Download - Carbohydrates 2011
Chapter 11:Carbohydrates: Structure and Biological Function
MARIA CRISTINA R. RAMOS, Ph.D. Professor , Department of
Chemistry
Selaginella lepidophylla- a desert plant which can adapt to dehydrating condition by synthesizing trehalose,
and can resume normal metabolism when water is available-hence its nickname is “resurrection plant.
Roles of Carbohydrates
• Storage of metabolic fuel
• Structural framework of nucleic acids
• Structural elements in the cell walls of bacteria and plants
• Linked to proteins and lipids which are involved in cell recognition
Monosaccharides are classified by their number of
carbon atoms
Hexose
Heptose
Octose
TrioseTetrose
Pentose
FormulaName
C3H6 O3C4H8 O4
C5H1 0O5
C6H1 2O6
C7H1 4O7C8H1 6O8
Monosaccharides Stereoisomers
Carbohydrates contain many chiral carbons
OPTICAL ISOMERS
• Stereosiomers can be distinguished using plane of polarized light
Naming stereoisomers
Based on the structure of L- and D-glyceraldehyde
O H O H C C H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
D- sugars are the most common carbohydrates. D-
refers to the right hand orientation of the OH on
the chiral carbon farthest from the carbonyl carbon
L-sugars: L refers to the left hand orientation of the
OH on the chiral carbon farthest from the carbonyl
carbon
EXERCISE
• Draw the possible structure(s) of
1. aldotetrose
2. ketotetrose
Glucose
• The most common sugar
• Known as dextrose
• Also known as blood sugar
C
C OHH
C HHO
C OHH
C OHH
CH2OH
D-glucose
OH
Galactose
• One of the monosaccharides in the disaccharide lactose
• Found in plant gums and pectins polysaccharides
• An epimer of glucose at C4• Converted to glucose during metabolism
Fructose
• One monosaccharide in the disaccharide sucrose (table sugar).
• Called levulose or fruit sugar • Found in honey and fruits • Sweeter than sucrose and glucose • Commercially prepared as high-
fructose sugars from corn starch for sweetness
• An epimer of glucose at C2• Readily converted to glucose in metabolism
by isomerization
C HHO
C OHH
C OHH
CH2OH
CH2OH
C O
D-fructose
CARBOHYDRATES IN CYCLIC STRUCTURES
Because of the tetrahedral nature of carbon bonds, pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar.
The representation above reflects the chair configuration of the glucopyranose ring more accurately than the Haworth projection.
Fructose forms either
a 6-member pyranose ring, by reaction of the C2 keto group with the OH on C6, or
a 5-member furanose ring, by reaction of the C2 keto group with the OH on C5.
CH2OH
C O
C HHO
C OHH
C OHH
CH2OH
HOH2C
OH
CH2OH
HOH H
H HO
O
1
6
5
4
3
2
6
5
4 3
2
1
D-fructose (linear) -D-fructofuranose
The formation of hemiketals and hemiacetals results in an asymmetric carbon atom.
Isomers that differ only in their configuration about the new asymmetric carbon are called anomers, the new assymetric carbon is called anomeric carbon.
a-anomer has the hydroxyl group at the right side of the anomeric carbon
-anomer has the hydroxyl group at the left side of the anomeric carbon
Anomers in aqueous solution freely interconvert between the and forms, unless the hydroxyl group attached to the
anomeric carbon is linked to another molecule. This process of interconvertion is
called MUTAROTATION
EXERCISE
D-Mannose exists in aqueous solution as a mixture of and forms. Draw the Haworth projection formula of the two anomers.
Reactions of Monosaccharides
Oxidation-Reduction
• Oxidation
Oxidation using a mild oxidizing agent
Oxidation using a strong oxidizing agent
Oxidation to Uronic Acids
CHO
CH2OH
OHHHHOOHHOHH
CHO
COOH
OHHHHOOHHOHH OH
OH
COOHO
HOHO
D-Glucose
enzyme-catalyzedoxidation
D-Glucuronic acid(a uronic acid)
Reduction to alditol
Reduction of ribose to 2’-deoxyribose by ribonucleotide reductase
Reaction with dilute base
Reaction with a concentrated base-
fragmentation
Esterification
Amino Derivatives
Formation of Glycosides
• Treatment of a monosaccharide, all of which exist almost exclusively in a cyclic hemiacetal form, with an alcohol gives an acetal
HH OH
HHO
HOH
OH
H
CH2OHO
CH3OHH
+
-H2O
OCH2OH
H
OH
OCH3H
HOH
OHH
H
OCH2OH
H
OH
HH
HOH
OHH
OCH3
(-D-Glucose)-D-Glucopyranose
Methyl -D-glucopyranoside(Methyl -D-glucoside)
anomeric carbon
+
+
Methyl -D-glucopyranoside(Methyl -D-glucoside)
glycosidicbond
Disaccharides
Maltose
– present in malt, the juice from sprouted barley and other cereal grains
– maltose consists of two units of D-glucopyranose joined by an -1,4-glycosidic bond
– maltose is a reducing sugar
Lactose
– lactose is the principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk
– it consists of D-galactopyranose bonded by a -1,4-glycosidic bond to carbon 4 of D-glucopyranose
– lactose is a reducing sugar
Sucrose
– is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets
– sucrose is a nonreducing sugar
Sweeter than sugar, and no calories
N-Glycosides
French Fries a la Plastic
EXERCISE
A. Draw the chair conformations for -D mannopyranose and -D-mannopyranose
B. Draw the chair conformation of 1. methyl - and -D-mannopyranoside2. -D-mannopyranosyl-(1-4)- -D-glucopyranoside
Polysaccharides
Storage Polysaccharides
Glycogen
• is the energy-reserve carbohydrate for animals
• glycogen is a branched polysaccharide of approximately 106 glucose units joined by -1,4- and -1,6-glycosidic bonds
• the total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle
Starch
• starch can be separated into amylose and amylopectin
• amylose is composed of unbranched chains of up to 4000 D-glucose units joined by -1,4-glycosidic bonds
• amylopectin contains chains up to 10,000 D-glucose units also joined by -1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by -1,6-glycosidic bonds
Structural Polysaccharides
CelluloseCellulose
• is a linear polysaccharide of D-glucose units joined by -1,4-glycosidic bonds
• it has an average molecular weight of 400,000 g/mol, corresponding to approximately 2200 glucose units per molecule
• cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds
• this arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength
• it is also the reason why cellulose is insoluble in water
Plant Polysaccharides
Hemicellulose chains are shorter than cellulose and may be branched
Pectin
• Is a heteropolymer containing galacturonate and rhamnose residues
• Used a gelling agent in the preparation of jams and jellies
Chitin
• Present in exoskeletons of arthropods
• Composed of N-acetylglucosamine, linked in (1-4)
Bacterial Cell Walls
• The bacterial cell wall is a unique structure which surrounds the cell membrane.
• Maintaining the cell's characteristic shape• Countering the effects of osmotic pressure• Consist of many layers of peptidoglycan
connected by amino acid bridges
Peptidoglycan
• is composed of an alternating sequence of N-acetyl-muraminic acid (NAM) and N-acetylglucosamine (NAG) joined by -1,4-glycosidic bonds.
O
NHO
OCH2OH
OO
NHO
CH2OH
O
O=C O=CCH3 CH3
CHH3C
C=O
NH
Ala
Gln
Lys
Ala
L
D
L
D
C=O
NH-(Gly)5C----
(CH2)4NH-C-(Gly)5-NH-----
To tetrapeptide side chainsO
O
• in Staphylococcus aureus, the cross link is a tetrapeptide
• this tetrapeptide is unusual in that it contains two amino acids of the D-series, namely D-Ala and D-Gln
• each tetrapeptide is cross linked to an adjacent tetrapeptide by a pentapeptide of five glycine units
The NAM-NAG polysaccharide is in turn cross-linked by
small peptides
Bacterial polysaccharides form a biofilm
A biofilm is attached to a surface and
harbors a community of embedded bacteria
that contributes to biofilm production and
maintenance
A Pseudomonas aeruginosa biofilm
Glycoproteins
Components of the cell membrane, where they play variety of roles in cell adhesion
Glycoprotein Functions
• Glycoproptein and Cancer
• Protein Turnover
• Viral Growth
• Antifreeze Glycoprotein
Protein turnover is initiated by removal of carbohydrate residues by hydrolysis
Specific enzymes are responsible for
oligosaccharide assembly
• Glycosyltransferase-catalyzes the synthesis of oligosaccharides through the formation of glycosidc bond
Protein glycosylation takes part in the lumen of the endoplasmic
reticulum and in the golgi complex
Processing of an N-linked oligosaccharides
PROTEOGLYCANS contain long
glycosaminoglycans
Glycosaminoglycans (GAGs) are anionic polyanionic polysaccharides chains
made of repeating disaccharide units
• play important roles in the structure and function of connective tissues
• polysaccharides based on a repeating disaccharide where one of the monomers is an amino sugar and the other has a negative charge due to a sulfate or carboxylate group
• Glycosaminoglycans tend to be negatively charged, because of the prevalence of acidic groups.
• GAGs are usually attached to proteins to form proteoglycans
Common GAGs
• heparin: natural anticoagulant
• hyaluronic acid: a component of the vitreous humor of the eye and the lubricating fluid of joints
• chondroitin sulfate and keratan sulfate:: components of connective tissue
Chondroitin 6-sulfate
• The repeating disaccharide units of N-acetylgalactosamine ang glucoronic acid
• can be sulfated in either 4 or 6 position of GalNAc
• They are prominent components of cartilage tendons, ligaments and aorta
• Palate is made from chondroitin sulfate
LECTINS
• Proteins that bind specific carbohydrate structures
• Are ubiquitous, being found in plants, animals and microorganisms