Protein Structure & Function (AQA A Level Biology)

Proteins: Structures & Functions

• There are four levels of structure in proteins, three are related to a single polypeptide chain and the fourth level relates to a protein that has two or more polypeptide chains

• Polypeptide or protein molecules can have anywhere from 3 amino acids (Glutathione) to more than 34,000 amino acids (Titan) bonded together in chains

Primary

• The sequence of amino acids bonded by covalent 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. 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)

The primary structure of a protein. The three-letter abbreviations indicate the specific amino acid (there are 20 commonly found in cells of living organisms)

Secondary

• 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

• There are two shapes that 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

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

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

The secondary structure of a protein with the α-helix and β-pleated sheet shapes highlighted. The magnified regions illustrate how the hydrogen bonds form between the peptide bonds

Tertiary

• 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

The tertiary structure of a protein with hydrogen bonds, ionic bonds, disulphide bonds and hydrophobic interactions formed between the R groups of the amino acids

Quaternary

• 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

The quaternary structure of a protein. This is an example of haemoglobin which contains four subunits (polypeptide chains) working together to carry oxygen

Summary of Bonds in Proteins Table