Protein Structure & Function (AQA A Level Biology): Revision Note

Proteins: structure & function

Function

• Proteins are macromolecules made from individual monomer units, amino acids

• Proteins perform a wide range of essential roles in all living organisms due to their diverse structures

• They are therefore vital for structure, transport, communication, defence, movement, and catalysis in all living cells:

  • Enzymes – biological catalysts that speed up metabolic reactions (e.g. amylase, DNA polymerase)

  • Transport proteins – carry substances (e.g. haemoglobin transports oxygen; channel proteins in membranes)

  • Structural proteins – provide support (e.g. collagen in connective tissues; keratin in hair and nails)

  • Hormones – regulate processes (e.g. insulin controls blood glucose levels)

  • Antibodies – part of the immune response, recognising and neutralising pathogens

  • Contractile proteins – enable movement (e.g. actin and myosin in muscles)

Structure

• There are four levels of structure in proteins

  • Three are related to a single polypeptide chain

  • The fourth level relates to a protein that has two or more polypeptide chains

• Protein molecules can have anywhere from three amino acids (Glutathione) to more than 34,000 amino acids (Titin) bonded together in chains

Primary structure

• The sequence of amino acids bonded by peptide bonds is the primary structure of a protein

• DNA of a cell determines the primary structure of a protein by instructing the cell to add certain amino acids in specific quantities in a certain sequence, during translation. This affects the shape and, therefore, the function of the protein

• The primary structure is specific for each protein (one alteration in the sequence of amino acids can affect the function of the protein)

Secondary structure

• The secondary structure of a protein is held together by hydrogen bonds that form between the -NH region of one amino acid and the -C=O region of another

  • The hydrogen of -NH has an overall positive charge, while the oxygen of -C=O has an overall negative charge

• Hydrogen bonds are relatively weak, so they can be broken easily by high temperatures and pH changes

• Two shapes can form within proteins due to the hydrogen bonds:

  • α-helix

  • β-pleated sheet

• The α-helix shape occurs when the hydrogen bonds form between every fourth peptide bond

• The β-pleated sheet shape forms when the protein folds so that two parts of the polypeptide chain are parallel to each other, enabling hydrogen bonds to form between the folded layers

• Secondary structure is the highest level of structure for some fibrous proteins, e.g. collagen and keratin

Tertiary structure

• Further conformational change of the secondary structure leads to additional bonds forming between the R groups (side chains)

• The additional bonds are:

  • hydrogen bonds between R groups

  • disulphide bonds between cysteine amino acids

  • ionic bonds between charged R groups

  • weak hydrophobic interactions between non-polar R groups

• This structure is common in globular proteins such as enzymes and antibodies

Quaternary structure

• Occurs in proteins that have more than one polypeptide chain working together as a functional macromolecule, for example, haemoglobin

• Each polypeptide chain in the quaternary structure is referred to as a subunit of the protein