Lecture notes
Structural Carbohydrates
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- University Of Southampton (UOS)
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Structural CarbohydratesCarbohydrates are most abundant biomolecules on earth
In one year- Photosynthesis converts 100x 10 9 metric tonnes of CO2 and H2O to cellulose and other plant
products
and its ~60-70% carbohydrate
The main source of energy for all living organisms
Carbohydrates serve as structural components- cellulose and chitin
Carbohydrate polymers (Gylcans)- serve as structural and protective elements in cell walls of bacteria and
plants and in connective tissues of animals
o Other carbohydrate polymers- skeletal joints and participate in recognition and adhesion between
cells
o Complex carbohydrate polymers covalently attached to proteins or lipids- act as signals- determine
intracellular location or metabolic fate – Glycoconjugates
Carbohydrates are polyhydroxy aldehydes or ketones or substance- yield such compounds on hydrolysis
Carbohydrates can be divided into three major groups on basis of their structures:
Simple sugars- consist of monosaccharides, disaccharides and oligosaccharides- often function as metabolic
intermediates in energy conversion pathways
o Monosaccharides- consist of single polyhydroxy aldehyde or ketone unt
- Most abundant is six-carbon sugar D- glucose – dextrose
o Oligosaccharides- consist of short chain of monosaccharide units/ residues joined by glycosidic
bonds
o Disaccharides- with two monosaccharides units
Polysaccharides- consist of either glucose homopolymers- e.g. cellulose or disaccharide heteropolymers-
e.g. chitin or keratan sulphate- in which one of two sugars hexosamine- monosaccharide containing amino
group
Glycoconjugates- proteins or lipids with covalently attached glycans- play critical role in cellular
communication
o Glycosyltransferases- covalently linked glycans to proteins and lipids
o Glycosidases- remove glycans through hydrolysis reactions
The basics
Monosaccharides
Monosaccharides are either aldehydes or ketones with two or more hydroxyl groups. Many of carbon atoms to
which hydroxyl groups are attached are Chiral centres- give rise to many stereoisomers
In general- molecule with n chiral centres can have 2n- stereoisomers
Chiral centres- atom bonded to four different chemical groups
- Lack plane of symmetry and exist as two optical isomers- enantiomers
Enantiomers- exist in nature as either- D-form (right handed) or L-form (left
handed)- differentially rotate polarised light
Epimers- two monosaccharides that differ in position of OH group around only one
carbon atom
Hemiacetel- bond between alcohol and aldehyde group of aldose sugar
-C5 OH group of D-glucose attacks C1 aldehyde group- form cyclic hemiacetal- C1 of D-glucose becomes new chiral
, centre
When cyclic form of glucose has OH group at C 1 on same side of ring as CH2OH- called β-D-glucose
OH group of C1 on opposite side of ring as CH2OH- α-D-glucose
In aqueous solution- equilibrium is established between α-D-glucose and β-D-glucose conformation at ratio of
about 40:60 for α-D-glucose to β-D-glucose
Hemiacetal C1 of cyclic D-glucose- anomeric carbon and β-D-glucose and α-D-glucose- referred to anomers- differ
only at anomeric carbon
2 enantiomers D and L- C2-5= chiral centres, C5- determining for glucose and fructose- both in D conformation
2 anomers at C1 position α and β
Glucose
Galactose
Fructose
Addition of Hydroxyl groups from within same molecule – generate CYCLIC forms
having four or more backbone carbons
- Ring closure creates new chiral centre
Monosaccharides are colourless, crystalline solids- freely soluble in water but insoluble in non-polar solvents.
Most have sweet taste
Backbone of monosaccharides are unbranched carbon chains- all carbon atoms are linked by single bonds.
In open chain form- one of carbon is double bonded to oxygen atom- form carbonyl atom, each of ither carbon
atoms has hydroxyl group
- Hydroxyl group at end- Aldehyde
- Hydroxyl group at other position- Ketone
Disaccharides
Structural carbohydrates and polysaccharides- glycans- in which monosaccharide units are joined as in disaccharides
Disaccharides- consist of two monosaccharides joined covalently by glycosidic bond- formed when hydroxyl group
of one sugar reacts with anomeric carbon of another
- Formation of acetal from hemiacetal and alcohol- result in glycoside
Glycosidic bonds are hydrolysed by acid but resist cleavage by base- disaccharides can be hydrolysed to yield their
free monosaccharide components by boiling with DILUTE ACID
Maltose- contains D-glucose residues joined by glycosidic linkage between C1 one glucose residue and C4 of
another
o Disaccharide retains free anomeric carbon, maltose is reducing sugar –
Glc(α1→4)Glc
Lactose- yields D-galactose and D-glucose on hydrolysis
o Lactose is reducing disaccharide- Gal (β1-4)Glc
Sucrose- disaccharide of glucose and fructose- formed by plants
o Sucrose contains no free anomeric carbon atom- Non-reducing sugar
o Glc (α1-2 β)Fru
Polysaccharides
Polysaccharides (Glycans)- differ from each other in identity of their recurring monosaccharide units- in
length of their chains, in types of bonds linking units and in degree of branching
o Homopolysaccharides- contain only single monomeric species
Serve as storage forms of monosaccharide- used as fuels, STARCH, GLYGOGEN
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