Protein1 of the 3 macronutrients

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

C

H

NH2

R

COOH C

H

NH2

R

COOH

H2O

CONH

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