FS1 Revision Pack
Amino Acids and Proteins
Proteins are chains of amino acids, linked by peptide bonds. Amino acids consist of a carboxylic group, an amine group, an R group and a hydrogen, al linked by a carbon atom. Therefore all amino acids, except glycine, have a chiral carbon centre. They can form L and D enantiomers.
About half of the amino acids are needed to be consumed in the diet, as the body can’t synthesise them. These are referred to as the essential amino acids.
Amino acids are also amphoteric compounds. They can react as either an acid or a base. This will depends on the pH of the media they are in. In an acidic environment, the carboxylic group would be unionised and the amine group would be ionised. In a basic environment, the carboxylic group would be ionised and the amine group would be unionised. At neutral pH, both groups would be ionised, therefore there would be no net charge. The point at which this occurs is known as the isometric point. Compounds with this ability are known as zwitterions.
Amino acids will join in condensation reactions, two amino acids would from a dipeptide. More can join to form a polypeptide chain. Polypeptides of 100+ amino acids are proteins. Proteins have seven functions:
5) Metabolic activity
6) Communication and regulation
7) Defence and protection
A proteins structure has four levels to it:
1) PRIMARY – simple sequence of amino acids in the chain.
2) SECONDARY – hydrogen bonds between different parts of the chain. Forms an alpha helix of beta pleated sheet.
3) TERTIARY – complex coiling/folding occurs to give final 3D shape. Interactions occur between R groups in amino acids.
4) QUARTERNARY – interactions between individual polypeptide subunits to form whole protein. Other molecules can be incorporated to protein.
There are two types of protein:
1) Globular – Usually rounded and disperse in water. Haemoglobin is an example.
2) Fibrous – Linear strand or sheet like. Usually water insoluble and play a protective or structural role. Collagen is an example.
A proteins shape can be changed by temperature, pH and ionic composition of media. If this shape change is permanent, the protein is said to be denatured. Proteins can also exist with sugars (glycoproteins) or with lipids (lipoproteins).
Two classes of nucleic acids;
- RNA – Ribonucleic acid made up of ribonucleotides.
- DNA – Deoxyribonucleic acid made up of deoxyribonucleotides.
Sugar and Polysaccharides
A basic unit of a carbohydrate is known as a monosaccharide. These join together by covalent bonds known as glycosidic bonds. Polysaccharides are formed when 10+ monosaccharides join together. Some examples are glycogen and starch, which act as energy reserves in animals and plants. Also, cellulose is another, which plays a structural role in plant cell walls.
Typically water insoluble. Less oxygen content than…