Download Carbohydrates

Carbohydrates
Monosaccharides: 1 sugar unit
Disaccharides: 2 sugar units
Oligosaccharides: 3 – 10 sugar units
Polyscaccharides: More than 10 sugar
units
* The sugar units are connected by a
bridging oxygen atom known as the
glycosidic link
Monosaccharide Nomenclature
Classification based on location of the double bond C=O
The family-name ending -ose indicates a carbohydrate
Monosaccharide Classification
using # of C atoms in the chain
Can be either a ketose or an aldose sugar
Enantiomers - Non-superimposable mirror images of a
Chiral molecule
* A chiral carbon is attached to 4 different groups
adapted from
ull.chemistry.uakron.edu
The D and L classification is from optical activity
Physical Properties
Optical Activity
Ability to rotate planar polarized light
Dextrorotatory (D- isomer)
-Also written as (+) isomer
-Rotates light to the right
Levorotatory (L-isomer)
-Also known as (-) isomer
-Rotaes light to left
- Usually white in color. Tastes sweet. Soluble in water (why?)
Note- Each pair of enantiomers have a different name
Using Fischer projections
In a carbohydrate Fischer projection the Aldehyde CHO or the Ketone C=O
Is always placed on top. The dotted lines are connected to atoms that are
Projected above the plane, while the solid wedges are atoms that are
behind the plane
The result is that -H and -OH groups projecting above the page are on the left
and right of the chiral carbons, and groups projecting behind the page are
above and below the chiral carbons
Some Important Monosaccharides
• D- Glucose- Most common. Also known
as blood sugar or grape sugar. Main
energy source and the brain requires a
constant concentration in blood.
• D-Fructose- Usually found in fruits and
sweeter than table sugar
• D-Ribose- Used in the biosynthesis of
RNA.
Cyclic Monosaccharides
Monosaccharides have a aldehyde/ketone and an
alcohol in the same molecule In the same molecule,
so it forms an intramolecular hemiacetal linkage
Definitions
Anomers: Cyclic sugars that differ only in positions of
substituents at the hemiacetal carbon (the anomeric carbon)
the α form has the -OH on the opposite side from the -CH2OH (below the plane of the ring)
the β form has the -OH on the same side as the -CH2OH (above the plane of the ring)
Q: Is the hemiacetal carbon a ‘chiral carbon’ ?
Haworth Projections
The monosaccharides are shown as cyclic and planar rings in a
Haworth projection unlike Fischer projections
(the six-membered ring are generally in a chair conformation)
Diagram adapted from www.rpi.edu/molbiochem
D and L forms
How to distinguish between D and L forms of Glucose ?
Diagram adapted from www.rpi.edu/molbiochem
Monosaccharide: Fructose
A ketohexose, can also form a 5-membered ring in solution
Monosaccharide: Ribose
A 5 carbon aldehyde sugar, i.e aldopentose
Both molecules are part of larger biomolecules, such as, coenzymes.
Ribose is part of RNA and Deoxyribose is a part of DNA (Deoxyribo-)
Reactions of Monosaccharide
We will look at three types:
•
Reducing sugar reactions (Lab)!!!
•
Phosphate ester formation
•
Glycoside bond formation
Reducing sugar reactions
Aldehyde sugars get oxidized to carboxylic acid with the help of an
oxidizing agent.* Note: we refer to the open-chain form of the aldose.
These sugars are known as reducing sugars
The hemiacetal group must be present in a reducing sugar
Phosphate esters
The –OH groups of sugars can add phosphate groups to form phosphate esters.
** Will see these esters a lot when we look at carbohydrate metabolism !
Glycoside bond formation
This reaction can also happen between two monosaccharide sugars
methyl glucoside (methyl-glucopyranose).
Disaccharides
1,4-Glycosidic Links
Polysaccharides
1,4-glycoside linkage between glucose molecules
Q. Which end is the reducing sugar ?
Q. Is it a a galactosidic linkage or a β linkage ?
Branched Polysaccharides
Distinguish between 1,6 and 1,4 glycosidic links
Glucosamines
An Amino group replaces the Hydroxyl (-OH) group.
The amine is ‘acetylated’ on the right
In Summary
Monosaccharides are simple sugars
-An ‘aldose’ is an aldehyde, a ‘ketose’ is a ketone
-Triose: 3 carbons ; Tetrose: 4 carbons etc…
-Aldohexose: an aldehyde sugar with 6 carbons
When writing D sugars: The lowest –OH group goes to
the right, L-sugar the lowest –OH is on left
Cyclic sugars: Cyclization by internal hemiacetal formation. Forms between C1 and C5.
Haworth projections: Showing the ring sugars in a flat structure.
Anomers: α and β anomers from Haworth (ring) structures. In the α form the –OH on C1
is on the opposite of –CH2OH (C6).
In the β form they are on the same side of the ring.
A reducing sugar: A carbohydrate that gets oxidized itself to carboxylic acid. It reduces
the oxidizing agent used. (Generally aldoses are reducing sugars).
Mutarotation: change in the specific rotation of a cyclic monosaccharide as it reaches
an equilibrium between its α and β anomeric forms.
Practice:
-Distinguishing acetals and hemiacetals in sugars
-Identifying 1,4-, 1-2 Glycosidic linkages
-Writing hydrolysis reactions of disaccharides
- Writing Structures of phosphate esters
- Oxidation/reduction reactions of sugars
The monosacchararide shown below is
classified as a(an)
1.
2.
3.
4.
aldotriose
aldopentose
ketotriose
ketopentose
What is the maximum number of aldotetrose
stereoisomers?
1.
2.
3.
4.
The cyclic structure below is
1.
2.
3.
4.
α-D-galactose
α-L-galactose
β-D-galactose
β -L-galactose
One
Two
Three
Four
The cyclic structure below is
1.
2.
3.
4.
α-D-galactose
α-L-galactose
β-D-galactose
β -L-galactose
The disaccharide maltose is shown below. Classify the
connection between the two α-D-glucose units as an α1,4 link or a β-1,4 link, and indicate whether maltose is
a reducing or a nonreducing sugar.
1.
2.
3.
4.
α-1,4 link, nonreducing
α-1,4 link, reducing
β -1,4 link, nonreducing
β -1,4 link, reducing
In N-linked glycoproteins, the sugar is usually attached
to the protein by a bond to the N-atom in a side-chain
amide. Which amino acid below can form such a bond?
1.
2.
3.
4.
Cysteine
Glutamine
Threonine
Tyrosine
The polysaccharide that provides structure in
plants is
1. Amylopectin
2. Amylose
3. Cellulose
4. Glycogen