Explain the formation of a protein Classify proteins according to their chemical structure Describe the classification of protein Name a food source for the following proteins albumin caseinogen ID: 1034788
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2. Revision questionsOutline the basic chemical structure of an amino acid.Explain the formation of a protein.Classify proteins according to their chemical structure.Describe the classification of protein.Name a food source for the following proteins: albumin caseinogen, collagen, actin, gluten, myosin, gelatin.Differentiate between an essential and non-essential amino acid (give four examples of each).What do you understand by the biological value of protein?What do you understand by supplementary/complementary value of proteins? (Give an example)Give an account of the properties of protein (with a culinary use where possible). Outline the biological functions of protein. What is the RI value of protein? What is the energy value of protein?What do you understand by the deamination of protein?Outline the digestion and absorption of protein.
3. 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 atomH= Hydrogen atom NH2 = Amino group (basic)COOH = Carboxyl group (acidic)R = Variable group (changes for every amino acid)Q1. Outline the basic chemical structure of an amino acid
4. Glycine The amino acid glycine: The R group = H (one hydrogen atom)
5. Q2. Explain the formation of a proteinThe bond that links one amino acid to another is called a peptide link or bond.Each time two amino acids link together, a water molecule is taken away. The linking process is therefore 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.The amino group (NH2) of one amino acid links with the carboxyl group (COOH) of another amino acid.The amino group (NH2) loses one H and the carboxyl group (COOH) loses 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.
6. 1. Acid reacts with the NH2 of another amino acid. 2. The amino group (NH2) loses one H and the carboxyl group (COOH) loses an OH group.3. The remaining CO and NH join together to make a dipeptide.
7. Q3. Classify proteins according to their chemical structurePrimary structureThe number and sequence 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.
8. Secondary structure Involves the folding of the protein chain into a spiral or zigzag shape.This structure is caused by cross-links that form between different chains or within the one chain. There are different types of cross-links: Hydrogen links – where a hydrogen atom in one chain bonds Disulfide links – when two sulphur atoms bond with an oxygen atom in another chain.A disulfide link A hydrogen link
9. Tertiary structureThis refers to the three-dimensional folding of the chain. This structure can be globular or fibrous. The shapes give certain properties to the protein. Fibrous: In these the protein chain takes on a straight, coiled or zigzag 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 zigzag structure.
10. Q4. Describe the classification of proteinSIMPLE CONJUGATED DERIVED These proteins are formed due to a chemical or enzyme action on a protein, i.e. rennin acts on caseinogen and makes casein PROTEIN + NON-PROTEIN Protein + lipid = lipoprotein (lecithin) Protein + phosphate = phosphoprotein (casein) Protein + nucleic acid = nucleoprotein (DNA) Protein + colour pigment = chromoprotein (haemoglobin) ANIMAL PLANTClassified Classified GLUTENINS: Soluble in acids & alkaliaccording according Glutenin in wheatto shape to solubility PROLAMINES: Soluble in alcoholFIBROUS GLOBULAR gliadin in wheatCollagen Albumin
11. Q5. Name a food source for the following proteinsAlbumin = Egg/meatCaseinogen = Milk/cheeseCollagen = Meat/fishActin = Meat/fishGluten = Wheat/cerealsMyosin = Meat/fishGelatin = Meat/fish
12. Q6. Differentiate between an essential and non-essential amino acid (give four examples of each)There are over 20 different amino acids.Eight are essential amino acids that cannot be made by the body and must be eaten, e.g. valine, leucine, isoleucine, lysine, phenylalanine, tryptophan, threonine and methionine. There are two extra essential amino acids for children, i.e. arginine and histidine.The rest are non-essential amino acids. These can be made by the body, e.g. cysteine, alanine, asparagine, glutamine, glycine, proline and tyrosine.
13. Q7. What do you understand by the biological value of protein?The biological value of a protein is a measure of the quality of the protein and is expressed as a percentage.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 two types of protein foods: 1. High biological value: contain all essential amino acids, complete proteins, animal sources. 2. Low biological value: lack some essential amino acids, incomplete protein, plant sources.
14. Q8. What do you understand by supplementary / complementary value of proteins? (give an example)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 vice versa.This complementary value of protein means that vegans can get all their essential amino acids without eating animal foodExample: Bread is lacking lysine but is high in methionine. Beans are lacking methionine but are high in lysine. By eating beans on toast, both essential amino acids are included in the meal.
15. Q9. Give an account of the properties of protein (with a culinary use where possible)Denaturation is caused by: a) heat b) chemicals c) agitationIt is often an irreversible process. DenaturationDenaturation is a change in the nature of the protein. The protein chain unfolds, causing a change to the structure.
16. Heat – Most proteins coagulate/set when heated, e.g. egg white coagulates at 60°C; egg yolk coagulates in the stomach at 68°C. Chemicals – Acids, alkali, alcohol and enzymes cause changes to the protein structure, e.g. lemon juice added to milk causes the milk protein caseinogen to curdle.Agitation – This is also known as mechanical action. It involves whipping or whisking the protein. This results in the protein chain unfurling and partial coagulation. 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.
17. 3. The Maillard reaction The Maillard reaction is also known as non-enzymic browning. It occurs when food is roasted, baked or grilled. Examples include roast potatoes and toast. Amino acid + carbohydrates + dry heat = brown colour.Elasticity Certain proteins have an elastic property, e.g. gluten, the protein found in flour, enables bread to rise during cooking5. 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.
18. 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 and 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.
19. Q10. Outline the biological functions of protein Function Effect of deficiency 1. Structural proteins, e.g. keratin Production of cell membrane, muscles, skin and hairGrowth and repair Retarded growthDelayed healing of wounds 2. Physiologically active proteins, e.g. sterols Production of: HormonesEnzymesAntibodies Blood/plasma proteinsNucleoproteins Malfunctioning of body organs and systemsOpen to infection and illnesses 3. Nutrient proteins Supply the body with essential amino acidsExcess protein is used for body head and energyLack of energy In severe cases, can lead to kwashiorkor and marasmus
20. Q11. What is the RI of protein?Q12. What is the energy value of protein?RI 1 g of protein per kilogram of body weight.Child 30 ̶ 50 g/dayTeenager 60 ̶ 80 g/dayAdults 50 ̶ 75 g/dayPregnant or lactating 70 ̶ 85 g/dayEnergy value1 g of protein gives 4 kcal or 17 kJ energy
21. Q13. What do you understand by the deamination of protein? This is the process by which excess protein is used for energy.Leftover 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.
22. Q14. Outline the digestion and absorption of proteinOrgan JuiceEnzyme Substrate and productStomach Gastric juice PepsinRennin Proteins to peptidesCaseinogen to casein Small intestine DuodenumPancreatic juice Intestinal juice TrypsinErepsin Proteins to peptidesPeptides amino acids
23. Key words to revise AgitationAmino acidCondensationConjugated proteinsDeaminationDenaturationDipeptideDisulfide linksElasticityFoam formationGel formationHydrogen linksHydrolysisMaillard reactionPolypeptideSolubility