Protein
1 of the 3 macronutrients
Elemental Composition
Proteins are made up of atoms of:
• Carbon
C
• Hydrogen
H
• Oxygen
O
• Nitrogen
N
• and sometimes small amounts of 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 protein – Amino
Acids
• There are 20 different amino
acids but each has the same
basic structure.
C = carbon atom
H= Hydrogen atom
NH2 = Amino group
COOH = Carboxyl group
R = Variable group
e.g. in the amino acid Glycine the R
= H (one Hydrogen Atom)
H
COOH
C
R
NH2
Formation of a Peptide Bond
H
COOH
C
R
H
NH2
COOH
C
R
NH2
Formation of a peptide bond
• The Amino group (NH2) of one amino acid links with the
Carboxyl group (COOH) of another amino acid.
• The H from the NH2 Amino group and OH from the
Carboxyl group 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
• Each time 2 amino acids link together a water molecule is
formed this 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.
Biological Value of Protein
The Biological Value of a protein is a measure of the quality of the
protein and is expressed as a %.
High Biological Value
• Contain all essential Amino
Acids in the correct
proportion for the bodies
needs
• Complete proteins
• Animal sources
Low Biological Value
• Lack some essential amino
acids or do not contain them
in the correct proportion
• Incomplete protein
• Plant sources
Sources of protein
Animal protein
Cheese 26%
Meat 20%
Fish 17%
Eggs 12%
Milk 3%
Plant protein
Soya beans 40%
Nuts 24%
Rice 7 %
Peas 6%
Beans
Chicken
Peas
Lentils
Cereals
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 e.g. Lysine, Leucine,
Isoleucine.
• There are 2 extra essential amino acids for childrenArginine and Histidine
• The rest are Non-Essential Amino Acids these can be
made by the body e.g. Glycine, Cystine, Cysteine.
Supplementary Value of Protein
• When low biological value foods, that lack
essential amino acids, are eaten together they can
provide all the essential amino acids.
• 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.
Protein structure - Primary
• *use diagrams from textbook
instead, pg. 8 & 9, for first 3 slides
• 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.
Secondary Structure
*Use diagrams from textbook
• 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) Disulphide links which happen when 2
Sulphur atoms bond e.g. cysteine
(b) Hydrogen bonds where a Hydrogen
atom in one chain bonds with an Oxygen
atom in another chain.
Tertiary Structure
• 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.
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
FIBROUs
e.g.Collagen
GLOBULAR
e.g albumin
Classified
GLUTENINS : Soluble in acids & alkali
according
e.g. Glutenin in wheat
to solubility
PROLAMINES: Soluble in alcohol
e.g. gliadin in wheat
Properties of Protein
1.Denaturation
 Denaturation is a change in the
nature of the protein
 The protein chain unfolds,
causing a change to the
structure
 Denaturation is caused by a)
heat, b) chemicals and
c) agitation
 It is often an irreversible
process
A.Heat
 Most proteins coagulate/set
when heated.
 E.g. Egg white coagulates at
60˚C; egg yolk coagulates in the
stomach at 68˚C
B. Chemicals
 Acids, alkali, alcohol & enzymes
cause changes to the protein
structure
 E.g. Lemon juice added to milk
causes the milk protein
caesinogen to curdle
 E.g. Enzyme rennin coagulates
milk protein caesinogen in the
stomach
C. Agitation
 This is also known as mechanical
action
 It involves whipping or whisking
the protein
 This results in the protein chain
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
 Eg. roast potatoes
4. Elasticity
 Certain proteins have an
elastic property, e.g. Gluten, 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 can absorb large amounts
of water and, when heated, forms a
sol
 On cooling, this becomes solid & a
gel is formed
 A gel is a semi-solid viscous solution
 All gels have a three-dimensional
network whereby water becomes
trapped. This property is used in
making cheesecakes and soufflés
Gelatine
Heat is applied
Sol
As the protein
Uncoils water
becomes trapped
Water
Protein Matrix – the mixture has
set – it has become a gel
Properties of protein –7. Effects of Heat
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
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.
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 of energy
Digestion of protein
Part of
System
Digestive
Juice
Enzyme
Stomach
Gastric
juice
Rennin
Pepsin
Duodenum
Pancreatic Trypsin
Juice
Small
Intestine
Intestinal
Juice
Substrate
Product
Caseinogen Casein
Proteins
Peptones
Protein
Peptidase Peptones
Peptones
Amino acids
Absorption & Utilisation
• 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. Differentiate 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.
Exam Questions- 2011 & 2006, Q 1 (HL)
(OL) Q1 2007