Proteins

• Proteins are a diverse class of biological molecules
• They are polypeptides built of long chains of monomers called amino acids
• All amino acids contain; carbon, nitrogen, oxygen and hydrogen
• Many also contain sulphur or phosphorous

Structure of amino acids

• All amino acids have the same general structure
• The only difference between each amino acid is the nature of the R group. There are 20 different R groups giving 20 different amino acids
• The R group represents a side chain from the central carbon. It can be anything from a simple hydrogen atom to a more complex structure

Dipeptide

• Two amino acids bond to form a dipeptide
• A peptide bond forms between the carboxylic group and the amino group of the other amino acid
• This is a condensation reaction so a molecule of water is released 

Polypeptides

• Three or more amino acids joined together by peptide bonds form a polypeptide
• There are up to four levels of protein structure; primary, secondary, tertiary and quarternary. Each of these play an important role in defining the overall structure and function of the protein

Primary

• Determined by the type, number and sequence of amino acids in the chain
• If the sequence changes (due to a mutation in the DNA) then the protein will be non functional even though the type and number of amino acids stays the same

Secondary 

• The polypeptide chain can then twist to form either an alpha helix or a beta pleated sheet
• Hydrogen bonds between the peptide bonds maintain the shape of the structure

Tertiary

• A tertiary structure forms when the secondary structured polypeptide folds into a specific 3 dimensional shape
• This structure is held in place by hydrogen bonds, ionic bonds and disulphide bonds

Quaternary

• Formed when two or more polypeptides with a tertiary structure combine
• The polypeptides are held together by hydrogen bonds, ionic bonds and disulphide bonds
• An example of a quaternary structured protein is haemoglobin. It is made up of 4 tertiary structured polypeptides bonded together (2 alpha helix and 2 beta pleated sheet)

Proteins can be divided into two types;

• Fibrous 
• Globular

Fibrous

• Have a secondary structure 
• Long parallel polypeptide chains of amino acids joined by peptide bonds twisted into an alpha helix or beta pleated sheet and held in position by hydrogen bonds
• Several long polypeptide chains twist together to form long, rope like fibres
• Fibrous proteins are insoluble, so cannot dissolve

They have a structural function. Examples include; 

• Collagen - The main component of connective tissue found in ligaments, tendons and cartilage 
• Keratin - The main component of hard structures such as hair, nails, hooves and claws. Also a resistant protein in skin cells which helps prevent entry of pathogens

Globular Proteins 

• Have a tertiary or quaternary structure 
• Long polypeptide chains tightly fold into a specific 3D shape held in position by hydrogen bonds, ionic bonds and disulphide bonds between R groups of amino acids
• Globular protiens are soluble in water so they can dissolve and diffuse into water

They have a biochemical function relying on the molecule's 3D shape. Examples include;

• Enzymes - Tertiary structured globular proteins that catalyse chemical reactions 
• Transport proteins - Such as haemoglobin, myoglobin and those embedded in membranes
• Hormones - Such as oestrogen and insulin
• Antibodies

Denaturing Proteins

• If the bonds that maintain a proteins shape are broken, the protein will stop working properly and is denatured
• Changes is pH, temperature and salt concentration can all denature a protein

• Fibrous proteins lose their structural strength when denatured wheras globular proteins become insoluble

Testing for Proteins

• Add Biuret agent (Copper sulphate solution and sodium hydroxide)
• A purple/lilac colour shows protein is present
• If it stays blue it is a negative result