• 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
• The 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 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

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 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)
  • Disulphide?(only occurs between cysteine amino acids)
  • Ionic?(occurs 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.

Disulphide

• Disulphide bonds?are?strong?covalent bonds that form between?two cysteine R groups?(as this is the only amino acid with a sulphur atom)
• These bonds are the strongest within a protein but occur less frequently, and help stabilise the proteins
• These are also known as disulphide bridges
• Disulphide bonds can be broken by oxidation
• This type of bond is common in proteins secreted from cells eg. insulin

Ionic

• Ionic bonds?form between?positively charged?(amine group -NH3+) and?negatively charged?(carboxylic acid -COO-)?R groups
• Ionic bonds are stronger than hydrogen bonds but they are not common
• These bonds are broken by pH changes

Hydrogen

• Hydrogen bonds?form between strongly polar?R groups. These are the weakest bonds that form but the most common as they form between a wide variety of R groups

Hydrophobic interactions

• Hydrophobic interactions?form between the?non-polar (hydrophobic) R groups?within the interior of proteins

Tertiary structure determines function

• A polypeptide chain will fold differently due to the interactions (and hence the bonds that form) between?R groups
• Each of the?twenty amino acids?that make up proteins has a unique?R group?and therefore many different interactions can occur creating a vast range of protein configurations and therefore functions

The interactions that occur between the R groups of amino acids determines the shape and function of a protein. These interactions are found within tertiary structures of proteins.

Quaternary structure

• Quarternary structure exists 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