Download Functions of carbohydrates

Functions of carbohydrates
 energy stores, fuels, and metabolic
intermediates
 structural framework of RNA and DNA
 structural elements in the cell walls of
bacteria and plants
 linked to many proteins and lipids
Carbohydrates
• aldehyde or ketone compounds with multiple
hydroxyl groups
• constructed of carbon (carbo-) plus hydrogen
and oxygen (-hydrate)
• simplest carbohydrates = monosaccharides, or
simple sugars
 Cn(H2O)n
• make up most of the organic matter on Earth
• Starch consists of chains
of linked glucose
molecules.
• These chains are broken
down into individual
glucose molecules
• Glucose catabolism
used to generate ATP
and building blocks for
other molecules.
Sugars can be aldoses or ketoses
D-aldoses
• contain an aldehyde
group
• have Dglyceraldehyde at the
asymmetric center
farthest from the
aldehyde group
D-ketoses
• have one fewer
asymmetric center than
aldoses with the same
number of carbons
• D-Fructose is the most
abundant ketohexose
Examples of triose sugars
• Monosaccharides often represented by Fischer projections
• Most commonly occurring sugars are in the D conformation
6 – carbon aldoses
Epimers
• Sugars that differ in a single chiral center
• Examples:
1. Galactose and glucose (C-4 epimers)
2. Glucose and mannose (C-2 epimers)
Pentoses and hexoses cyclize to form
furanose and pyranose rings
• open-chain forms usually cyclize into rings
• an aldehyde can react with an alcohol to form a
hemiacetal
Ketoses
• a ketone can react with an alcohol to form a
hemiketal
• C-2 keto group can react with C-6 OH group to form a
6-membered cyclic hemiketal or the C-5 OH group to
form a 5-membered cyclic hemiketal
• cyclic hemiacetal =six-membered
ring
– called pyranose because of its
similarity to pyran
• Cyclic hemiketal = 5-membered
– Called furanose because of similarity
to furan
Pyranose formation
Two anomeric forms, designated α and β, can result because
of new chiral center at C1
Three alternative configurations of D-glucose
• An equilibrium mixture of glucose contains
~1/3 α anomer
2/3 β anomer
<1% of the open-chain form
• Glucose stored by plants (starch) and animals
(glycogen)
• Also present in cellulose; provides structural integrity
to the cells
Most biologically important sugars are six-carbon
hexoses that are structurally related to D-glucose
• Haworth
projection an
oversimplification
because pyranose
ring is not planar
• Most stable
conformation =
chair
Furanose formation
• open-chain form of fructose cyclizes to a 5-membered ring
when the C-5 OH group attacks the C-2 ketone to form an
intramolecular hemiketal
• Two anomers are possible
Fructose
• forms both pyranose and furanose rings
• pyranose form predominates in fructose free
in solution
• furanose form predominates in many fructose
derivatives
Other common monosaccharides
Galactose - found in milk as part of the disaccharide lactose
Ribose - five carbon sugar contained in RNA and DNA
• form furanose rings
Reactions of monosaccharides
• Oxidation - provides E for organisms
• Produces sugar acids
• For hexoses:
1. Oxidation at C1 → aldonic acid
 Detected by Benedict’s, Barfoed’s or Seliwanoff’s test
2. Oxidation at C6 → uronic acid
3. Oxidation at C1 and C6 → aldaric acid
Oxidation of sugars
Sugars that react are called reducing sugars; those that
do not are called nonreducing sugars
Lactones
• Produced from oxidation of aldose
e.g., α-D-glucose → lactone in presence of
silver-ammonia complex
– basis of Tollen’s test
– Positive result = silver mirror
Glycoside formation
• Glycoside = non-reducing sugar
– C1 not free because linked with R group
– Usually, -OH group bonded to anomeric C in cyclic
form
Hemiacetal + R – OH → full acetal/glycoside
Anomeric carbon atom reacts with
the hydroxyl group of methanol
Exhaustive methylation
• R-OH reactions only affect anomeric C’s
• Other R groups can be methylated using
dimethylsulfate
– Used to determine presence of glycosidic linkages
Reduction reactions
• Formation of alditols
– C=O to C – OH
– E.g., sorbose → sorbitol
Xylose → xylitol
• Formation of sugar alcohols
– C – OH to C – H
– e.g., β-D-ribose → β-D-deoxyribose
Esterification reactions
• -OH groups of sugars behave exactly like all
other alcohols
• May reach with acids, acid derivatives,
phosphates, etc
• e.g., phosphates esterified to ribose or
deoxyribose
Modified monosaccharides
•
•
Carbohydrates can be modified by the addition of substituents
other than hydroxyl groups.
Such modified carbohydrates are often expressed on cell
surfaces.
Formation of sugar sulfates
• Sulfates esterified at C-2, C-4 or C-6 of aldoses
• Found mostly in proteoglycans of ECM
• Presence of sulfate groups mean that sugar is
negatively charged at physiological pH
The major glycosaminoglycans
Polysaccharides
Carbohydrates
• aldehyde or ketone compounds with multiple
OH groups
• constructed of carbon (carbo-) plus hydrogen
and oxygen (-hydrate)
• simplest carbohydrates = monosaccharides, or
simple sugars
 Cn(H2O)n
• make up most of the organic matter on Earth
Polysaccharides
• play vital roles in energy storage, and in maintaining
structural integrity of an organism
• May be
1. Homopolysaccharides
– all of the monosaccharides are the same
– e.g., starch, glycogen
2. Heteropolysaccharides
– >1 type of monosaccharide
– Usually just 2 alternating monosaccharide units
Complex carbohydrates formed by linkage
of monosaccharides
• Oligosaccharides formed by O-glycosidic linkages
between monosaccharide units
• Since monosaccharides have multiple OH groups,
various glycosidic linkages are possible
Glycosidic bonds
• A large no. of different
glycosidic bonds can be formed
between 2 sugar residues
• For example, glucose could be
bonded to fructose by any of
the following linkages:
o α(1→1)
o α(1→2)
o α(1→3)
o α(1→4)
o α(1→6)
o β(1→1)
o β(1→2)
o β(1→3)
o β(1→4)
o β(1→6)
Common disaccharides
Common disaccharides
Starch
• nutritional reservoir in plants; makes up >50% of
carbohydrates ingested by humans
• 2 forms: amylose and amylopectin
1. Amylose
– unbranched
– consists of glucose residues in α-1,4 linkage.
2. Amylopectin
– branched form, but less dense compared to
glycogen
– Has α-1,6 linkage per 30 α-1,4 linkages
• Both forms rapidly hydrolyzed by α-amylase
Glycogen
• Storage form of
glucose in animals
• Main chain: glucose
linked by α-1,4glycosidic bonds
• Branches: formed by α1,6-glycosidic bonds
Cross section of a
glycogen molecule
Branch point in glycogen
• 2 chains of glucose molecules joined by α-1,4-glycosidic bonds
are linked by an α-1,6-glycosidic bond to create a branch point
• α-1,6-glycosidic bond forms every 10 glucose units
Cellulose
• the other major polysaccharide of glucose in plants
• one of the most abundant organic compounds in the
biosphere
• unbranched polymer of glucose residues joined by β1,4 linkages
• serves a structural rather than a nutritional role
– Mammals lack cellulases; cannot digest wood, vegetable
fibers
Glycosidic bonds determine polysaccharide structure
• β-1,4 linkages favor straight chains, which are optimal for
structural purposes
• α-1,4 linkages favor bent structures, which are more suitable
for storage.
Chitin
• long-chain polymer of a N-acetylglucosamine
• main component of the exoskeletons of arthropods, such as
crustaceans and insects
Dextran
• synthesized from sucrose
by certain lactic-acid
bacteria
– e.g., Leuconostoc
mesenteroides,
Streptococcus mutans
• Found in dental plaque
• Similar to amylopectin
– main chains formed by
α(1→6) glycosidic linkages
– side branches attached by
α(1→3) or α(1→4) linkages
Dextran: uses
• used in eye drops as a lubricant
• used to replace lost blood
when replacement blood is not
available
• BUT must be used with caution;
does not contain electrolytes
• Present in certain IV fluids to
1. solubilise other factors, e.g.,
iron (=iron dextran).
2. Provide energy: digested into
glucose and free water
Peptidoglycan
• Makes up bacterial cell walls
• linear polysaccharide chains
cross-linked by short peptides
• mechanical support
• prevents bacteria from
bursting in response to their
high internal osmotic pressure
• Penicillin inhibits cross-linking
transpeptidase
Formation of crosslinks in peptidoglycan
• catalyzed by glycopeptide transpeptidase
• Penicillin mimics d-Ala-d-Ala moiety of normal substrate
– Bound penicillin forms covalent bond with Ser at active site
of enzyme
– penicilloyl-enzyme does not react further
Glycoproteins
• proteins that contain oligosaccharide chains
covalently attached to their polypeptide sidechains
• sugars may be attached to
1. N in asparagine side chain (N-linkage)
2. O in the side chain of serine or threonine (Olinkage)
Glycosidic bonds between proteins and carbohydrates
N -linked oligosaccharides
Blood group antigens
Blood group antigens
• Carbohydrates attached to glycoproteins,
glycolipids on the surfaces of RBCs
– Antigenic determinants
• If an antigen not normally present in a person
is introduced, the person's immune system
recognizes it as foreign
Glycosaminoglycans
• have disaccharide repeating units containing a
derivative of an amino sugar
– either glucosamine or galactosamine
• At least one of the sugars in the repeating unit has a
negatively charged carboxylate or sulfate group
• usually attached to proteins to form proteoglycans
The major glycosaminoglycans
Proteoglycans
• Almost all GAGs covalently attached to protein in the
form of proteoglycans
• distinguished from other glycoproteins by the nature,
quantity, and arrangement of their sugar side chains
• Found in animal cells
The linkage between a GAG chain and its core protein in a
proteoglycan molecule
Glycoproteins vs proteoglycans
1. Glycoproteins
 contain 1–60% carbohydrate by weight
 numerous relatively short, branched oligosaccharide
chains
2. Proteoglycans
 can contain as much as 95% carbohydrate by weight
 Most carbs = long, unbranched GAG chains, each
typically ~ 80 sugars long
Proteoglycan functions
• GAGs, proteoglycans can associate to form huge
polymeric complexes in the ECM
• GAG chains can form gels of varying pore size, charge
density
– may serve as selective sieves to regulate the traffic of
molecules and cells according to their size, charge, or both
• Also associates with fibrous matrix proteins, e.g.,
collagen
• Aggrecan
– major component of cartilage
– MW ~ 3 × 106 daltons with over 100 GAG chains
• Decorin
– secreted by fibroblasts
– has a single GAG chain