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Proteins are huge three-dimensional molecules whose building blocks or monomers are the variety of different amino acids found in nature Human hair Feather  The significance of proteins cannot be over-estimated, since they are intimately connected with all phases of the chemical and physical activities of the living cell  Proteins function as enzymes, hormones, antibodies and oxygen transporters and they form the bulk of skin, hair, feathers, nails and cartilage

amino group carboxylic acid group Variable group C C O O

Two Amino Acids Peptide bond CONDENSATION -H 2 O

Individual amino acids display a tetrahedral shape due to the angles of the bonds between the atoms There are twenty different naturally occurring amino acids that differ from one another by virtue of the R group The simplest of the amino acids possesses a hydrogen atom for its R group. This amino acid is called glycine

This amino acid, known as alanine, possesses a CH 3 group as its R group This amino acid possesses one acid and one basic group. Overall, this amino acid is a neutral molecule R group The amino group is a basic group The carboxylic acid group is an acidic group

R group This amino acid has a carboxylic acid group in the R group. Therefore, it has two acid groups and one basic group. Overall, this amino acid is an acidic molecule This amino acid has an amino (basic) group in the R group. Therefore, it has two basic groups and one acid group. Overall, this amino acid is a basic molecule

R group This amino acid has sulphur in its R group. This amino acid is called cysteine There are only two sulphur-containing amino acids; these are cysteine and methionine There are only two sulphur-containing amino acids; these are cysteine and methionine

Amino Acid Sequence Polypeptide chains form when amino acids bond together in a particular sequence. The primary structure of a protein is the number, type and sequence of amino acids that make up this linear chain together with the peptide bonds that hold them together. Different proteins have different primary structures. Different proteins are made up of different types, numbers and sequences of amino acids making up the primary chain. Peptide bond

The primary chain of amino acids possesses an amino end (N terminus) and a carboxyl end (C terminus)

Secondary Structure The secondary structure of a protein is determined by the spatial arrangement of the atoms that form the backbone or linear chain of the protein The amino acid chain can coil into a helix shape or form a shape called the beta pleated sheet The helix and beta pleated sheet shapes are secondary structures of protein molecules Alpha helixBeta pleated sheet

The Alpha Helix The amino acid chain coils into a right-handed helix and hydrogen bonds form between oxygen and hydrogen atoms that have been brought into close proximity These hydrogen bonds help to stabilise this secondary structure Hydrogen Bond

The Beta-Pleated Sheet The amino acid chain folds back upon itself many times forming anti- parallel chains. The oxygen and hydrogen atoms that have been brought into close proximity form hydrogen bonds The hydrogen bonds help to stabilise this secondary structure

Myoglobin molecule Haem group All globular proteins display tertiary structure. Once the secondary structure has formed, the molecule bends and folds into a 3-D globular shape Myoglobin is a globular protein found in muscle cells. This tertiary shape is the highest level of structure for this protein and a variety of bonds help to stabilise its structure

The Haemoglobin Molecule Quaternary structure is a level of structure displayed by proteins that consist of more than one polypeptide chain Haemoglobin is a protein displaying quaternary structure Haemoglobin consists of four polypeptide chains that are held together by weak Van der Waals forces Each polypeptide chain contains an iron containing Haem group that binds to molecules of oxygen

BONDS THAT STABILISE SECONDARY & TERTIARY STRUCTURE BONDS THAT STABILISE SECONDARY & TERTIARY STRUCTURE As the chains of amino acids bend & fold to form secondary & tertiary structures, various atoms are brought into close proximity and form bonds Hydrogen and oxygen atoms from both the main chain and the R groups may form hydrogen bonds The R groups of two amino acids contain sulphur atoms. When these atoms are in close proximity they form disulphide bridges Many of the carboxylic acid and amino groups form charged groups in solution. Oppositely charged groups form ionic bonds Many hydrophobic R groups tend to cluster towards the interior of the protein molecule forming hydrophobic interactions

DA solution of the material to be tested is placed in a test tube DA little sodium hydroxide solution is added to clear the solution DA small amount of weak copper sulphate solution is added down the side of the tube

A purple/violet colour indicates the presence of a protein