Transcript Reactions of Sugars Two major reactions: 1. Cyclization 2. Glycosylation

Reactions of Sugars
Two major reactions:
1. Cyclization
2. Glycosylation
Other reactions:
Reduction
Oxidation
Glycosides: Digitoxin
• One component of the heart drug digitalis
• A cardiac glycoside; classified as a steroid
• Digitalis: powdered leaves of Digitalis pupurea (foxglove)
• Excessive use of digitalis can produce xanthopsia (yellow vision)
O
O
HO
O
HO
OH
O
O
O
HO
O
OH
Digitoxin
O
Glycosides: Sucralose/Splenda
• Carbohydrate-based sweetener
• Made from sugar
• 600 times sweeter than sugar
OH
HO
HO
Cl
OH
O
O
HO
OH
OH
O
O
O
OH
OH
Sucrose
Cl
OH
OH
O
OH
OH
Sucralose
Cl
Sucralose/Splenda
• Does not metabolize to produce energy, thus no calories
• Only low calorie sweetener made from sugar
• Heat stable - used in cooking and
baking
Cl
OH
O
HO
Cl
OH
O
Discovery story:
• In 1976 by Dr. Hough’s lab
at King’s College
• From 1980 onwards, collaboration
with Tate & Lyle, a British sugar company,
and McNeil Specialty Products
O
OH
OH
Sucralose
Cl
Roles of Polycarbohydrates in Biological Systems
1. Structural elements
2. Address labels for proteins
Structural
• Glucose is polymerized in vivo to form two different linear polymers:
cellulose and starch
-glu-glu-glu-glu-
Cellulose: 1,4-b-linkage (see depiction on board):
• Rigid, rod-like (like a 2 x 4 plank)
• Many opportunities for H-bonding (the “glue” that pulls the polymer chains
together)
• Very strong, so trees can grow >300 feet tall!
Structural Role of Carbohydrates
Starch (potatoes): 1,4-a-linkage (see depiction on board):
•
•
•
•
•
a-linkage imparts a helical shape overall to the polymer - very different
from cellulose
Intramolecular H-bonding, not so much intermolecular H-bonding, so polymer
chains are not “glued” together
Water soluble, cellulose is not
Can be processed by mammals - a food source
Mammals contain enzymes which cleave (hydrolyze) the 1,4-a-linkage to
produce glucose  fuel! These same enzymes do not touch the 1,4-b-linkage
of cellulose, but bacteria can hydrolyze cellulose. Bacteria live in some
mammals, so these animals can eat plants.
Carbohydrates: Address Labels
• Many proteins need to be chaperoned to their final destinations in cell
compartments from their point of synthesis in the ribosome.
• These proteins are tagged with an address label that directs it to its
proper location.
• Address labels are oligosaccharides (carbohydrate chain 5-20 units long).
• Address labels have very precise H-bonding pattern which encodes the
address’ information. This code is “read” by receptor proteins.
Amino Acids, Peptides & Proteins
amine; basic end
a-amino acid:
H2N
R
carboxylic acid; acidic end
O
OH
H
(S) absolute stereochemistry
side chain
O
H3N
R
O
H
zwitterionic form (internal acid/base reaction):
overall neutral
salt
H2O soluble
pH = 7.3 (within cell; isoelectric point depends on R)
Amino Acids
O
O
• Are >500 naturally
occurring amino acids
identified in living
organisms
• Humans synthesize
10 of the 20 they use.
The other 10 are called
essential amino acids.
H2N
H2N
OH
OH
H
H
H
Valine (hydrophobic,
bulky)
Glycine (achiral)
O
O
H2 N
OH
H2NH2CH2CH2CH2C
H2N
OH
H
H
Lysine (basic, nucleophilic, often
used in catalysis)
O
H2 N
CO2H
Aspartic acid (acidic, used in
catalysis)
O
OH
H
NH
OH
SH
Cysteine (nucleophilic, used in
catalysis, controls shape of
protein)
H
Proline
Amino Acids, Peptides & Proteins
Peptides & proteins:
• Derived from amino acids through peptide or amide bonds.
• The amine and acid ends of amino acids couple to form amide (peptide) bonds
in peptides/proteins/enzymes. See board for further discussion.
• Proteins fold into well-defined structures. The hydrophobic residues
segregate to the water-free interior, while the polar/charged residues favor
the exterior.
Aspartame
Discovery story:
• In 1965 by Jim Schlatter
working on discovering new
treatments for gastric
ulcers.
• Made a dipeptide intermediate,
which he spilled on his hand
• Tested the dipeptide in coffee
CH3
O
O
Methyl ester
H
Phenylalanine
HN
H3N
O
H
O
• 4 calories per gram
• 180 times sweeter than sugar
Aspartic acid
O
Aspartame
Aspartame: A Dipeptide
CH3
O
O
Methyl ester
H
Phenylalanine
HN
H3N
Two main constituents:
Phenylalanine
Aspartic acid
O
Aspartic acid
H
O
O
Goal:
1. Make the methyl
ester of phenylalanine
2. Make a peptide (amide)
bond between phenylalanine
and aspartic acid
Overall - two main steps to this synthesis