7.3 Protein structure This could be you!

Secondary Structure

Other bonds Disulphide bonds.- Between Cysteine molecules…..as these have Sulphur in them! Ionic bonds. – Between ionised amine and carboxylic acid group. Different pHs can break them! Hydrophobic interactions. – Between non- polar side chains.

Tertiary Structure This is the 3-D shape of the protein – the protein folds, and bonds with itself. It will often form a compact, globular structure

Type of bonding in tertiary structure…

Ionic bonds also occur between ionised amine and carboxylic acid groups R groups (just to be confusing) often contain amine and carboxylic acid groups (like the amino acid itself). These groups will ionise, forming ammonium (NH 3 + ) and a negative acid group (COO - ). Where oppositely charged amino acids are found close together, an ionic bond will form

Type of bonding in tertiary structure…

Bonding – Hydrogen bonds Between polar groups Fairly weak Groups that form H bonds are said to be dipolar Shared electrons spend more time around N and O, these are more negative and H and C are more positive. Opposites attract!

Type of bonding in tertiary structure… Hydrophobic interactions. These occur between non-polar R groups The non-polar groups mutually repel water and other polar groups and results in a net attraction of the non-polar groups for each other. In many cases this results in the non-polar side chains of amino acids being on the inside of a globular protein, while the outside of the protein contains mainly polar groups.

Quaternary Structure 2 or more polypeptide chains Held together again by the 4 types of bonds. Not all proteins have a quaternary structure. Examples of proteins that do include haemoglobin.

A protein whose molecules curl up into a ball is known as a globular protein. Proteins commonly found in water (as cellular proteins will invariably be) will often be globular and their hydrophobic regions will point into the centre of the molecule, whilst the hydrophilic regions will remain on the outside. Water molecules are thus excluded from the centre of the folded protein.