Levels of Protein Structure

• Proteins are relatively?large,?complex molecules?that contain?one or more chains?of amino acids known as?polypeptides
• The?three-dimensional arrangement?of polypeptide chains dictates a protein's structure and function
• There are?four levels of structure in proteins
  • Three levels are structural aspects of a single polypeptide chain
  • The fourth level relates to a protein that has more than polypeptide chain

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 specific, ordered sequence
• This affects the?shape,?and therefore the?function,?of the protein
• The primary structure is?specific?for each protein
• Some mutations can lead to the incorrect amino acid being incorporated into the polypeptide chain which 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 is the?formation of complex shapes?within the polypeptide chain
• Secondary structure of a protein occurs due to?weak hydrogen bonds
  • Hydrogen bonds form between?carboxyl?(C=O) groups and?amino?(H-N-H) groups
  • The bonds usually form?between non-adjacent amino acids?resulting in a change in shape of the linear polypeptide chain
• There are?two shapes?that can form within proteins due to the hydrogen bonds:
  • Alpha-helix (or?α-helix)
  • Beta-pleated sheet (or?β-pleated sheet)

Polar and non-polar amino acids are relevant to the bonds formed between R groups

• Tertiary structure refers to how the polypeptide chain folds to form a complex, three-dimensional shape
• Tertiary structure gives proteins a?very specific shape?that is important for function
  • Such as?receptor sites?on cell membranes and?active sites?in enzymes
• Folding results from?interactions between R groups?(side chains) of the amino acids and the surrounding environment
• A number of different interactions between R-groups contribute to the tertiary structure
  • Hydrogen bonds?form between polar R-groups
  • Hydrophobic interactions?form between the R-groups of non-polar amino acids?within the interior of proteins?to avoid contact with water
  • Covalent bonds?form between the R-groups of cysteine amino acids to form disulphide bridges
  • Ionic bonds?form between positively and negatively charged R-groups

Quaternary structure

• Large proteins often consist of?multiple polypeptide chains?functioning together as a larger biologically active macromolecule
  • Each polypeptide chain is referred to as a?subunit?of the protein
• Many proteins also contain non-polypeptide components (prosthetic groups) and are classed as?conjugated proteins
• Quaternary structure refers to how polypeptides and other components are arranged
  • This relates closely to function
  • Proteins with only one polypeptide chain do not have a quaternary structure
• Haemoglobin?is a conjugated protein, having quaternary structure, as it consists of?multiple polypeptide chains?(making?four subunits) each with a prosthetic group
  • There are two pairs of identical polypeptide chains (α–globins?and?β–globins)
  • Each subunit has a prosthetic?haem?group which contains an?iron atom (Fe)

Summary of Bonds in Proteins Table