Download Reducing sugars

Sugars
Alice Skoumalová
Repetition:
1. Glucose, fructose and galactose (structure)
2. Optical isomerism - definition (D- and L-glucose)
3. Formation of a cyclic glucose (anomers)
4. Reducing sugars - definition
5. The most important disaccharides (composition)
6. The most important polysaccharides (structure)
Important sugars in an organism:
1. Glucose
2. Glycogen
3. Glycoproteins
4. Glycolipids
5. Proteoglycans
An introduction to carbohydrates:
 a large class of naturally occurring polyhydroxy aldehydes
and ketones
 monosaccharides (simple sugars): from 3 to 7 carbons +
one aldehyde or ketone functional group
 chiral compounds (having right- or left-handedness with two
different mirror-image forms) because they contain carbons bonded to
four different groups
An aldose
A ketose
Monosaccharides, disaccharides, polysaccharides
Modified saccharides
The D and L families of sugars:
Enantiomers - „mirror images“ (rotate polarized light in opposite directions →
optical activity)
Fischer projection:
a mirror
D sugar → the OH group on the chiral carbon farthest from the
carbonyl group pointing to the right in a Fischer projection
L sugar → the OH group on the chiral carbon farthest from the
carbonyl group pointing to the right in a Fischer projection
2n of possible stereoisomers (half that many pairs of enantiomers)
n - the number of chiral carbons (e.g. glucose - 16 stereoisomers)
The structure of glucose:
Mutarotation
 change in rotation of plane-polarized
light resulting from the equilibrium
between cyclic anomers and the openchain form of a sugar
Anomers
 cyclic sugars that differ
only in positions of
substituents at the
anomeric carbon
Reactions of the monosaccharides:
Reaction with oxidizing agents:
Reducing sugars react in basic solution with a mild oxidizing agent
In basic solutions, all monosaccharides are reducing sugars
Reaction with alcohols (glycoside formation):
Glycosidic bond → bond between the anomeric carbon of a monosaccharide and
an -OR group
Hydrolysis of a disaccharide (during digestion of carbohydrates)
Important monosaccharides
Aldoses:
Pentoses
 in RNA and
NADH
 in polysaccharides in the walls
of plant cells
Hexoses
 „blood“ sugar (energy)
 in cellulose and starch
 in glycogen (as a
source of energy in an
organism)
 in glycolipids and
glycoproteins
 in lactose (milk),
glycolipids and
glycoproteins
 converted to glucose
 galactosemia
Ketoses:
D-Ribulose
D-Fructose
 an intermediate in the pentose
phoshate pathway
 in fruit juices and in honey
 in sucrose
 converted to glucose
Deoxyaldoses:
2-deoxy-D-ribose
 in DNA
Acetylated amino sugars:
N-Acetyl-D-glucosamine
N-Acetyl-D-galactosamine
 in glycoproteins
Acidic sugars:
D-Glucuronic acid
N-Acetylneuraminic acid (sialic)
 in glycosaminoglycans in connective
tissue
 conjugation of bile acids
 in glycoproteins
Important disaccharides
Maltose
 a breakdown product of the
starch
Lactose
 milk sugar (4,5% - 7%)
 lactose intolerance (reduced
activity of lactase)
Sucrose
 cane sugar, beet sugar
 non-reducing sugar
Important polysaccharides
Cellulose
 ß-D-Glucose, ß-1,4 link
 the fibrous substance that provides structure in
plants
 humans cannot hydrolyze cellulose
Starch
 α-D-Glucose
 source of energy in plants
 fully digestible - an essential part of the human diet (the grains wheat, potatoes, rice)
1. Amylose (20%, soluble in water)
 α-1,4 link
2. Amylopectin (80%, not water soluble)
 α-1,6 branches (every 25 units)
Glycogen
 α-D-Glukose, α-1,4 and α-1,6 link
 source of energy in animals (liver, muscles)
Other important sugars
1. Glycosaminoglycans
Hyaluronic acid
Chondroitin sulfate
 25,000 disaccharide units
 in tendons and cartilage
 form very viscous mixture
 in connective tissue, synovial fluid,
vitreous humour
Glucuronic acid
Glucuronic acid
N-Acetylglucosamine
N-Acetylgalactosamine
sulfate
Heparin
 contains sulfate groups (negative charges)
 anticoagulant - prevents the clotting of blood (binds to the clotting
factors)
2. Proteoglycans
 aggregates of proteins and glycosaminoglycans in the extracellular matrix
 highly hydrated and resilient (cartilage)
3. Glycoproteins
 on the surfaces of cells (receptors, blood type)
Forms:
Blood type
 cell-surface carbohydrates
Summary:
1. Epimers, enantiomers, anomers (examples)
2. The most important reaction of monosaccharides
3. The importance of glucose
4. Important disaccharides
5. Important polysaccharides
6. Glycosaminoglycans and proteoglycans - structure, function
7. Glycoproteins - structure, function