Carbohydrates
Carbohydrates
• “Hydrates of carbon” (C, H,
& O)
• Polyhydroxy aldehydes
(ALDOSES) or ketones
(KETOSES)
• Usually Cx(H2O)y
• “Sugars”
• Single unit:
Monosaccharide
• Two units: Disaccharide
• Three units: Trisaccharide,
etc.
• Many units: Polysaccharide
Carbohydrates
• Functions
– Energy stores, fuels, and metabolic
intermediates
– Ribose and deoxyribose serve as structural
framework to RNA and DNA
– Structural elements in the cell walls of
bacteria and plants
– Linked to lipids and proteins
• Mediates interactions among cells
• Mediates interactions between cells and other
elements in the cellular environment
Monosaccharides
• The simplest carbohydrates; aldehydes or
ketones that have two or more hydroxyl groups
Monosaccharides
• Fischer projections of monosaccharides
• D,L designation refers to the configuration of
the highest-numbered asymmetric center
• D,L only refers the stereocenter of interest
back to D- and L-glyceraldehyde!
• D,L do not specify the sign of rotation of
plane-polarized light!
Aldehydes and Ketones Can React with
Alcohols to Form Hemiacetals and Hemiketals
Glucose Cyclizes into a Hemiacetal
Fructose Cyclizes into a Hemiketal
Ribose and Deoxyribose
Conformation of Pyranose and Furanose
Rings
Glycosidic Bonds
Glucopyranoside Has Different Reactivity
Compared to Glucose
Modified Monosaccharides
• Addition of substituents other than alcohols;
often found on cell surfaces
Disaccharides
Polysaccharides
• Energy storage and structural roles
• Are homopolymers if all of the
monosaccharides are the same,
heteropolymers if not.
Some Common Polysaccharides
•
Starch: glucose polymer (alpha)
– Potato, rice, wheat, corn
•
Glycogen: branched glucose
polymer
– Animal storage
•
Cellulose: glucose polymer (beta)
– Plant structures, paper, cotton,
wood
•
Chitin: Modified glucose (N-Acetyl
Glucose)
– Fungi cell wall, insect exoskeleton
Polysaccharides
Starch
A plant storage polysaccharide
• Two forms: amylose and amylopectin
• Most starch is 10-30% amylose and 7090% amylopectin
• Branches in amylopectin every 12-30
residues
• Amylose has alpha(1,4) links, one
reducing end
Starch
• Amylose-unbranched
Starch
• Amylopectin-branched
Starch
A plant storage polysaccharide
• Amylose is poorly soluble in water, but
forms micellar suspensions
• In these suspensions, amylose is helical
– iodine fits into the helices to produce a blue
color
Why branching in Starch?
Consider the phosphorylase reaction...
• Phosphorylase releases glucose-1-P products
from the amylose or amylopectin chains
• The more branches, the more sites for
phosphorylase attack
• Branches provide a mechanism for quickly
releasing (or storing) glucose units for (or from)
metabolism
Glycogen
•
•
•
•
•
The glucose storage device in animals
Glycogen constitutes up to 10% of liver mass and
1-2% of muscle mass
Glycogen is stored energy for the organism
Only difference from starch: number of branches
Alpha(1,6) branches every 8-12 residues
Like amylopectin, glycogen gives a red-violet color
with iodine
Dextrans
A small but significant difference from starch and glycogen
• If you change the main linkages between
glucose from alpha(1,4) to alpha(1,6), you
get a new family of polysaccharides dextrans
• Branches can be (1,2), (1,3), or (1,4)
Dextrans
A small but significant difference from starch and glycogen
• Dextrans formed by bacteria are
components of dental plaque
• Cross-linked dextrans are used as
"Sephadex" gels in column
chromatography
• These gels are up to 98% water!
Structural Polysaccharides
Composition similar to storage polysaccharides, but small
structural differences greatly influence properties
• Cellulose is the most abundant natural polymer on
earth
• Cellulose is the principal strength and support of
trees and plants
• Cellulose can also be soft and fuzzy - in cotton
Cellulose
Structural Polysaccharides
Composition similar to storage polysaccharides, but small
structural differences greatly influence properties
• Beta(1,4) linkages make all the difference!
• Strands of cellulose form extended ribbons
Other Structural
Polysaccharides
• Chitin - exoskeletons of crustaceans,
insects and spiders, and cell walls of fungi
– similar to cellulose, but C-2s are N-acetyl
– cellulose strands are parallel, chitins can be
parallell or antiparallel
Other Structural
Polysaccharides
• Alginates - Ca-binding polymers in algae
– Eg. CARAGEENAN
• alternating 3-linked-α-D-galactopyranose and 4-linked-β-Dgalactopyranose units
• Sulfonated, uses cations to gel; Common types 
• Agarose and agaropectin - galactose polymers
• Glycosaminoglycans - repeating disaccharides
with amino sugars and negative charges
Glycosaminoglycans
• Made of disaccharide repeating units containing
a derivative of an amino sugar, either
glucosamine or galactosamine
Glycosaminoglycans
• Present in the animal cell surface or the
extracellular matrix
• Usually attached to proteins to form
proteoglycans
• Proteoglycans resemble polysaccharides more
than proteins
• Function as lubricants and structural
components in connective tissue, mediate
adhesion of cells to the extracellular matrix, and
bind factors that stimulate cell proliferation
Oligosaccharides are Made by
Glycosyltransferases
Effects of Glycosyltransferases
Illustrated by the ABO Blood Group
http://en.wikipedia.org/wiki/Blood_type
Glycoproteins
• Carbohydrates attached to proteins
• Carbohydrates are a much smaller percentage
of the weight of glycoproteins than of
proteoglycans
• Present in cell membranes
– Cell adhesion and the binding of sperm to
eggs
• Present in soluble proteins
Linking Carbohydrates to Proteins
Elastase is a Soluble Glycoprotein
Protein Glycosylation in the Lumen of the
ER and in the Golgi Complex
Proteins from the ER are Carried to the Golgi
Complex for Further Glycosylation
Peptidoglycans in bacteria
http://en.wikipedia.org/wiki/Gram_stain, http://microvet.arizona.edu
Lectins
• Ubiquitous proteins that bind to a specific
saccharide
• Main function is facilitate cell-cell contact
C-type Lectin Binding Domain
Influenza Virus Hemagglutinin Recognize Sialic
Acid Residues on Glycoproteins
CARBOHYDRATES
• Extra carbohydrates is stored in the liver
and muscle tissues as glycogen
• Carbohydrates supply 4 kcal of energy per
gram
• “Good” and “Bad” carbohydrates
• Glycemic index
On the “Sugarfree” Phenomenon
• Saccharin
– Oldest artificial
sweetener
• Aspartame and
Acesulfame-K
– Not carbohydrates
• Sucralose
– “Splenda”