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Module 2 2.1.2 Biological Molecules Proteins By Ms Cullen Proteins • Proteins are a group of organic compounds. • Our bodies need essential proteins, from our diet and non-essential proteins which the body can synthesise. (Despite the names the body needs all of them to function!) • Proteins have many functions - Proteins • The test for protein is the biuret test. • An equal volume of biuret reagent is added to crushed food. • Biuret is made up of sodium hydroxide and copper sulphate, when these are added to protein they will turn lilac in colour. • Which solution contains proteins? Amino Acids • Amino acids are the monomers of all proteins. Q: What is a monomer? • Amino acids can not be stored by the body so they are deaminated (amino group removed) in the liver to form urea. • All amino acids contain the elements Carbon, Hydrogen, Oxygen and Nitrogen (some may contain sulphur as well). • Amino acids condense together to form a polypeptide chain. • Protein molecules consist of 1 or more of these polypeptide chains. Amino acid structure Week 14 Amino acids have: • A central Carbon atom • A carboxyl group COOH • A Hydrogen atom • An amino group- NH2 • An ‘R’ group. Basic amino acid structure - glycine Q: What is the R group in glycine? A: Dipeptides and Polypeptides • Two amino acids bond together to form a dipeptide. • This occurs by a condensation reaction, where water is lost. • To do this the hydroxide from the carboxyl group of one amino acid reacts with the hydrogen from the amine group of the other amino acid to form water. • The water is removed. • The new bond is known as a peptide bond, a type of covalent bond. • http://www.biotopics.co.uk/as/aminocon.html • More and more amino acids can join together in this way to form polypeptide chains, the ‘backbones’ of all protein molecules. Condensation and hydrolysis Hydrolysis • Polypeptide changes can be split back into amino acids by the addition of water. • This is known as an hydrolysis reaction – ‘splitting by water’. • http://www.biotopics.co.uk/as/dipeptidehydr olysis.html Primary structure of Proteins • This is the order of specific amino acids found in a polypeptide chain. • Only peptide bonds are involved. • Frederick Sanger was the first Scientist to work out the primary structure of a protein, insulin, it took him 10 years! (19451955) Insulin consists of 2 chains linked together by disulphide bridges. Secondary structure of Proteins • This is where the polypeptide folds to form either an α-helix or a β-pleated sheet. • Hydrogen bonds hold the coil in place. • Although H bonds are weak there are many of them. Tertiary structure of Proteins The tertiary structure is stabilised by 4 types of bonds: 1. 2. 3. 4. Quaternary structure of Proteins • The quaternary structure of a protein is the final 3D structure. • It is formed by all the polypeptide chains making up the protein molecule. • The protein haemoglobin is formed by 4 polypeptide chains. (square parts represent haem – the non-protein part) Copy and complete the table below: Level Primary Secondary Tertiary Quaternary Description Bonds involved Which proteins are they found in? Haemoglobin – a conjugated protein • Haemoglobin consists of 4 polypeptide subunits. • 2 are known as α chains, and 2 are β chains. • The haemoglobin molecule is a water-soluble globular protein. • The haem group, which contains an iron (Fe2+ ) ion, is the part that binds to oxygen. • It is not an amino acid and is known as a prosthetic group. • Haemoglobin is a conjugated protein as it is a globular protein with a prosthetic group attached. Catalase – an enzyme • Catalase is a globular protein, it’s quaternary structure contains 4•haem prosthetic groups. • The presence of iron II prosthetic groups allows the enzyme to speed up the break down hydrogen peroxide. • Hydrogen peroxide is often a waste product of metabolic reactions, it is harmful to cells and therefore the enzyme catalase is necessary to prevent it accumulating in cells. • Catalase is found in particularly high concentrations in liver cells – hepatocytes. Carbonic anhydrase – an enzyme • Carbonic anhydrase is a globular protein; an enzyme that catalases the conversion of carbon dioxide into hydrocarbonate ions in a cell. It is a reversible reaction. CO2 + H20 ⇌ HCO3- + H+ • There are a whole family of carbonic anhydrases all based around different proteins, but all of them have a zinc ion bound up in the active site, the prosthetic group. Collagen – a fibrous protein • Collagen is made up of 3 polypeptide chains wound around each other. • Each of the chains is a coil composed of about 1000 amino acids. The most abundant of these is glycine. • Hydrogen bonds form between chains to give strength. • Collagen molecules are linked together by covalent bonds between molecules, adding to the strength. • We call this a collagen fibril. • Many fibrils from a collagen fibre. Collagen structure The functions of Collagen Essentially to provide mechanical strength: • in artery walls • tendons which connect muscle to bone • bones – with the addition of minerals • cartilage and connective tissue Collagen is now being used for cosmetic purposes. Injecting into the lips gives a fuller appearance. Other fibrous Proteins Keratin • Found in hair, nails and skin. • This fibrous protein contains a lot of the amino acid cysteine, which contains sulfur. • The large number of disulfide bridges in it’s structure makes keratin insoluble, inflexible and strong. • Hair has less cysteine, and therefore less disulfide bridges than nails making it more flexible. Other fibrous Proteins Elastin • Found in elastic fibres. • Elastic fibres are found in blood vessels and walls of alveoli. • Elastin is flexible and allows these structures to expand, but also to recoil to their original shape and size. • Composed of many stretchy molecules called tropoelastin. • This gives a cross-linked, stable structure to elastin. • Elastin gives strength and elasticity to the skin and other tissues. Activity Dissect chicken leg and identify areas where collagen, elastin and haemoglobin would be found. Fibrous v Globular Proteins Complete the table below: Fibrous Proteins Structure Shape and chain length Solubility Stability Function Globular Proteins Compare and contrast the structure and function of haemoglobin and collagen.