Proteins

Have many roles in the body: Enzymes Hormones Muscles Antibodies Form haemoglobin, hair, skin, nails etc Composed of Carbon, Hydrogen, Oxygen & Nitrogen (some have S & P) They are long polymers composed of amino acids

Amino Acids There are 20 naturally occurring amino acids Structure is: R – group varies in different amino acids

Amino acids are linked together to form proteins A condensation reaction takes place between the amino group of one aa and the carboxyl group of another. The bond formed is a PEPTIDE BOND

C C C C C C C C glycine glycine

PRIMARY STRUCTURE

Protein structure Primary Structure ala – gly – cys – ala – phe – phe – cys – gly – ala - The sequence of amino acids in the chain Held together by peptide bonds Order of amino acids is important as Determines the shape and therefore the function of the protein Peptide bonds

Amino acids do not lie flat Secondary Structure Amino acids do not lie flat Chains of amino acids fold to form helices and b pleated sheets held together by H-bonds H bonds

alpha helix

beta pleated sheet

Tertiary Structure Complex 3D shape of molecule Occurs when polypeptide helix twists and folds around itself Held together by hydrogen bonds -which are weak ionic bonds -between R-groups with positive or negative charges, which are quite strong disulphide bridges -covalent S-S bonds between two cysteine amino acids, which are strong.

tertiary structure

Quaternary Structure When a number of polypeptide chains link together Example – haemoglobin has 4 polypeptide chains

Primary Protein Structure The sequence of a chain of amino acids Secondary Protein Structure Occurs when the sequence of a chain of amino acids are linked by hydrogen bonds Tertiary Protein Structure Occurs when bonds form between alpha helices and pleated sheets Quaternary Protein Structure As a protein containing of more than one amino acid chain

Protein shape in relation to function The final 3D structure of proteins results in 2 main classes of protein Fibrous Globular

Fibrous Contain polypeptides that bind together to form long fibres or sheets Physically tough Insoluble in water Tend to have a structural function Eg collagen – the most widespread structural protein in the body. Gives strength to bones, tendons, skin and ligaments. 3 polypeptide chains twisted into a triple helix.

Collagen fibrils

Globular Individual molecules with complex tertiary or quaternary structures Spherical (globular) in shape Most are soluble in water Tend to have a biochemical rather than structural function Eg enzymes, haemoglobin Because the 3D structure is held together with weak bonds they are very susceptible to extremes of temperature (this can break the bonds and alter the shape of the protein)

A globular protein

Haemoglobin Has a quaternary structure Made up of 4 polypeptide chains It is a conjugated protein as each polypeptide chain is associated with an iron containing haem group It role is to carry oxygen around the body

quaternary structure polypeptide 1 polypeptide 2 polypeptide 4 polypeptide 3 haemoglobin

Conjugated proteins These are proteins that are joined to other non-protein groups. The non-protein group is called a prosthetic group. Glycoprotein = protein + carbohydrate Haemoglobin = 4 polypeptide chains, each with an iron containing haem group.

Primary Protein Structure The sequence of a chain of amino acids Secondary Protein Structure Occurs when the sequence of a chain of amino acids are linked by hydrogen bonds Tertiary Protein Structure Occurs when bonds form between alpha helices and pleated sheets Quaternary Protein Structure A protein containing of more than one amino acid chain

Primary Protein Structure The sequence of a chain of amino acids Secondary Protein Structure Occurs when the sequence of a chain of amino acids are linked by hydrogen bonds Tertiary Protein Structure Occurs when bonds form between alpha helices and pleated sheets Quaternary Protein Structure A protein containing of more than one amino acid chain