
derivatives
--- linear or branched polymers
--- hundreds or even thousands of
monosaccharide units
--- molecular weights range up to 1 million or
more
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Structure and chemistry of
monosaccharide
• 3~7 carbon atoms
• Aldoses or ketoses (aldehyde function or ketone group)
• Trioses
•Hexoses are the most abundant
sugars in nature
•The simplest monosaccharides are
water soluble, and most taste sweet
Stereochemistry is a prominent
feature of monosaccharides
•Chiral centers (對掌中心、不對稱中心)
•Fisher projection
--- D (+; plus) --- D-glucose or D(+)-glucose; dextrorotatory
--- L (-; minus) --- D-fructose or D(-)-fructose; levorotatory
•Enantiomer (鏡像異構物)
•Diastereomer (非對映〔異構〕體 )
•Epimer (差向異構體 ):非鏡形異構物的一種，兩個結構式
中，只有一個OH基的方向不同，其
他的OH基方向皆相同
Monosaccharides exist in cyclic
and anomeric forms (異位異構物 )
•Fischer projections are useful for presenting
the structure of particular monosaccharides
and their stereoisomers
•Haworth projection is a common way of
representing the cyclic structure of
monosaccharide with a simple threedimensional perspective.
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Hemiacetals
(半縮醛)
六環異構物
Pyranose (吡喃糖)
Anomer
(異頭物 )
Hemiketals (半羧酮 )
五環異構物
Furanose
Monosaccharides can be converted to
several derivative forms
•A variety of chemical and enzymatic reactions
produce derivatives of the simple sugars
--- sugar acids
--- sugar alcohols
--- deoxy sugars
--- sugar esters
--- amino sugars
--- muramic acid and neuraminic acid
--- acetals ketals and glycosides
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Aldonic acid
Sugar acids
•Sugars with free anomeric carbon atoms
are reasonably good reducing agents and
reduce hydrogen peroxide, ferricyanide,
certain metals (Cu2+ and Ag2+) and other
agents  sugar acids
•Reducing sugars
Uronic acid
Aldaric acid
Gluconic acid
Deoxy sugars
Sugar alcohols
•Prepared by the mild reduction (with NaBH4
or similar agents) of the carbonyl groups of
aldoses and ketoses
•Alditols --- Sweet tasting
•Are monosaccharides with one or more hydroxyl
groups replaced by hydrogens
•Is a constituent of DNA in all living thing
•Also occur frequently in glycoproteins and
polysaccharides
Cyclic alcohol,
a components of lipids
Sweeten sugarless gum and mints
High toxic Arrow poison
Flavin coenzyme
Sugar esters
Amino sugar
•Phosphate esters of glucose, fructose, and other
monosaccharides are important metabolic
intermediates
•The ribose of nucleotides --- ATP and GTP
•Contain an amino group (instated of a hydroxyl
group) at the C-2 position
•In many oligosaccharides and polysaccharides
5-position
amino group
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Muramic acid and neuraminic acid
C-3
•Are components of the polysaccharides of
cell membranes of higher organisms and
bacterial cell walls
The hydroxyl group of a lactic acid
at C-3
C-1
•Are glucosamines linked to three-carbon
acids at C-1 or C-3 positions
Acetals, ketals, and glycosides
Alcohols
•Pyranose and furanose forms
monosaccharides react with alcohol
Glycosides
•Reaction at the C-1 carbon
•Glycosidic bond --- between
anomeric carbon atom and the oxygen
atom of the alcohol
Structure and chemistry of
oligosaccharides
•Relative complexity of oligosaccharides and
polysaccharides in higher organisms
Disaccharides are the simplest
oligosaccharides
• Consist of two monosaccharide units linked by glycosidic
bond
• Each individual unit in an oligosaccharide is term a residue
Oxidation-reduction reaction
•The oligosaccharides and polysaccharides
are similar to protein
Nonreducing end
•Both form complicated structures based on a
small number of different building blocks (身
體組織的建構物 )
Homodisaccharides
(contain one kind of monosaccharide)
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Maltose (麥芽糖)
• Is produced from starch (澱粉) (a polymer of -D-glucose
produced by plants)
• Diastase (澱粉糖化酵素)
--- catalyzes hydrolysis of starch to maltose
• Isomatlose is obtained in the hydrolysis of some polysaccharides
(such as dextran ---高分子葡萄糖聚合物)
• Cellobiose (纖維雙醣) is obtained from the acid hydrolysis of
cellulose
Sucrose
•A disaccharide
-D-Lactose
•Is the principal carbohydrate in milk and is of
critical nutritional importance to mammals in
the early stages of their lives
•Is formed from D-galactose and D-glucose
via a (14) link
•Broken down into galactose and glucose by
lactase
A variety of higher oligosaccharides occur
in nature
Cycloamyloses (環狀糊精 )
•Composed of fructose and glucose
•Hydrolyzed by invertase
Structure and chemistry of
polysaccharides
•Glycosidically linked sugar residues
•Contain polymeric saccharide structure linked via
covalent bonds to amino acids, peptides, proteins,
lipids, and other structure
•Polysaccharides = Glycans
•Homopolysaccharides (homoglycan) --- one kind
of monosaccharide
•Heteropolysaccharides --- more than one kind of
monosaccharide
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The functions of polysaccharides
•Serve energy storage
•Structure
•Protection
Store of energy
•Storage polysaccharides are an important
carbohydrate form in plants and animals
(starch, glycogen, and dextran)
•Starch
--- -amylose (10%~30% in nature)
(直鏈澱粉) (12~30 residues)
--- amylopectin (70%~90% in nature)
(支鏈澱粉) (24~30 residues)
•Chloroplasts & amyloplasts (澱粉體) in plant
•In animal, digestion and use of plant
starches begin in the mouth with salivary amylse (an endoamylase) (splits 14
glycosidic linkages)
Starch phosphoeylase
•In plants and microoganisms, cleaves
disaccharide (maltose) units -amylse (an
exoamylose)
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•Glycogen
--- the major storage polysaccharide in animals
• Dextran
--- (16)-linkage of D-glucose; found in yeast and bacteria
--- mainly in the liver (10% of liver mass) and
skeletal muscle (1%~2% of muscle mass)
--- (16)-linkage; 8~12 glucose units
--- hydrolyzed by both - and -amylases (yielding
glucose and maltose)
--- also hydrolyzed by glycogen phosphorylase to
release glucose-1 phosphate
Structure and strength
•Cellulose (纖維素) (structure polysaccharides)
Extended ribbon
--- is the most abundant natural polymer
found in the world
--- cell walls in plant, providing physical
structure and strength
--- cotton, almost pure cellulose
--- a linear homopolymer of D-glucose units
linked by (14)
•Cellulose
--- resistant hydrolysis (acid
or digestive tract amylases)
--- most animal (including
human) can not digest
cellulose
--- ruminant (反芻動物)
animals can digest cellulose
--- cellulase (a glucosidase)
Bacteria live in here
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•Chitin (幾丁質)
--- is present in the cell walls of fungi and
exoskeletons of crustaceans, insects, and
spiders
--- (14) linkage
--- difference between cellulose and chitin
parallel (all the reducing ends
together at one end of a packed bundle
and all nonreducing ends together at the
other end)
National cellulose seems to occur only in parallel arrangements
antiparallel (each sheet of chains having
the chains arranged oppositely from the
sheets above and below
•Chitin
--- three forms, sometimes all in the same
organism
--- -chitin: all-parallel arrangement
--- -chitin: antiparallel arrangement
--- 
-chitin: pairs of parallel sheets separated
by single antiparallel sheet
--- the earth’
s second most abundant
carbohydrate polymer
•Alginates
--- a family of novel extended ribbon structure that
bind metal ions, particularly calcium
--- brown algae (phaeophyceae) (海藻)
--- poly(-D-mannuronate)
linked chain formed from -D-mannuronic acid
--- poly(-L-guluronate)
linked chain formed from -L-guluronic acid
•Agarose
poly(-L-guluronate)
--- an important polysaccharide
mixture isolated from marine
red algae (Rhodophyceae)
--- agar
--- agarose and agaropectin
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•Glycosaminoglycans
--- a class of polysaccharides known as
glycosaminoglycans is involved in a variety of
extracellular (and sometimes intracellular)
functions
--- one of monosaccharide units is an amino sugar
The highest net negative charge
Consist of as many as 25,000
disaccharide units
--- one (or both) of the monosaccharide units
contains at least one negatively charged sulfate or
carboxylate group
Polysaccharides provide strength
and rigidity to bacterial cell walls
•Peptidoglycan is the polysaccharide of bacterial
cell walls
•Bacteria (Gram stain): Gram-positive & Gramnegative
•Peptidoglycan: bacterial cell walls have a strong,
protective peptide-polysaccharide layer (murein;
胞壁質) (14) linkage
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Glycoprotein
Glycoprotein
Glycoprotein
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Antifreeze glycoprotein (AFGPs)
N-linked oligosaccharides
O-linked glycoprotein
Affect the physical properties and functions of a protein
N-linked oligosaccharides
Oligosaccharide cleavage
can serve as a timing device
for protein degradation
Proteoglycans modulate processes
in cells and organisms
Proteoglycans
•A family of glycoproteins
•Carbohydrate moieties are predominantly
glycosaminoglycans
•The functions of proteoglycans
--- Binding to other protein
--- Modulate cell growth processes
--- Make cartilage flexible and resilient (彈性彎曲)
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Binding to other protein
Binding to other protein
Make cartilage flexible
and resilient
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