Elements: C, H, O and N and sometimes others such as Sulphur

Proteins are polymers composed of amino acid monomers.

General formula of an amino acid:

The R group can be anything from a simple hydrogen atom to the most complex of ring molecular structures. The R group gives different amino acids different characteristics of solubility in fats or water, and attraction or repulsion to or from other amino acids. Below is the simplest amino acid, glycine where a single hydrogen is the R group.

The presence of sulphur in the ring structure allows for the formation of sulphur cross-bonds in the formation of complex 3D protein shapes. These sulphur bonds are called Disulphide bonds. The more disulphide bonds there are in the protein then the more heat stable the protein's shape will be.

Amino acids have an acid group (COOH) at one end and a basic group (NH2) at the other so they can act as acids, bases or as neutral substances according to the conditions they find themselves in. They are therefore AMPHOTERIC molecules. Also amino acids may lose a hydrogen atom from the COOH end to form COO- and H+. The H+ can be attracted to the NH2 group to give NH3+, and as a result the molecule can be a positive ion or a negative ion depending upon the conditions it is in. The molecule is therefore referred to as a ZWITTERION. Amino acids can be polymerised by condensation forming peptide bonds. These bonds form between the C of the COOH group and the N of the NH2 group. In this way dipeptides (2 amino acids), then polypeptides (many amino acids) are formed and then these polypeptides can combine to form proteins.

 

The primary structure of a protein is the sequence of amino acids in its polypeptide chain.

The secondary structure of the protein results from the chain forming an a-helix that is a spiral molecule, or a b-pleated sheet.

The spiral molecule may then fold into its tertiary structure that gives it a complex 3D shape.

The quaternary structure is given by the close association of different 3D polypeptides held together by hydrogen bonding, hydrophilic and hydrophobic forces, which gives a highly complex shape. An example of such a complex quaternary structure is that of haemoglobin that consists of 2 a-polypeptide chains and 2 b-chains in close association with each other.

 

Proteins are important for their catalytic, structural, contractile and buffering functions. They are an integral part of all life functions. The complex 3D shape of proteins can be damaged by heat, radiation, high or low pH, heavy metal ions such as lead, organic solvents and detergents. If a protein loses its shape too badly it may denature and lose its most useful properties and be unable to regain them. Given the correct conditions a denatured protein can renature itself by folding spontaneously back into its ideal shape, but this is rare.

The final shape of the protein is governed by the positions of individual amino acids along the polypeptide sequence. Therefore any change in the amino acid sequence may alter the final protein shape and its functional ability.

The test for protein is the Biuret test:

Sodium, or Potassium Hydroxide is added to the crushed food material. This dissolves the protein. Then very dilute (0.5%) Copper Sulphate solution is added and a lilac or purple colour indicates the presence of protein. A blue colour indicates that protein is not present.