Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a...
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Transcript of Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a...
Monosaccharides - simple sugars with multiple OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose.
Disaccharides - 2 monosaccharides covalently linked.
Oligosaccharides - a few monosaccharides covalently linked.
Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units.
I (CH2O)n or H - C - OH
I
Carbohydrates (glycans) have the following basic composition:
Monosaccharides
Aldoses (e.g., glucose) have an aldehyde group at one end.
Ketoses (e.g., fructose) have a keto group, usually at C2.
C
C OHH
C HHO
C OHH
C OHH
CH2OH
D-glucose
OH
C HHO
C OHH
C OHH
CH2OH
CH2OH
C O
D-fructose
D vs L Designation
D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde.
The lower representations are Fischer Projections.
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
L-glyceraldehydeD-glyceraldehyde
L-glyceraldehydeD-glyceraldehyde
Sugar Nomenclature
For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group.
Most naturally occurring sugars are D isomers.
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 & L sugars are mirror images of one another.
They have the same name, e.g., D-glucose & L-glucose.
Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc.
The number of stereoisomers is 2n, where n is the number of asymmetric centers.
The 6-C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars).
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
Hemiacetal & hemiketal formation
An aldehyde can react with an alcohol to form a hemiacetal.
A ketone can react with an alcohol to form a hemiketal.
R'O
H
R
C
H
OO C O
R
H
R'
H
alcohol aldehyde hemiacetal (C-O-C-O-H)
R'O
H
R
C
R''
OO C O
R
R''
R'
H
alcohol ketone hemiketal (C-O-C-O-H)
Ring Closure in glucose…hemiacetal formation
CH2OH
O
OH
OH
OH
OH
C
CH2OH
O
OH
OH
OH
O
H
O
OH
OH
OH
OH
CH2OH
Ring Closure in Fructose…hemiketal formation
CH2OH
CC
CH2OH
OOH
OH
O
HCH2OH
C
C
CH2OH
O
OH
OH
OH
CO
C
CH2OH CH2OH
OHOH
OH
Anomers
• Anomeric carbon is carbonyl carbon
• Depending on orientation during nucleophilic attack, can get two orientations
• If ring closes with OH below ring, then is called α
• If ring closes with OH above ring, then is called β
Anomer formation in glucose
CH2OH
O
OH
OH
OH
OH
H
C
CH2OH
O
OH
OH
OH
O
H
HO
OH
OH
OH
OH
CH2OH
H
CH2OH
O
OH
OH
OH
OH
H
C
CH2OH
O
OH
OH
OH
O
H
H
O OH
OH
OH
OH
CH2OH
H
alpha glucose
beta glucose
Draw the following in the closed form… do the first as α galactose
an the second as β riboseCH2OH
OH H
OH
H OH
OH
O
H
H
CH2OH
H
H
OH
O
OH
OH
H
galactose
ribose
Sugar derivatives
sugar alcohol - lacks an aldehyde or ketone; e.g., ribitol.
sugar acid - the aldehyde at C1, or OH at C6, is oxidized to a carboxylic acid; e.g., gluconic acid, glucuronic acid.
CH2OH
C
C
C
CH2OH
H OH
H OH
H OH
D-ribitol
COOH
C
C
C
C
H OH
HO H
H OH
D-gluconic acid D-glucuronic acid
CH2OH
OHH
CHO
C
C
C
C
H OH
HO H
H OH
COOH
OHH
Sugar derivatives
amino sugar - an amino group substitutes for a hydroxyl. An example is glucosamine.
The amino group may be acetylated, as in N-acetylglucosamine.
H O
OH
H
OH
H
NH2H
OH
CH2OH
H
-D-glucosamine
H O
OH
H
OH
H
NH
OH
CH2OH
H
-D-N-acetylglucosamine
C CH3
O
H
N-acetylneuraminate (N-acetylneuraminic acid, also called sialic acid) is often found as a terminal residue of oligosaccharide chains of glycoproteins.
Sialic acid imparts negative charge to glycoproteins, because its carboxyl group tends to dissociate a proton at physiological pH, as shown here.
NH O
H
COO
OH
H
HOH
H
H
RCH3C
O
HC
HC
CH2OH
OH
OH
N-acetylneuraminate (sialic acid)
R =
Reducing Sugars
• Sugars that contain aldehyde groups that are oxidized to carboxylic acids are classified as reducing sugars.
• Common test reagents are : • Benedicts reagent (CuSO4 / citrate) • Fehlings reagent (CuSO4 / tartrate)
• They are classified as reducing sugars since they reduce the Cu2+ to Cu+ which forms as a red precipitate, copper (I) oxide.
• So any sugar that contains a hemi-acetal will be a reducing sugar.
Hemiacetals are reducing sugars
• In equilibrium with the aldehyde
CH2OH
O
OH
OH
OH
OH
O
OH
OH
OH
OH
CH2OH
Fructose, however is reducing
CH2OH
C
C
CH2OH
O
OH
OH
OH
CHOH
C
C
CH2OH
OH
OH
OH
OH
C
C
C
CH2OH
O
OH
OH
OH
OH
H
fructose ene-diol intermediateglucose
Acetal/ketal FormationR'O
COH
H
R
R''O
H
R'O
CR''OH
R
OH2
alcohol hemiacetal acetal
+
R'O
COH
R''
R
R'''O
H
R'O
CR'''OR''
R
OH2
alcohol hemiketal Ketal
+
Glycosidic BondsAcetal or ketal linkage between sugars
C
O
OHOH
OH
OH
CH2OHH
O
OHOH
OHO
CH2OH
H
O
OHOH
OH
O
CH2OH
O
OH
OH
OH
CH2OH
OH2
hemiacetal alcohol acetal
+
Disaccharide nomenclature• Sugars named• Linkages named as to from what carbon on sugar 1
to what carbon on sugar 2• Anomeric carbon has to be specified as α or β• Next slide has glucose-glucose in an α 1,4
linkage– Carbon 1 is α and attaches to carbon 4 on the next glucose
• Also shown is β 1,4 linkage– Carbon 1 is β and attaches to carbon 4 on the next glucose
Cellobiose, a product of cellulose breakdown, is the otherwise equivalent anomer (O on C1 points up).
The (1 4) glycosidic linkage is represented as a zig-zag, but one glucose is actually flipped over relative to the other.
Disaccharides:
Maltose, a cleavage product of starch (e.g., amylose), is a disaccharide with an (1 4) glycosidic link between C1 - C4 OH of 2 glucoses.
It is the anomer (C1 O points down).
O
OHOH
OH
O
CH2OH
O
OH
OH
OH
CH2OH
O
OHOH
OH
O
CH2OH
O
OH
OH
OH
CH2OH
alpha 1,4 glucose-glucose (maltose)
beta-1,4-glucose-glucose (cellobiose
Draw the following
• Glucose-glucose in an α-1,6 linkage
• Glucose-galactose in an α-1, β -1 linkage– (glucose is alpha and galactose in beta)
Answers
• Glucose-glucose in α 1,6 linkage
• Glucose α -1 – galactose β - 1
OH
O
OH
OH
CH2OH
O
CH2
OH
O
OH
OH
OH
O
OOH
OH
OH
CH2OH
O
OH
OH
OHCH2OH
Other disaccharides include:
Sucrose, common table sugar, has a glycosidic bond linking the anomeric hydroxyls of glucose & fructose.
Because the configuration at the anomeric C of glucose is (O points down from ring), the linkage is (12).
Lactose, milk sugar, is composed of galactose & glucose, with (14) linkage from the anomeric OH of galactose. Its full name is -D-galactopyranosyl-(1 4)--D-glucopyranose
Lactose Intolerance
• Lactose or milk sugar occurs in the milk of mammals - 4-6% in cow's milk and 5-8% in human milk. It is also a by product in the the manufacture of cheese.
• Lactose intolerance is the inability to digest significant amounts of lactose, the predominant sugar of milk. This inability results from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine. Lactase breaks down the lactose, milk sugar, into glucose and galactose that can then be absorbed into the bloodstream.
Polysaccharides
Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch. Glucose storage in polymeric form minimizes osmotic effects.
Amylose is a glucose polymer with (14) linkages. It adopts a helical conformation.
The end of the polysaccharide with an anomeric C1 not involved in a glycosidic bond is called the reducing end.
H O
OH
H
OHH
OH
CH 2 OH
HO H
H
OHH
OH
CH 2 OH
H
O
HH H O
OH
OHH
OH
CH 2 OH
HH H O
H
OHH
OH
CH 2 OH
H
OH
HH O
OH
OHH
OH
CH 2 OH
H
O
H
1
6
5
4
3
1
2
a m y lo s e
Amylopectin is a glucose polymer with mainly (14) linkages, but it also has branches formed by (16) linkages. Branches are generally longer than shown above.
The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur.
H O
OH
H
OHH
OH
CH2OH
HO H
H
OHH
OH
CH2OH
H
O
HH H O
OH
OHH
OH
CH2
HH H O
H
OHH
OH
CH2OH
H
OH
HH O
OH
OHH
OH
CH2OH
H
O
H
O
1 4
6
H O
H
OHH
OH
CH2OH
HH H O
H
OHH
OH
CH2OH
HH
O1
OH
3
4
5
2
amylopectin
Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin. But glycogen has more (16) branches.
The highly branched structure permits rapid release of glucose from glycogen stores, e.g., in muscle during exercise. The ability to rapidly mobilize glucose is more essential to animals than to plants.
H O
OH
H
OHH
OH
CH 2OH
HO H
H
OHH
OH
CH 2OH
H
O
HH H O
OH
OHH
OH
CH 2
HH H O
H
OHH
OH
CH 2OH
H
OH
HH O
OH
OHH
OH
CH 2OH
H
O
H
O
1 4
6
H O
H
OHH
OH
CH 2OH
HH H O
H
OHH
OH
CH 2OH
HH
O1
OH
3
4
5
2
glycogen
Review quiz
• Which hormone signals the release of glycogen into glucose?
• What was the mechanism of signal transduction?
Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with (14) linkages.
1. How does the structure in cellulose affects it bonding?
2. Animals lack the enzymes needed to break down these linkages. How do animals, like cattle, subsist on cellulose?
.
.
OOH
OH
OH
O
CH2OH
O
OH
OH
O
CH2OH
O
OH
OH
O
CH2OH
O
OH
OH
O
CH2OH
O
OH
OH
O
CH2OH
O
OH
OH
OH
CH2OH
Answer 1
• The van der Waals interactions cause cellulose chains to be straight & rigid, and pack with a crystalline arrangement in thick bundles called microfibrils
• The role of cellulose is to impart strength and rigidity to plant cell walls, which can withstand high hydrostatic pressure gradients. Osmotic swelling is prevented.
Answer 2
• Animals, like cattle, have commensal bacteria in their gut that can break the linkages of cellulose.
•Similar to cellulose(14) linkages.
•The monomer is N-acetyl-D-glucosamine•Plays a structural role •Structural components of exoskeleton of invertebrates
ChitinO O
NH
OH
O
CH2OH
C
O
CH3
O
NH
OH
O
CH2OH
C
O
CH3
O
NH
OH
OH
CH2OH
C
O
CH3
O-linked oligosaccharide chains of glycoproteins vary in complexity.
They link to a protein via a glycosidic bond between a sugar residue & a serine or threonine OH.
O-linked oligosaccharides have roles in recognition, interaction, and enzyme regulation.
H O
OH
O
H
HNH
OH
CH2OH
H
C CH3
O
-D-N-acetylglucosamine
CH2 CH
C
NH
O
H
serine residue
Oligosaccharides that are covalently attached to proteins or to membrane lipids may be linear or branched chains.
H O
OH
O
H
HNH
OH
CH2OH
H
C CH3
O
-D-N-acetylglucosamine
CH2 CH
C
NH
O
H
serine residue
•N-acetylglucosamine (GlcNAc) is a common O-linked glycosylation of protein serine or threonine residues. •Many cellular proteins, including enzymes & transcription factors, are regulated by reversible GlcNAc attachment. •Often attachment of GlcNAc to a protein OH alternates with phosphorylation, with these 2 modifications having opposite regulatory effects (stimulation or inhibition).
N-linked oligosaccharides of glycoproteins tend to be complex and branched. First N-acetylglucosamine is linked to a protein via the side-chain N of an asparagine residue in a particular 3-amino acid sequence.
H O
OH
HN
H
H
HNH
OH
CH2OH
H
C CH3
O
C CH2 CH
O HN
C
HN
O
HC
C
HN
HC
R
O
C
R
O
Asn
X
Ser or ThrN-acetylglucosamine
Initial sugar in N-linked glycoprotein oligosaccharide
Many proteins secreted by cells have attached N-linked oligosaccharide chains.
Carbohydrate chains of plasma membrane glycoproteins and glycolipids usually face the outside of the cell.
They have roles in cell-cell interaction and signaling, and in forming a protective layer on the surface of some cells.