Bio 98 - Lecture 11
Carbohydrates
a.k.a. Sugars,
Carbs…
I. Definition of a carbohydrate
General formula: (CH2O)n , “hydrated carbon”
Example: C6H12O6 is glucose
Many carbohydrates have more complex formulas &
contain amino, phosphate, sulfate & other groups
II. Functions
1. Fundamental source of metabolic energy for most
life forms.
2. Component of many important biomolecules.
1. Carbohydrates as an energy source
sunlight
bacteria
plants
photosynthesis
carbohydrate + O2
CO2 + H2O
animals, plants
respiration
ATP
ADP
2. Biological molecules
Cellulose - wood, plant fiber, etc.
Chitin - exoskeleton of arthropods
Cell walls of bacteria & yeast
Glycoproteins, glycolipids - cell membranes
DNA, RNA - deoxyribose and ribose
Carbohydrates are often polymers
Monosaccharides: glucose, ribose, fructose, etc.
Oligosaccharides: di-, tri-, tetra-, etc.
Sucrose is a disaccharide: glucose + fructose
Polysaccharides: can be linear or branched (i.e. starch)
Monosaccharide nomenclature
1. Carbon number: triose, .., pentose, hexose, .., octose
2.
Aldoses
and
ketoses
C#
D-glucose
1
HC=O
2
HC-OH
3
HO-CH
D-fructose
H2C-OH
C=O
HO-CH
4
HC-OH
HC-OH
5
HC-OH
HC-OH
6
H2C-OH
H2C-OH
an aldohexose
a ketohexose
Asymmetric (chiral) carbons generate many distinct
monosaccharides; compare mannose, glucose, and
galactose, which are all aldohexoses.
How many chiral carbons are in glucose?
O-chem terms relevant to monosaccharide structure
1. diastereomers - identical structures except for
configuration (chirality) at one or more carbons; e.g., all
aldohexoses are diastereomers of each other.
2. epimers - differ in chirality at only one carbon. Glucose
and galactose are epimers at carbon 4.
3. enantiomers - mirror images; designated as D- & L-;
no name change (like amino acids)!
How many aldohexose names are possible?
2 configuration choices at each of 4 asymmetric carbons;
however, half of these represent enantiomers (3. above).
Number of unique names = 24 / 2 = 8.
Ring structures
Most pentoses and hexoses
spontaneously form ring
(cyclized) structures in solution.
5-member ring: furanose
6-member ring: pyranose
When forming a ring, a
new chiral center is
created, giving rise to 2
possible anomers.
+
HC=O
HC-OH
HO-CH
HC-OH
..
HC-OH
H2C-OH
Dglucose(linear
form)
Linear form
Haworth
projection
hemiacetal
a and b
anomers
a-D-glucopyranose
In solution rapid mutarotation occurs
b-D-glucopyranose (~66%)
Linear
D-glucose
(1%)
a-D-glucopyranose (~33%)
Disaccharides
1. Mainly found in plants
2. Three common disaccharides
• sucrose - sugar cane, sugar beets
• lactose - milk sugar
• maltose - malted (germinating) barley, wheat
3. 2 monosaccharides joined covalently by an
O-glycosidic bond
Common disaccharides are produced by enzymecatalyzed condensation/dehydration reactions
Can be either
a or b due to
mutarotation
Notice:
there is no
hemiacetal
Polysaccharides - aka Glycans
1. homopolysaccharides vs
heteropolysaccharides
2. can be branched or unbranched
3. used by animals and plants as a
compact storage form of CHOs
4. common examples
• starch - plants, roots and seeds
• glycogen - liver of mammals
• cellulose - plant fiber, wood
Starch = amylose + amylopectin
found in corn, rice, potato, wheat and barley
a linear polysaccharide of glucose units
Amylopectin - a branched form of amylose
Structure of starch
glycogen (animal starch) is like starch, but more highly branched
High-fructose corn syrup
High-fructose corn syrup is produced by milling corn to produce
corn starch, then enzymatically processing that starch to yield
corn syrup, which is almost entirely glucose, followed by adding
other enzymes that change a portion of the glucose into fructose.
1. Corn starch is treated with alpha-amylase to produce shorter
chains of sugars called oligosaccharides.
2. Glucoamylase - which is produced by Aspergillus, a fungus, in
a fermentation vat — breaks the sugar chains down even further
to yield the simple monosaccharaide glucose.
3. The enzyme xylose isomerase (aka glucose isomerase) then
converts some of the glucose to a mixture of about 42% fructose
and 50–52% glucose with some other sugars mixed in.
http://en.wikipedia.org/wiki/High-fructose_corn_syrup
http://en.wikipedia.org/wiki/High-fructose_corn_syrup_and_health
High-fructose corn syrup
1. Corn starch is treated with alpha-amylase to produce shorter
chains of sugars called oligosaccharides.
High-fructose corn syrup
3. The enzyme glucose isomerase then converts some of the
glucose to a mixture of about 42% fructose and 50–52% glucose
with some other sugars mixed in.
https://wiki.geneseo.edu/display/food/Group+7-+High+Fructose+Corn+Syrup+-+Societal+Beliefs+vs.+Science
Protein glycosylation:
A post-translational modification
• Sugars covalently attached to the
polypeptide as oligosaccharide
chains containing 4 to 15 sugars
• Sugars frequently comprise 50% or
more of the total molecular weight of
a glycoprotein
• Most glycosylated proteins are either
secreted or remain membranebound
• Glycosylation is the most abundant
form of post-translational
modification
• Glycosylation confers resistance to
protease digestion by steric
protection
• Important in cell-cell recognition
Blood group antigens on the erythrocyte surface
• The O substance is a
tetrasaccharide which is missing the
5th residue and does not elicit an
antibody response (non-antigenic).
• The A antigen and B antigen are
pentasaccharides which differ in
composition of the 5th sugar residue