Module 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 ‏ Amino acids have: A central Carbon atom A carboxyl group - COOH A Hydrogen atom An amino group- NH 2 An ‘R’ group. Week 14

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. 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’. olysis.html 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! ( ) 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:

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: LevelDescriptionBonds involvedWhich proteins are they found in? Primary Secondary Tertiary Quaternary

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 (Fe 2+ ) 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. CO 2 + H 2 0 ⇌ HCO 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 ProteinsGlobular Proteins Structure Shape and chain length Solubility Stability Function

Compare and contrast the structure and function of haemoglobin and collagen.