Protein 1 of the 3 macronutrients

Elemental Composition Proteins are made up of atoms of: Carbon C HydrogenH OxygenO NitrogenN 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 NH 2 = Amino group COOH = Carboxyl group R = Variable group e.g. in the amino acid Glycine the R = H (one Hydrogen Atom) C H NH 2 RCOOH

Formation of a Peptide Bond C H NH 2 RCOOH C H NH 2 RCOOH

Formation of a peptide bond The Amino group (NH 2 ) of one amino acid links with the Carboxyl group (COOH) of another amino acid. The H from the NH 2 Amino group and OH from the Carboxyl group bond together to form a water molecule (H 2 O) 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 proteinPlant protein Cheese 26%Soya beans 40% Meat 20%Nuts 24% Fish 17%Rice 7 % Eggs 12%Peas 6% Milk 3%Beans ChickenPeas 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 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 PLANT Classified ClassifiedGLUTENINS : Soluble in acids & alkali according according e.g. Glutenin in wheat to shape to solubility PROLAMINES: Soluble in alcohol FIBROUs GLOBULAR e.g. gliadin in wheat e.g.Collagen e.g albumin

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 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 As the protein Uncoils 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 heatExamples Coagulation: protein sets and then hardens Hard boiling eggs Colour changeMyoglobin 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 indigestibleOvercooked 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 ProteinProvides 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 NH 2 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 EnzymeSubstrateProduct StomachGastric juice Rennin Pepsin Caseinogen Proteins Casein Peptones Duodenum Pancreatic Juice TrypsinProteinPeptones Small Intestine Intestinal Juice PeptidasePeptonesAmino 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 & 2006, Q 1 (HL) (OL) Q1 2007