Protein Structure

Protein Structure

General Structure

  • All composed of the same basic unites – amino acids
  • 20 different amino acids
  • Plants can make all these amino acids, animals can only make some
  • Animals obtain the other amino acids through diet; essential amino acids


  • Amine group; -NH2
  • Carboxylic acid group; -COOH
  • Hydrogen; -H
  • All attached to a central carbon atom
  • Each amino acid has a different, variable, side chain (residual group)


Primary Structure:

  • The specific sequence of amino acids in the polypeptide chain
  • Two amino acids join in a condensation reaction
  • Forming a dipeptide with a peptide bond between the two subunits
  • This process is repeated to form polypeptide chains
  • A protein is made of one or more polypeptide chains


Secondary Structure:

  • Interactions between the amino acids in the polypeptide chain cause the chain to twist into an α-helix or fold into a β-pleated sheet.
  • Hydrogen bonds between the carboxylic and amine group


Tertiary and Quaternary Structure:

  • Polypeptide chain bends and folds to produce and precise 3-D shape
  • Chemical bonds and hydrophobic interactions between R groups maintain the tertiary structure
  • R group is polar when sharing uneven electrons
  • Polar R groups attract other polar molecules (water) and are therefore HYDROPHILIC
  • Non-polar groups are HYDROPHOBIC and are arranged to face the inside of the protein
  • Ionic bonds, hydrogen bonds, disulphide bonds
  • Single chain proteins stop at tertiary level



Conjugated Proteins:

  • These have another chemical group associated with their polypeptide chain


Globular Proteins:

  • Polypeptide chain is folded into a compact spherical shape
  • Soluble; due to hydrophilic side chains
  • Enzymes are globular proteins
  • 3-D shape is crucial to ability to form enzyme substrate complexes and catalyse reactions within cells & binding to other substances
  • Antibodies are globular


Fibrous Proteins:

  • Remain as long chains
  • They can be cross-linked for additional strength
  • Insoluble; important for structure