Elemental CompositionProteins 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 atomH= Hydrogen atomNH2 = Amino groupCOOH = Carboxyl groupR = Variable group
e.g. in the amino acid Glycine the R = H (one Hydrogen Atom)
C
H
NH2
R
COOH
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 Plant protein Cheese 26% Soya beans 40%Meat 20% Nuts 24%Fish 17% Rice 7 %Eggs 12% Peas 6%Milk 3% Beans
Chicken Peas LentilsCereals
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 children-
Arginine 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 PLANTClassified Classified GLUTENINS : Soluble in acids & alkaliaccording according e.g. Glutenin in wheatto shape to solubility
PROLAMINES: Soluble in alcoholFIBROUs GLOBULAR e.g. gliadin in wheate.g.Collagen e.g albumin
Properties of Protein1.DenaturationDenaturation is a change in
the nature of the proteinThe protein chain unfolds,
causing a change to the structure
Denaturation is caused by a) heat, b) chemicals and
c) agitationIt is often an irreversible
processA.HeatMost 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.SolubilityProteins are generally
insoluble in waterThere are two exceptions –
egg white in cold water & connective tissue, which is converted to gelatine in hot water
3.Maillard reactionMaillard 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 Protein6. 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
As the proteinUncoils water
becomes trapped Sol
Water
Protein Matrix – the mixture has set – it has become a gel
Properties of protein –7. Effects of Heat
Effect of heat ExamplesCoagulation: protein sets and then hardens
Hard boiling eggs
Colour change Myoglobin in meat - red to brown
Maillards reaction (dry heat)
Bread crust
Tenderising (moist heat) Collagen in meat changes to gelatine and fibres fall apart
Becomes indigestible Overcooked meat or cheese becomes tough and hard to digest
Biological Functions of ProteinFunction type Function Result of deficiency
Structural Function
Growth & repair of body cells muscles &skin
Retarded growthDelayed healing
Physiologically active protein
Making hormones, enzymes, antibodies, blood protein, nucleoprotein
Body organs & systems malfunction. Easily infected.
Nutritive Protein
Provides essential amino acids for the body.Excess protein used for energy
Lack of energy.Kwashiorkor, Marasmus
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/dayEnergy Value• 1g of protein gives 4kCal of energy
Digestion of protein
Part of System
Digestive Juice
Enzyme Substrate Product
Stomach Gastricjuice
RenninPepsin
CaseinogenProteins
CaseinPeptones
Duodenum Pancreatic Juice
Trypsin Protein Peptones
Small Intestine
Intestinal Juice
Peptidase 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 acid3. Explain how a peptide link forms4. 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