Key Biological Principle: Protein Structure is the key to Protein Function

• Proteins have a wide range of functions in living thins
• They are all composed of the same basic units; amino acids - there are 20 different types that commonly occur
• Plants can make all of them but animals only a few, obtaining the rest through diet - these are know as essential amino acids

Primary Structure:

• Two amino acids join in a condensation reaction to form a dipeptide, with a peptide bond forming between the two subunits
• This process can be repeated to form polypeptide chains which may contain 1000s of amino acids - a protein is made up of one or more of these polypeptide chains
• The sequence of amino acids in the polypeptide chain is known as the primary structure of a protein

Further Levels of Protein Structure:

• Interactions between the amino acids in the polypeptide chain cause the chain to twist and fold into 3D shapes
• Lengths of the chain may first coil into a-helices or come together in B-pleated sheets - known as the secondary structure
• The chain then folds into its final 3D shape, the tertiary structure

Secondary Structure:

• The chain of amino acids may twist to form an a-helix, a shape like an extended spring
• Within the helix, hydrogen bonds form between the C=O of the carboxylic acid and the -NH of the amine group of different amino acids, stabilizing the shape
• Several chains may link together, with hydrogen bonds holding the parallel chains in an arrangement known as a B-pleated sheet

Tertiary and Quaternary Structures:

• A polypeptide chain often bends and folds to produce a precise 3D shape
• Chemical bonds and hydrophobic interactions between R groups maintain this final tertiary structure of the protein
• An R group is polar when the sharing of electrons within it isn't quite even
• Polar R groups attract other polar molecules, like water, and are therefore hydrophilic. The non-polar groups are hydrophobic
• Non-polar, hydrophobic R groups are arranged so they face the inside of the protein, excluding water from the centre of the molecule
• A protein may be made up of several chains held together (quaternary structure)
• E.g., haemoglobin is made up of four polypeptide chains held tightly together - quaternary structure)

Conjugated Proteins:

• Some proteins are known as conjugated - they have another chemical group associated with their polypeptide chain(s)

Globular and Fibrous Proteins:

• In globular proteins the polypeptide chain is folded into a compact spherical shape
• These proteins are soluble due to the hydrophilic side chains that project from the outside of the molecules and are therefore important in metabolic reactions
• Enzymes are globular - their 3D shape is crucial to their ability to form enzyme-substrate complexes and catalyse reactions
• The 3D shape of globular proteins are critical to their roles in binding to other substances
• Examples include transport proteins with membranes and the oxygen-transport pigments haemoglobin and myoglobin
• Antibodies are also globular and rely on their precise shapes to bind to the microorganisms that enter our boides
• Fibrous proteins don't fold up into a ball shape but remain as long chains
• Several polypeptide chains can be cross-linked for additional strength
• These insoluble proteins are important structural molecules - Keratin in hair and skin, and collagen in skin, tendons, bones and cartilage are examples