12/1/2014

Today:

◦ Polyhydroxy Aldehydes &
Ketones
◦ Ring Form of Carbohydrates
◦ Disaccharides: Acetal Linkages
◦ Polysaccharides
◦ Glycoconjugates
Problem Set 8 DUE
Wednesday, Dec. 3rd at 11
pm
Carbohydrates are Everywhere!
Maltose: Sugar obtained from
the hydrolysis of starch
Hydrates of Carbon:
Cn(H2O)n
Sucrose: Common table sugar
Ribose: Forming part of the
backbone in DNA
Lactose: Sugar found in milk
Carbohydrate Structure
Polyhydroxy Aldehydes and Ketones:
• Aldehydes and ketones with many hydroxy groups (—OH)
Aldose
C4 = tetrose
C5 = pentose
C6 = hexose
Aldotriose
Ketose
Ketotriose
Simple sugars have HIGH water solubility
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Different Representations of the Same Molecule:
Skeletal Structure: with
dashes and wedges to indicate
stereochemistry
Fischer Projections:
Haworth Projections:
Convenient way of drawing
cyclic carbohydrates
D- & L- Sugars:
Homochirality: The preference for one chiral form over another
Look at the position of the OH group on the stereocenter farthest
the C=O bond: D- sugars have the OH on the RIGHTHAND SIDE of
the Fischer projection
Enantiomers:
NONsuperimposable
mirror images
D-sugars naturally
occur in nature
L-sugars have the last
OH group on the left
Straight Chain vs. Cyclic Forms:
Aldehyde/Ketone + Alcohol = Hemiacetal/Hemiketal
INTRAmolecular reactions: a reaction between two functional
groups in the same molecule: the C=O & OH react.
Equilibrium favors the cyclic product
in solution
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Haworth Projections & alpha/beta Anomers:
Stereochemistry of Cyclic Hemiacetals
NEW tetrahedral, stereogenic
carbon formed in product:
TWO ISOMERS POSSIBLE (aka ANOMERS)
α (Alpha)
*: Anomeric Carbon
*
*
No stereocenter present
at C=O before reaction
β (Beta)
Haworth Projections & alpha/beta Anomers:
Stereochemistry of Cyclic Hemiacetals
Alpha/beta anomers will influence the
digestibility of complex carbohydrates.
Formation of a Disaccharide:
Hemiacetal + Alcohol = Acetal
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Formation of a Polysaccharide:
A polymer of monosaccharides
Carbohydrates are Everywhere!
Chitin: cell walls in
fungi & exoskeletons
in insects
Polysaccharides:
Starch: polymers of glucose
stored for energy in plants
Glycogen: polymers of glucose
stored for energy in the liver & muscles
Blood Types: characterized by different
carbohydrates on the surface of red blood cells
Glycosidic Linkages:
Connecting Monosaccharides together to form Disaccharides or
Polysachharides
Maltose: a disaccharide
of two glucose molecules
α (1→4)
linkage
Start numbering
closest to the
anomeric carbon
(the acetal carbon)
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Glycosidic Linkages:
Lactose: a disaccharide of
glucose & glacatose
1. Classify the acetal as
an alpha anomer or
beta anomer.
2. On each sugar: Start
numbering closest to the
anomeric carbon (the
acetal carbon)
• To digest lactose: the β 1,4-linkage must be
hydrolyzed.
• This requires a specific enzyme, LACTASE.
• People lacking lactase cannot digest lactose.
Digesting Carbohydrates:
Hydrolyzing Glycosidic Linkages
The first step in digesting
sugars: glycosidic bonds are
hydrolyzed, which allows the
carbohydrates to break into
their subunits. These simpler
subunits can then be
absorbed and metabolized.
Most animals lack the
enzymes necessary to break
BETA-glycosidic linkages
Amylose:
• 20-30% of Starch
• 500-20,000 glucose units in length
• Linear chain of glucose units form a
helical strcutre
Amylopectin:
•
•
•
70-80% of Starch
2,000-200,000 glucose
units in length
Branches every 20-30 glucose units
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Glycogen:
a HIGHLY BRANCHED polymer of glucose
The same as Amylopectin
but with more branch
points: every 7-12 units
Polymeric Structure:
Varying degrees of Branching
The digestive enzymes responsible for hydrolysis break
polysaccharides down on the ends of the polymer chains
Higher degree of branching makes a
polysaccharide MORE digestible.
Formation of a Polysaccharide:
A polymer (continuous chain) of simple sugars (monosaccharides)
connected together
• Starch & Glycogen: contain glucose rings joined in α
1,4- & 1,6-linkages
• Cellulose: contain glucose rings joined in β 1,4-linkages
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α linkages are readily
hydrolyzed by the
digestive enzymes of
most animals.
Starch vs. Cellulose:
Glycosidic Linkages
β linkages can only be
hydrolyzed by enzymes
present in certain
bacteria & fungi.
Both cows & termites have
bacteria present in their
digestive system that allow
them to break down
cellulose.
Cellulose is the most abundant polymer on Earth
Cellulose vs. Chitin:
Substitution an the C-2 OH for an Amide
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Polysaccharides on the Surface of Cell Membranes:
Glycoproteins (Sugars + Proteins) & Glycolipids (Sugars + fats)
Because of all their possible structural variation, carbohydrates allow specific
recognition between cells using relatively small molecules.
Blood Types:
Different Polysaccharides on the Cell Surface
• Short polysaccharide chains allow one type of
red blood cell to be distinguished from another.
Glycoconjugates:
Sugar Molecules Covalently Attached to Other Molecules
• Glycosaminoglycans: HIGHLY POLAR, capable of forming a gellike matrix to act as a lubricant and shock absorbed in connective
tissues & joints
Glycoconjugates play important roles
in cell-to-cell recognition & signaling,
the immune system, fertilization,
brain development and inflammation.
• Glycolipids: Fats with attached
carbohydrates
• Glycoproteins: Proteins with attached
Carbohydrates
•
Mucin: secreted by the respiratory &
digestive tracts, high molecular weight
proteins with dense sugar coatings, which
provide immense water holding capacity
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Carbohydrate Antigens as Markers of Cancer Cells:
Complex carbohydrates coat all cell surfaces where they are used as recognition
molecules for critical functional interactions with other cells, pathogens, and
biomolecules. The entire compliment of these structures, known as the GLYCOME,
may be one of the least studied and most complicated of the molecular
classifications in humans.
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