Proteins
The Building Blocks of our Body
© PDST Home Economics
Elemental Composition
Proteins are made up of atoms of:
Carbon
C
Hydrogen
H
Oxygen
O
Nitrogen
N
and sometimes Phosphorus (P), Sulphur (S) and Iron
(Fe)
Nitrogen is needed for growth.
Proteins are the only nutrients that contain the
element nitrogen.
These elements are bonded together in small
molecules called amino acids.
Amino acids are bonded together into long chains
called proteins
Basic Structure of proteins –
Amino Acids
Proteins are made up of
chains of small molecules
called amino acids.
There are 20 different amino
acids but each has the same
basic structure.
C = carbon atom
H= Hydrogen atom
NH2 = Amino group (basic)
COOH = Carboxyl group
(acidic)
R = Variable group, changes
for every amino
For example in the amino acid
Glycine the R group = H (one
Hydrogen Atom)
Essential Amino Acids
There are over 20 different amino acids.
8 are Essential Amino Acids that cannot be made
by the body and must be eaten.
There are 2 extra essential amino acids for
children.
The rest are Non-Essential Amino Acids these
can be made by the body
Essential and Nonessential Amino
Acids
Essential Amino acids cannot be made by the body and
must be obtained from food
Vincent’s
Lovely
Valine
Lysine
Mother
Methionine
Took
Trytophan
Him
Histidine (children)
Threonine
To
Leucine
London
Isoleucine
In
a
Pram
Arginine (children)
Phenylalnine
8 essential for adults and 10 for children
Peptide Bonds or Links
Amino acids join together into long chains called proteins.
Just like the 26 letters of the alphabet can form
thousands of words by linking in different ways; the 20
amino acids can form many different proteins
The bond that links one amino acid to another is called a
peptide link or bond
Each time 2 amino acids link together a water molecule is
taken away and so the linking process is called
Condensation.
The reverse of condensation is called Hydrolysis, water
molecules are added to protein chains and split the amino
acids apart, this happens during protein digestion.
Peptide Bonds or Links
The Amino group (NH2) of one amino acid links with
the Carboxyl group (COOH) of another amino acid.
The Amino group (NH2) looses one H, the Carboxyl
group (COOH) looses an OH group.
The H and OH bond together to form a water
molecule (H2O)
The remaining CO and NH bond together to form a
peptide link.
Two amino acids joined is called a Dipeptide
Many amino acids joined in a chain is called a
Polypeptide
Amino Acids Bonding to form
protein
Protein structure - Primary
Order and
number of amino
acids in a protein
chain for
example the
protein insulin
has over 50
amino acids in its
chain arranged in
a definite order.
Protein structure
- Secondary
Involves the folding of the
protein chain into a spiral or
zig-zag shape
This structure is caused by
crosslinks that form
between different chains or
within the one chain.
There are different types
of cross-links
(a) Disulfide links which
happen when 2 Sulphur
atoms bond .
(b) Hydrogen bonds where a
Hydrogen atom in one chain
bonds with an Oxygen atom
in another chain.
Protein structure - Tertiary
This refers to the 3 dimensional
folding of the chain. This structure
can be globular or fibrous. The
shapes give certain properties to the
protein
Globular : In these the protein chain
is rolled up like a ball of wool. This
structure makes the protein soluble.
This type of protein is found in body
cells, myoglobin in meat, albumin in
egg, haemoglobin in blood.
Fibrous: In these the protein chain
takes on a straight, coiled or zig-zag
shape. These shapes make the
protein insoluble and stretchy or
tough. Gluten in wheat and elastin in
meat have a coiled structure.
Collagen in meat has a zig-zag
structure.
Sources of protein
Animal protein
Cheese
Chicken
Meat
Fish
Eggs
Milk
Plant protein
Soya beans
TVP
Nuts
Lentils
Peas
Beans
Cereal
PROTEIN CLASSIFICATION
SIMPLE
CONJUGATED
DERIVED
These proteins are formed due
to a chemical or enzyme action on a
protein : i.e: Rennin acts on
caesinogen and makes caesin
PROTEIN + NON-PROTEIN
Protein + Lipid = Lipoprotein (lecithin)
Protein + Phosphate = Phosphoprotein (caesin)
Protein + nucleic acid = Nucleoprotein (DNA)
Protein + Colour Pigment = Chromoprotein (Haemoglobin)
ANIMAL
PLANT
Classified
according
to shape
Classified
according
to solubility
FIBROUS
Collagen
GLOBULAR
albumin
GLUTENINS : Soluble in acids & alkali
Glutenin in wheat
PROLAMINES: Soluble in alcohol
gliadin in wheat
Properties of protein – effects of
heat on protein
Effect of heat
Examples
Coagulation: protein sets
and then hardens
Hard boiling eggs
Colour change
Myoglobin in meat - red to
brown
Bread crust
Maillards reaction (dry
heat)
Tenderising (moist heat)
Becomes indigestible
Collagen in meat changes to
gelatine and fibres fall
apart
Overcooked meat or
cheese becomes tough and
hard to digest
Properties of Protein
1. Denaturation
B. Chemicals
Denaturation is a change in the
Acids, alkali, alcohol & enzymes
nature of the protein
cause changes to the protein
The protein chain unfolds,
structure
causing a change to the
E.g. Lemon juice added to milk
structure
causes the milk protein
Denaturation is caused by a)
caesinogen to curdle
heat, b) chemicals and
E.g. Enzyme rennin coagulates
c) agitation
milk protein caesinogen in the
stomach
It is often an irreversible
process
C. Agitation
A. Heat
This is also known as mechanical
action
Most proteins coagulate/set
when heated.
It involves whipping or whisking
the protein
E.g. Egg white coagulates at
60˚C; egg yolk coagulates in the
This results in the protein chain
stomach at 68˚C
unfolding & partial coagulation
Properties of Protein
2. Solubility
Proteins are generally insoluble
in water
There are two exceptions – egg
white in cold water & connective
tissue, which is converted to
gelatine in hot water
3. Maillard reaction
Maillard reaction is also known
as non-enzymic browning. It
occurs when food is roasted,
baked or grilled
Amino Acid + Carbohydrates +
Dry heat = Brown Colour
Examples include roast
potatoes, toast
4. Elasticity
Certain proteins have an
elastic property, e.g. Gluten,
the protein found in flour,
enables bread to rise during
cooking
5. Foam Formation
When egg white is whisked, air
bubbles are formed as the
protein chains unravel
Whisking also produces heat,
which slightly sets the egg
white
This foam will collapse after a
while, unless it is subjected to
heat
This property is used to make
meringues
Properties of Protein
6. Gel formation
Collagen, when heated, forms
gelatine
Gelatine
Gelatine can absorb large
Heat is applied
amounts of water and, when
heated, forms a sol
As the protein
On cooling, this becomes solid
Uncoils water
& a gel is formed
becomes trapped
Sol
A gel is a semi-solid viscous
Pro gel
solution
All gels have a threeWater
dimensional network whereby
water becomes trapped. This
property is used in making
Protein Matrix – the mixture has set – it
cheesecakes and soufflés
has become a gel
Biological Functions of Protein
Function type
Function
Result of deficiency
Structural
Function
Growth & repair
of body cells
muscles &skin
Retarded growth
Delayed healing
Physiologically
active protein
Making hormones,
enzymes,
antibodies, blood
protein,
nucleoprotein
Body organs &
systems
malfunction.
Easily infected.
Nutritive Protein
Provides essential Lack of energy.
amino acids for
Kwashiorkor,
the body.
Marasmus
Excess protein
used for energy
Deamination
This is the process by
which excess protein is
used for energy.
Left over amino acids
are brought to the liver
The NH2 group is
broken off, changed to
ammonia, then to urea
and then excreted.
The rest of the
molecule is converted to
glucose and used for
releasing energy.
Biological Value of Protein
The Biological Value of a protein is a measure of the
quality of the protein and is expressed as a %.
It is decided by the number of essential amino acids a
protein contains in proportion to how much of them the
body needs.
There are 2 types of protein foods:
(a) High Biological value, contain all essential Amino Acids,
complete proteins, animal sources.
(b) Low Biological value, lack some essential amino acids,
incomplete protein, plant sources
Biological Value of different proteins
Source
Animal
Food
HBV
LBV
Eggs
100%
Ovalbumin, Livetin, Vitelin
Milk
95%
Casein, Lactalbumin,
Lactoglobulin
Meat
8090%
Collegen, Elastin, Myosin,
Fish
Actin, Myosin, Collegen
Gelatine
Plant
Protein
Soya
Beans
0%
Gelatine
74%
Rice
67% Oryzenin
Wheat
53% Gluten
Maize
40% Zein
Supplementary Value of Protein
When low biological value foods, that lack
essential amino acids, are eaten together
they can provide all the essential amino
acid.
The essential amino acids missing in one
food can be made up for by being present
in the other food and visa versa.
This complementary value of protein
means that vegans can get all the
essential amino acids without eating
animal food
Example; Bread is lacking Lysine but is
high in Methionine. Beans are lacking
Methionine but high in Lysine. By eating
beans on toast both essential amino acids
are included in the meal.
RDA Protein & Energy value
RDA
1gram of protein per kilogram of body weight.
Child 30-50g/day
Teenager 60-80g/day
Adults 50-75g/day
Pregnant or lactating 70-85g/day
Energy Value
1g of protein gives 4kCal or 17 kJ energy
Digestion of protein
Part of
System
Physical
Digestion
Mouth
Chewed
Stomach
Churned
Digestive
Juice
Gastric
juice
Enzyme
Rennin
Pepsin
Duodenum
Pancreatic Trypsin
Juice
Small
Intestine
Intestinal
Juice
Substrate
Product
Caseinogen Casein
Proteins
Peptones
Protein
Peptidase Peptones
Peptones
Amino acids
Absorption & Assimilation of
Amino Acids
Amino Acids are absorbed into blood capillaries in the
villi of the small intestine.
These capillaries connect into the portal vein which
carries the amino acids to the Liver.
From here the Amino Acids will be sent to (a) replace
& repair body cells, (b) form new cells, antibodies,
hormones, enzymes or (c) be deaminated
Questions??
1 What is the elemental composition of protein?
2 Draw the chemical structure of an amino acid
3 Explain how a peptide link forms
4 What are essential amino acids?
5 List the biological functions of protein.
6 What is meant by ‘biological value’ of protein?
7 Difference between denaturation & deamination
8 List (a) the energy value (b) the RDA of protein?
9 List 4 sources of (a) HBV and (b) LBV protein.
10 Describe the digestion of protein in humans.