Biology AS:Proteins.

Amino acids are the basic monomer units of a protein,which combines to make up a polymer called a polypeptide.Polypeptides can be combined to form proteins.

About 100 different amino acids have been identified, 20 of these occur naturally in proteins.

Every amino acids has a central carbon atom to which are attached four different chemical groups:

Amino group (NH2) This is a basic group from which the amino part of the name amino acid is derived.

Carboxyl grup (COOH) this is an acidic group which gives the amino acid the acid part in its name.

R group- a variety of different chemical groups.Each amino acid has a different R group.

Amino acid monomers can combine to form a dipeptide. (di meaning two). This process is called a condensation reaction.In this reaction, a molecule of water is removed.

The water is made by combining an OH from the carboxyl group of one amino acid with an H from the amino group of another amino acid.

The two amino acids then become linked by a new peptide bond.

The peptide bond can be broken by the addition of water (hydrolysis) Whihc gives its two constituent amino acids.

Primary:Many amino acid monomers can be joined together in a process called polymerisation.The resulting chain of many hundreds of amino acids is called a polypeptide.

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

It is the primary structure of a protein that determines the ultimate shape and hence its function.

Secondary: The chain is twisted into a 3-D shape, which can be a coil (alpha-helix) or a beta pleated sheet. Both these shapes can co-exist in a protein.The shapes are held by hydrogen bonds, which are relatively weak.

Tertiary: The alpha helixes of the secondary structure can be twisted and folden even more to give the complex, and often unique 3-D structure of each protein. The structure is maintained by a number of different bonds, including:

Disulphide bonds - These are fairly strong and therefore not easily broken down.

Ionic bonds -Which are formed between any carboxyl and amino groupsthat are not involved in forming any peptide bonds.These are weaker than disulphide bonds.

Hydrogen - Which are numerous but easily broken.

Quaternary: This arises from the combination of a number of different polypeptide chains and associated non-protein (prothetic) groups into a large, complex protein molecule.

The most reliable test for proteins is the biuret test, which detects peptide links.

How you perform it:

Place a sample of the solution you want to test in a test tube, and add an equal volume of sodium hydroxide solution at room temperature.

Add a few drops of very dilute copper sulphate solution and mix gently.

If peptide links are present, the solution will turn purple (at the beginning it was blue) And if no protein is present, the solution remains blue.