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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 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 Nature of Proteins

C amino group carboxylic acid group Variable group Amino Acid Structure

Formation of a Dipeptide

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 The Variety of Amino Acids

The carboxylic acid group is an acidic group The amino group is a basic group R GROUP 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 The Variety of Amino Acids

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 SH C C The Variety of Amino Acids

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 Proteins - Levels of Structure

Secondary Structure 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 helixBeta pleated sheet Proteins - Levels of Structure

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 Secondary Structure of Proteins

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 Secondary 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 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 MYOGLOBIN MOLECULE HAEM GROUP Tertiary Structure of Proteins

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 The Haemoglobin Molecule Quaternary Structure of Proteins

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

DThe building blocks of proteins are monomers called amino acids DEvery amino acid possesses an amino end and a carboxylic acid end DThere are twenty different naturally occurring amino acids DAmino acids differ by virtue of the nature of their R groups DAmino acids bond together forming peptide bonds DWhen two amino acids bond during a condensation reaction, the resulting molecule is a dipeptide DWhen many amino acids bond together, the resulting molecule is referred to as a polypeptideSUMMARY

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

DThe 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 DTwo main types of secondary structure are found in proteins - the beta pleated sheet and the alpha helix DThe alpha helix forms when the linear chain coils into a right handed helix DThe beta pleated sheet forms when the linear chain folds back on itself many times DHydrogen bonds play a major part in stabilising the secondary structure of proteins DMany proteins bend and fold further to form globular TERTIARY STRUCTURES

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

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

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