How does the Primary Structure of a Protein Dictate its structure and function?

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Introduction

Proteins are complex organic compounds, which always contain nitrogen, carbon, hydrogen and oxygen. In some cases, they also contain sulphur. Proteins are macromolecules, which have a high molecular mass. They actually consist of chains of many amino acids. There are many proteins as there are an unlimited number of amino acid combinations that can be used to form a protein. In fact, 50% of a cell’s mass is protein. In the body, proteins are very important. The following are examples of proteins in the body:

Enzymes are proteins, which control metabolism; there is a different enzyme for every reaction in the body.
Antibodies are proteins, which help the body fight disease.
Actin and myosin are both proteins, which enable the muscles to contract.
Collagen is an important protein, its function is to hold us together, connective tissues such as tendons, cartilage and bone have strength, which is due to collagen.
Keratin is also a protein; it gives strength to hair, skin and nails.
Each human cell has a unique combination of proteins and other chemicals on its surface. The body will reject cells if the proteins on the surface are completely different. This is especially the case when an organ transplant is done, the body will reject an organ unless careful matching is done.

Two amino acids join together to form a dipeptide. A chain of amino acids is known as a polypeptide. A protein consists of one or more polypeptides. The bond between two amino acids is called a peptide bond. This is formed between the carboxyl part of one amino acid to the amine group of another. They join together in a condensation reaction. The diagram to the right is an example of such a reaction:

The structure of proteins can be studied on four levels. These are primary, secondary, tertiary and quaternary.

Primary Structure

The number and sequence of amino acids determines the primary structure. The sequence is specific for each protein. Due to the different properties of each amino acid, the sequence will determine its final structure and hence its biological function. In the body, there are 20 amino acids, which make up the proteins that occur there. A single change in the amino acid sequence can cause a major alteration in the protein’s function. This is the case in sickle cell anaemia, where the protein haemoglobin has been changed.

The primary structure relates to the function of the protein as well. Certain proteins such as enzymes have to have a precise shape, as the active site will need to fit the substance it needs to catalyse. If there is a change in the primary structure, i.e. the amino acid sequence is changed; the shape will change and cause the enzyme not to work properly. Also, the overall structure of the protein will have changed, for example if the protein contains two cysteine residues, and only one is there, there will be no disulphide bonds and the shape will have changed.

Secondary Structure

There are two types of secondary structure in proteins, the helix and the pleated sheet. The attraction of the R groups within the same chain can cause the chain to twist into a "right handed" coil. This helix is held together by hydrogen bonds between the hydrogen and oxygen atoms of the amino acid backbone (amino groups and carboxyl groups). Below is a diagram to show how these bonds occur.

The β pleated sheet is formed when hydrogen bonds occur between the C=O and NH groups of one chain and the C=O and NH of the ...