Summary
Summary of Carbohydrates and Lipids
- Course
- Biochemistry (MCB2021F)
- Institution
- University Of Cape Town (UCT)
Notes are the summaries to the Carbohydrates and Lipids lecture series for MCB2021F
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Carbohydrates and glycoconjugates:Carbohydrates are the precursors of all other biomolecules, the covalently link to a variety
of other molecules. Glycoconjugates are important components of cells walls and
extracellular structures in plants, animals and bacteria. They are important in recognition
between cells or cellular structures by other molecules – good for cell growth, fertilization,
transformation of cells.
Biochemical role of carbohydrates:
1. Energy
2. Structural support
3. Protection
4. Recognition
Characteristics that make carbohydrates versatile and important:
1. At least one or more asymmetric centers
2. Ability to exist as linear or ring structures
3. Capacity to for polymeric structures via glycosidic bonds
4. Potential to form multiple hydrogen bonds with water and other molecules
Groups of carbohydrates:
1. Monosaccharides (also known as simple sugars) (CH2O)n – cannot be broken down
into smaller sugars
2. Oligosaccharides – consist of two to ten simple sugars. Four to six oligosaccharides
are usually bound covalently to other molecules such as glycoproteins
3. Polysaccharides – polymers of the simple sugars and their derivatives. Can be linear
or branched.
Monosaccharides are considered Aldoses or Ketoses:
Described whether they have an aldehyde or ketone functional group. the simplest aldose is
a glyceraldehyde and simplest ketose is a dihydroxyacetone – both only contain three
carbons and are therefore trioses, the naming follows either aldohexose, ketohexose,
aldopentose or ketopentose dependent on the number of carbons in the molecule.
Aldose with at least three carbons and ketose with at least four carbons will have a chiral
carbon. A chiral carbon is a carbon attached to four different molecules. When a molecule
has more than one chiral carbon, the molecule gets denoted with a D or L nomenclature. D
is when the chiral carbon has the hydroxyl group on the right hand side of the chiral carbon
(furthest away from the functional group) or L when the hydroxyl group is on the left hand
side of the chiral carbon.
D and L configuration indicate that the molecules are mirror images of each other
(enantiomers). If the molecule has more than two chiral carbons then more than two
stereoisomers can exist.
The number of stereoisomers = 2n where n is the number of chiral centers, half will be D and
half will be L
Pairs of isomers that have opposite configurations at one or more chiral centers but are not
mirror images of each other are known as diastereomers.
,Sugars that differ in configuration at one chiral center are described as epimers e.g. D-
mannose and D-talose are epimers.
D-glucose and D-mannose are epimers but D-glucose and D-talose are diastereomers.
Monosaccharides exist in cyclic and anomeric forms:
Monosaccharides have the ability to form cyclic structures with formation of an additional
asymmetric center.
Alcohols react with aldehydes to form hemiacetals – the result is a pyranose ring (six-
member ring (5C and 1O)); this reaction is catalyzed by acid (H+) or base (OH) and is readily
reversible.
Alcohols can react with ketones and form hemiketals – this results in a furanose ring (five-
member ring (4C and 1O)).
In aqueous solution, pyranose and furanose ring structures are preferred over linear
structures for monosaccharides. When ring structures are formed – the carbon atom that
carried the carbonyl functional group becomes the asymmetric carbon.
Isomers of monosaccharides that differ in only the configuration about that carbon are
called anomers, anomeric carbons. When the anomeric carbon is on the same side as the
Fischer projection as the oxygen atom at the highest numbered asymmetric carbon then the
configuration is alpha. When the hydroxyl is on the opposite side of the highest numbered
asymmetric carbon then the configuration is beta. This is an interchangeable process that is
known as mutarotation.
Beta configuration is more stable than alpha and alpha is more stable than linear.
Haworth projections are good for drawing sugars:
Denotes pyranose and furanose structures as hexagonal and pentagonal rings lying
perpendicular to the plane of the paper which thickened lines which indicate the side of the
ring closest to the reader.
Fischer projections are used to draw Haworth projections, if the substituents are drawn to
the left of the Fischer projection then they lie above the Haworth ring and substituents from
Fischer projection on the right are drawn below the Haworth ring.
D and L configuration is taken into consideration in Haworth projections – in D sugars the
anomeric hydroxyl group is drawn below the ring in the alpha anomer and above the ring in
the beta anomer. In L sugars the anomeric hydroxyl group is drawn above the ring in the
alpha anomer and below the ring in the beta anomer.
How to change a Fischer into a Haworth:
1. Number the carbons in Fischer projection – the functional group gets the first
number in a aldose and gets the lowest possible number in a ketose.
2. Rotate the Fischer projection 90o clockwise so that the functional group is on the
right-hand side and that the groups on the right hand-side of the Fischer projection
, point downwards and the groups on the left hand-side of the Fischer projection
point upwards.
3. Carbon five (or second last carbon from the left) needs a bond rotation – the last
carbon moves from the left of the molecule to the top position, the hydroxyl group
from the bottom moves to the left and the hydrogen from the top moves to the
bottom
4. The ring closes because the carbon five attacks the carbon one and breaks the
double bond oxygen, this allows for the protonation of the carbon one – oxygen to
become carbon bonded to a hydroxyl. The result is a five-carbon pyranose ring
structure.
5. All the groups pointing down in the rotated Fischer then point down in the pyranose
ring, all the groups pointing up in the rotated Fischer then point up.
6. The direction of the hydroxyl group on the carbon to the right of the oxygen
determines whether the ring is alpha or beta configuration. If pointed down then
alpha, if pointed up then beta.
Six carbon sugars have two hydroxyl groups that can react with the functional group and
thus can result in either a pyranose or furanose ring formation. A beta anomer in a pyranose
ring is in chair configuration.
If the hydroxyl is in the axial then it is below the ring, if the hydroxyl is in the equatorial then
it is above the ring. Equatorial is more stable than axial and requires/ uses less energy.
Monosaccharide derivatives:
Sugars with a free anomeric carbon are good reducing agents and will reduce oxidizing
agents, these reactions convert sugars into sugar acids. Carbohydrates that can reduce
oxidizing agents are referred to as reducing sugars, the sugar that reduces another
compound is itself oxidized.
Reducing sugars are found in benedicts test where the blue colour from copper turns to
brick red in the presence of a reducing sugar. Cu2+ à Cu+ and Cu2O + precipitate.
1. Sugar acids – aldose, aldehyde or OH group is oxidized to form carboxylic acid
2. Sugar alcohols – aldehyde/ ketone is reduced at hydroxyl group
3. Deoxy sugars – backbone of DNA, one or more oxygen in hydroxyl is removed and
leave hydrogen
4. Sugar phosphates – glucose metabolism, DNA and ATP
5. Amino sugars – one or more hydroxyl groups replaced by amino group
Disaccharides are the simplest form of oligosaccharides:
Disaccharides consist of two monosaccharides that are linked by a o-glycosidic bond in a
dehydration reaction, most abundant ones are sucrose, maltose and lactose. Each of these
disaccharides expect for sucrose is a reducing sugar because they have a free unsubstituted
anomeric carbon.
The end of the molecule that has the unsubstituted free anomeric carbon is known as the
reducing end (this end of the sugar is able to interact with another sugar and thus is not
bonded) and the other end that forms the bond with the other monosaccharide is known as
the non-reducing end. Sucrose is a non-reducing sugar.
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