The Four Levels of Protein Structure (Cambridge (CIE) AS Biology): Revision Note

Proteins: structures

• 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 structure

• 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 whole protein)

Secondary structure

• The secondary structure of a protein occurs when the weak negatively charged nitrogen and oxygen atoms interact with the weak positively charged hydrogen atoms to form hydrogen bonds

• 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 (between the oxygen of the carboxyl group and the hydrogen of the amine group)

• 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 parallel peptide bonds

• Most fibrous proteins have secondary structures (e.g. collagen and keratin)

• The secondary structure only relates to hydrogen bonds forming between the amino group and the carboxyl group (the ‘protein backbone’)

• The hydrogen bonds can be broken by high temperatures and pH changes

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 (these are between R groups)

  • Disulfide(only occurs between cysteine amino acids, sometimes referred to as a disulfide bridge)

  • Ionic (occurs between charged R groups)

  • Weak hydrophobic interactions (between non-polar R groups)

• This structure is common in globular proteins such as 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