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

C Amino Acid Structure Variable group amino group carboxylic acid group

Formation of a Dipeptide

The Variety of Amino Acids 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

Proteins - Levels of Structure Amino Acid Sequence Peptide bond 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

Secondary Structure Proteins - Levels of Structure Alpha helix The secondary structure of proteins is the arrangement in space 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 helix Beta pleated sheet

Tertiary Structure of Proteins All globular proteins display tertiary structure. Once the secondary structures have formed, the molecule bends and folds into a 3-D globular shape MYOGLOBIN MOLECULE HAEM GROUP 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

Quaternary Structure of Proteins 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 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

SUMMARY The building blocks of proteins are monomers called amino acids Every amino acid possesses an amino end and a carboxylic acid end There are twenty different naturally occurring amino acids Amino acids differ by virtue of the nature of their R groups Amino acids bond together forming peptide bonds When two amino acids bond during a condensation reaction, the resulting molecule is a dipeptide When many amino acids bond together, the resulting molecule is referred to as a polypeptide

Chains of amino acids numbering greater than 100 are generally referred to as proteins Individual amino acids may be neutral, basic or acidic Two amino acids, namely cysteine and methionine, possess sulphur atoms in their R groups The type, number and sequence of amino acids forming the original linear chain of a protein is termed the PRIMARY STRUCTURE OF A PROTEIN Different proteins have different primary structures The primary structure determines the final shape of the protein molecule

The linear chain of amino acids making up the primary structure of the protein bends and folds in various ways to form the SECONDARY STRUCTURE OF THE PROTEIN Two main types of secondary structure are found in proteins - the beta pleated sheet and the alpha helix The alpha helix forms when the linear chain coils into a right handed helix The beta pleated sheet forms when the linear chain folds back on itself many times Hydrogen bonds play a major part in stabilising the secondary structure of proteins Many proteins bend and fold further to form globular TERTIARY STRUCTURES

Myoglobin is a globular protein displaying the tertiary level of structure Myoglobin is a protein found in muscle cells Proteins consisting of more than one polypeptide chain display quaternary structure Haemoglobin is a protein consisting of more than one polypeptide chain Haemoglobin consists of four separate polypeptide chains held together by weak van der Waals forces Each polypeptide chain in haemoglobin contains a haem group that binds to molecular oxygen The role of haemoglobin is to transport oxygen molecules from the lungs to the body tissues

A variety of different bonds stabilise the secondary and tertiary structures of proteins Hydrogen bonds form between oxygen and hydrogen atoms within the main amino acid chain and between the R groups Disulphide bridges form between sulphur atoms in the R groups of amino acids such as cytseine Ionic bonds form between charged amino groups and charged carboxylic acid groups Hydrophobic interactions occur between R groups that have clustered towards the centre of protein molecule due to their hydrophobic nature