Proteins


These are chains of amino acids that make up around 18% of the body. There are 20 naturally occurring amino acids, each of which are coded for by the triplet code.

Structure of Amino Acids

Specification Reference

"Know the structure of amino acids (specific structures are not required)"
All amino acids have the same basic structure. They all contain an amine group (NH2) and a carboxyl group (-COOH). They also have an "r-group" which determines the properties of each amino acid. This is because it influences they way the amino acid folds and ultimately the resulting protein.

Formation of Peptide Bonds

Specification Reference

"Understand the formation of polypeptides and proteins (as amino acids linked together by peptide bonds in condensation reactions)."
Amino acids are joined together by a reaction between the amine group of one amino acid and the carboxyl group of another.
  • This occurs through a condensation reaction
  • Water is eliminated
  • A peptide bond forms
  • The resulting molecule is known as a dipeptide
More and more amino acids can join together to form a polypeptide chain which will eventually make up the primary structure of a protein.

Bonds in Proteins

Specification Reference

"Understand the role of ionic, hydrogen and disulfide bonding in the structure of proteins."
The peptide bonds in amino acids are strong but can be broken down through hydrolysis. Other bonds also form between the "r-groups" of different amino acids to from the 3D structures of proteins. Which type of bond occurs and where is dependant on the different atoms present in the chain.
  • Hydrogen bonds
  • Disulfide bonds
  • Ionic bonds

Protein Structure

Specification Reference

"Understand the significance of the primary, secondary, tertiary and quaternary structure of a protein in determining the properties of fibrous and globular proteins, including collagen and haemoglobin"
Primary Structure
  • A sequence of amino acids in a polypeptide chain
  • Joined together by peptide bonds
Secondary Structure
  • This is the arrangement of the polypetide chain into a regular, 3D repeating structure
  • Held together by hydrogen bonds
  • Examples of this include the alpha helix and the beta sheet
Tertiary Structure
  • Another level of 3D organisation
  • Chains are folded into further complicated shapes
  • Hydrogen, ionic and disulfide bonds hold these 3D structures in place
  • Globular proteins are an example of tertiary structures
Quaternary Structure
  • A level of organisation that is not found in all proteins
  • Only seen in proteins containing several polypeptide chains
  • This structure describes how separate polypetide chains join together
  • The bonds between these chains can easily be removed causing the protein to become denatured

Protein Structure and Function

Specification Reference

"Understand how the structure of collagen and haemoglobin are related to their function""

Fibrous Proteins

These are made up of long parallel polypeptide chains with little or no tertiary structure. They have few cross links and form fibres with important roles in the body such as connective tissue.

Collagen is an example of a fibrous protein that gives strength to tendons, ligaments, bone and skin. Collagen is extremely strong, the fibres have a high tensile strength due to the unusual structure it has. It is made up of three polypeptide chains which are arranged into a triple helix held together by a large number of hydrogen bonds. This results in collagen being a material of considerable strength making it suitable for its purpose as a connective tissue.

Globular Proteins

These are proteins that have a complex tertiary and sometimes quaternary structure. They fold into large and spherical shapes which affect their behaviour in water.

Globular proteins fold so that the hydrophobic parts are on the inside and the hydrophilic parts are on the outside. This makes them soluble in water. They also play an important role in holding molecules in position in the cytoplasm as well as making up antibodies.

Haemoglobin is the most well known globular protein. It is a very large molecule made up of 574 amino acids arranged into 4 polypeptide chains held together by disulfide bonds. Each chain is arranged around an iron containing haem group. It is the iron that enables the molecule to bind to and release oxygen. This makes haemoglobin suitable to transport oxygen around the body efficiently.