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Biomolecules lecture 3
Carbohydrates and proteins
Carbohydrates
•Organic molecules
•Carbohydrates and sugars usually have names
ending in "ose.“
•Containing Carbon Hydrogen and oxygen in a
ratio of 1:2:1
•General formula = (CH2O)N
•The term carbohydrate refers to the fact that
carbohydrates all contain carbon atoms which are
"hydrated".........C n(H2O)n.....
•The simplest carbohydrates are the
monosaccharide sugars e.g. glucose, fructose,
galactose (6 carbon atoms).
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• For convenience sugars are drawn as if their
carbon skeletons were linear
• However in aqueous solutions many
monosaccharides form rings
• Example: glucose
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Sugar Isomers
•Glucose has the formula C6H12O6
•Fructose and galactose have the same
formula
•These sugars are isomers, or alternative
forms of glucose
•Even though isomers have the same
formula, their atoms are arranged in
different ways ie their 3D structures are
different
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Stereoisomers
• Glucose and galactose have the
same structure apart from the 3D
orientation of one –OH group
• The –OH group positions are mirror
images of each other
• Galactose and glucose are called
stereoisomers
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• Seemingly minor differences in chemistry give isomers
different properties
• In this example, different arrangement of chemical
groups makes fructose taste sweeter than glucose
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Types of Carbohydrates
Monosaccharides:
Simple building blocks/sugars (e.g. glucose)
Disaccharides:
Contain 2 monosaccharide/sugar units
bound together (e.g. sucrose – table sugar)
Polysacchrides:
High molecular weight carbohydrates
Contain many (hundreds/thousands) monosaccharide/sugar
units to form long chains that are either straight or branched
(e.g. starch)
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Sucrose
•Sucrose / table sugar = most common disaccharide
•Main carbohydrate in plant sap and it nourishes all
of the plant
•Extracted from the stems of sugarcane/sugar beets
•Sucrose = 1 molecule of glucose + 1 molecule of
fructose
•Glucose is not very sweet but fructose is
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Disaccharides
Maltose
• Maltose, common in germinating seeds is used in
making beer and milk shakes
• It is composed of 2 glucose molecules
• Lactose is another disaccharide made of glucose
+ galactose
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Polysaccharides
• Complex carbohydrates
• Long chains of monosaccharides
• e.g. glycogen, plant starch, cellulose (found in plant cell
walls), and chitin (a constituent of the shells of insects).
• They are all homopolymers meaning they are all made of only
one sugar repeated over and over.
• Glycogen, plant starch and cellulose are made of glucose
only; chitin uses modified glucose (with nitrogen atoms
added).
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Glycogen
• Animals store excess glucose as glycogen
• Glycogen is made up of chains of glucose molecules,
extensively branched
• Important energy storage molecule in muscle and liver
• Stored in granules from which it can be hydrolysed to release
glucose when needed for energy
• Basis of carbo-loading-the consumption of large amounts of
starchy foods the night before an athletic event
• The starch is digested into glucose then converted into
glycogen in muscle and liver, which is then available for
rapid use during physical activity
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Starch
•Starch is used for energy storage in
plants
•Like glycogen, made of chains of glucose
molecules, but not branched
•Source in the diet: potatoes and grains
such as wheat, corn and rice
•Our salivary gland produces amylase, an
enzyme which breaks down amylose
(plant starch) into a disaccharide (maltose)
•Further digested into monomeric
components (glucose) in small intestine
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Cellulose
• Cellulose is the primary component of plant cell walls.
• Like starch, made up of glucose units, but linked together
differently: cannot be broken down by the enzymes found
in animals
• Cows and other ruminants can digest cellulose by means
of bacteria that live in their intestines which provide the
necessary enzymes
• In humans, cellulose is an important part of the diet, even
though we can’t digest it: it is dietary fibre, important for
digestive health.
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Glycolipids and Glycoproteins
•Polysaccharides can also combine with other classes of
macromolecules eg lipids and proteins to form glycolipids and
glycoproteins
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Proteins
•All proteins contain carbon, hydrogen, oxygen
and nitrogen
•Most common biomolecule, found in all cells and
all parts of cells
•Regulate many body processes: transportation,
protection (immune system), muscle contraction
(actin and myosin), structure (e.g. cytoskeleton,
collagen) and energy (enzymes involved in
metabolism)
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Structural proteins
Defensive
proteins
(antibodies)
Overview
of Protein
Function
Receptors
Contractile proteins
Hormones
Enzymes
Transport
proteins
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Proteins
•Amino acids are the basic building blocks of proteins
•All proteins are built from the same set of 21 amino
acids; chains of 15 – 10,000 amino acids
•Each amino acid has an amine group (-NH2), carboxyl
group (-COOH), a hydrogen atom and a side chain R
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Proteins
•Amino acids joined by a
dehydration reaction to
form a peptide bond
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Proteins
•There are 21 amino acids and each can be located at any
position along the protein chain
•This means that there can be a huge potential number of
different proteins with widely different biochemical properties
•The amino acid sequence of a protein determines the 3D
shape of the protein
•The shape of the protein determines its function
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R side group
R
Uncharged R groups
•The different properties of
amino acids in a protein
result from variations in the
structures of different R
groups.
•Uncharged R groups are
hydrophobic: they don’t like
to be near water
•They are found inside
proteins, or crossing through
the cell membrane
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R side group
•Charged R groups are
hydrophilic: they like to be
near water
•They are found on the
surface of proteins, in
contact with water
•Negatively charged amino
acids shown here: others are
positively charged (lysine,
arginine)
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Protein structure
• Primary structure of a protein refers to the linear array of
amino acids (i.e. the sequence of amino acids)
• Secondary structure : results from folding/bending of the
polypeptide chain – this folding is determined by where the
hydrophobic and hydrophilic amino acids are
• Once secondary structure has formed it folds back upon itself
to form an even more stable molecule: tertiary structure.
• If a protein consists of more than one amino acid chain, the
arrangement of the subunits is called the quaternary structure
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Protein structure
Levels of protein structure
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Enzymes
• An enzyme speeds up the rate of a chemical
reaction
• 3D shape contains an active site where
reactants attach.
• Enzyme can change shape to accommodate the
shape of specific reactants
• Names usually end in –ase eg lipase.
• Cofactors: combine with active site and make
nonfunctional enzymes functional
– Organic cofactors called coenzymes
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