Carbohydrates Flashcards
Functions of carbohydrates
1) Energy stores (glycogen, starch).
2) Components of structural material
(e.g. chitin for insect exoskeletons).
3) Parts of other important
macromolecules – DNA and RNA,
glycoproteins, and glycolipids.
4) Signalling function (recognition of cell surface molecules on other cells)
general formula for carbohydrates
C: H O:
1 : 2: 1
carbohydrates are hydrates of carbon with the
general formula Cx(H2O)x
Characteristics of monosaccharides
- Monosaccharides are colourless, crystalline solids that are
soluble in water and other polar solutions – in other words
monosaccharides are hydrophilic. - Monosaccharides have names ending with “ose”.
Glucose, Ribose, Deoxyribose…. - Most have a sweet taste
Continued
- The backbone of the monosaccharide is a carbon chain with
all carbons joined by single covalent bonds.
Glucose when solubilised, they form a ring structures
5- Monosaccharide chains contain a single carbonyl group
(C=O)
If it contains an aldehyde
carbonyl group.
aldose
D-Glyceraldehyde
If it contains a ketone
carbonyl group
ketose
Dihydroxyacetone
Common 6-carbon monosaccharides
Glucose
Most abundant sugar in nature; in many foods, e.g.
bread, pasta, fruit, cereals.
Most of the carbohydrates we eat are converted into
glucose for energy.
Fructose
Naturally occurring in foods such as honey & fruit.
Galactose
In nature primarily found as part of lactose (milk sugar).
Mannose A by-product of metabolism, also found in some fruitsand vegetables.
Monosaccharides have chirality
The chiral carbon which
determines whether a
carbohydrate is in the D
or L configuration is the
one furthest from the
carbonyl group
In glucose
Carbons 2-5 are chiral
C6 = Not a chiral carbon. Chiral
carbon should have 4
different atoms/groups
attached to it!
Which chiral carbon
determines D/L
configuration?
C5
D-glucose and L-glucose
are enantiomers.
Enantiomers
1- Do they have the same chemical formula? YES
2- Do they have chiral carbons? YES
3- Can they be superimposed? NO
4- Are they mirror images? NO
diastereomers.
1- Do they have the same chemical formula?
2- Do they have chiral carbons?
3- Can they be superimposed?
4- Are they mirror images?
YES
YES
NO
NO
These molecules are called diastereomers.
Hexoses…..
Same Chemical Forumula
How many diastereomers can a molecule have?
A molecule with n chiral carbons has (2n-2) diastereomers.
Example:
A molecule with 4 chiral carbons will have 24-2 = 14 diastereomers.
carbon derived from the carbonyl carbon is known as
the anomeric carbon
D-glucose
The ring has 6 members
Rings with 6 members are
called pyranose rings.
Fructose
Fructose is a ketose
The ring has 5 members
Rings with 5 members are
called furanose rings.
Common furanose monosaccharides
Deoxyribose is part of the structure of DNA
Ribose is part of RNA.
Forming carbohydrate polymers
Monosaccharides are joined together to form
disaccharides, oligosaccharides and polysaccharides by
condensation reactions.
Covalent bonds that form between a carbohydrate and
another molecule are called glycosidic bonds (this
other molecule does not need to be another
carbohydrate).
Maltose
Condensation reaction between α-D-glucose + either α or β-D-glucose
Covalent bond is α-1,4-glycosidic
Formula is: C12H22O11
Sucrose
Condensation reaction between α-D-glucose + b-D-fructose
Covalent bond is α-1,2-glycosidic
Formula is: C12H22O11
Lactose
Condensation reaction between b-D-galactose + a/b-D-glucose
Covalent bond is b-1,4-glycosidic
Formula is: C12H22O11
Cellobiose
Condensation reaction between b-D-glucose + b-D-glucose
Covalent bond is b-1,4-glycosidic
Formula is: C12H22O11
Why are some bonds named as a bonds and some b?
alpha glycosidic bonds are formed when the OH on the C1
(anomeric carbon) is below the ring
beta glycosidic bonds are formed when the OH on the C1
(anomeric carbon) is above the ring.
Starch
energy store of plants
Very large polymer of a-D-glucose.
Amylopectin: Branched polymer (every 24-30 glucose
molecules there is a branch).
Glucose monomers are linked through a-1,4-
glycosidic bonds.
The bond linking branches to the main chain is an
a-1,6-glycosidic bond.
Amylose: Linear polymer. Glucose monomers are
linked through a-1,4-glycosidic bonds.
Glycogen
energy store of animals
Very large polymer of a-D-glucose with
many branches
Branched polymer (every 8-12 glucose molecules
there is a branch).
Glucose monomers are linked through a-1,4-
glycosidic bonds.
The bond linking branches to the main chain is
an a-1,6-glycosidic bond.
Cellulose
for structural support in plant cell walls
Very large, unbranched polymer of b-D-glucose.
Most abundant polysaccharide on earth.
Humans cannot digest cellulose. We do
not have the correct enzymes (called
cellulases) to break the b-1,4-glycosidic
bonds.
Chitin
structural component of insects, molluscs and
fungi cell walls
Polymer of: N-acetylglucosamine
(shortened to GlcNAc).
This molecule contains:
b-D-glucose
Amino group
Acetyl group
Continued
Chitin is a polymer of N-acetylglucosamine
The covalent bond linking the GlcNAc monomers is b-1,4-glycosidic.
GLYCOSYLATION
Cellular process that attaches an oligosaccharide to a protein.
Can occur as the protein is being made or is added afterwards
(post-translational modification).
Estimated that more than half of proteins made in eukaryotic
cells are glycosylated.
Continued
Creates a diversity of structures – vastly increases the
number of functions proteins can perform.
Position the carbohydrate is attached.
Types of sugars attached.
Short or long chains.
Branched or unbranched.
Glycoproteins
Structure is mostly protein with short, linear, oligosaccharides
added on.
Proteoglycan
Structure is mostly polysaccharides held together by a protein
chain.
Types of Glycoprotein
Glycoproteins are classified by the type of the glycosidic
bond that connects the carbohydrate to the protein.
There are 2 main types of glycoproteins:
N-linked and O-linked
~90% of glycoproteins are N-linked.
Production of glycoproteins
N-linked glycoproteins are
formed in the endoplasmic
reticulum and then the golgi.
O-linked glycoproteins are
made in the golgi.
N-linked glycoproteins
Connects the sugar and protein through a nitrogen atom that
is on either an asparagine or lysine amino acid.
Functions include:
Cell-cell adhesion.
Cell signalling.
Host-pathogen
recognition.
Only found in eukaryotes and
Archaea, not Bacteria
Cell surface glycoproteins are used by viruses
- Haemagglutinin and Neuraminidase are
receptors on the flu virus protein coat. - Haemagglutinin binds to specific N-linked
glycoproteins on the cell it wants to
infect. - Neuraminidase cleaves an oligosaccharide
in the cell membrane to disconnect the
virus.
O- linked glycoprotein
Connects the sugar and protein through an oxygen atom that is
on either a serine, threonine or hydroxy-lysine amino acid.
Found in eukaryotes, Archaea and Bacteria.
Functions include:
ABO blood groups.
Extracellular matrix component.
Shock absorption in joints.
Proteoglycans
The polysaccharide chains within the proteoglycans change the
properties of water to a slimy, gel-like texture.
This texture is a useful function in cartilage, mucus and
synovial fluid in joints.
They provide shock absorption because they are surrounded by
a lot of water and can release it when pressure is applied.
Proteoglycans also provide structural support as part of the
extracellular matrix in eukaryotic cells.
