| – Proteins are made from amino acids joined together to form a polypeptide. The polypeptide folds to form a protein. There are 20 types of amino acids.
– Examples of proteins: Enzymes, hormones and haemoglobin. – Proteins are made on ribosomes using DNA/mRNA code – Like carbohydrates – Amino acids are only made up of carbon, hydrogen and oxygen – Amino acids are the monomer units used to make the proteins. |
| This is the chemical structure of an amino acid.
– NH2 is the amine group (NOT AMINO) – The R is the side chain, they differ from each amino acid. Some may be sulphur, nitrogen and phosphorous – COOH is the carboxyl group – ALL amino acids have the same structure |
R Chains
- May be polar, non-polar, electrically charged, contain sulphur or selenium, hydrophilic or hydrophobic. These are factors which determine how the protein will fold. If the R chain contains sulphur then they will form a disulide bridge.
- Hydrogen – Weak bonds between positive and negatively charged atoms, used for folding proteins.
- Ionic – Strong bonds between strongly positive and strongly negative atoms.
- Hydrophilic – Amino acids will be found on the outside of the protein.
- Hydrophobic – Amino acids will be found on the inside of the protein
- They can differ through these factors: Size, polarity and charge which gives each amino acid different properties.
- The R group is significant as it is the variable group, it gives each amino acid it’s specific characteristics
Peptide bonds
- Amino acids join together via peptide bonds – the amine group joins the carboxyl group of another condensation reaction. Condensation reaction means that a water molecule is released during the process. Whereas hydrolysis is a reaction which water is used to break a bond.
A mark Is awarded for showing the peptide bond (polypeptide chain) and the H20 molecule.
| Primary structure
– A chain of amino acids in a specific order which are joined together by peptide bonds |
| Secondary structure
– Arrangement of 3D structure of polypeptide chain using hydrogen bonds. – These forms either an alpha helix or a beta pleated sheet (dependant on the hydrogen bonding) |
| Tertiary structure
– The folding of the protein into its final shape, it can be globular or fibrous – R groups interact e.g. Disulphide bonds (if sulufr is present), ionic bonds and hydrogen bonds |
| Quaternary structure
– Some proteins have several polypeptide chains which are associated subunits can be identical or different. – It is also when 2 or more polypeptides are joined together to form a protein |
| Globular | Fibrous |
| Haemoglobin | Keratin |
| Antibodies | Collagen |
| Hormones | Myosin |
Collagen is a fibrous protein due to the presence of both hydrogen and covalent bonds in its structure. Collagen molecules wrap around each other and form fibrils which form strong collagen fibres. Collagen forms the structure of bones, cartilage and connective tissue and is a main component of tendons which connect muscles to bones.
Haemoglobin is a water-soluble globular protein which consists of two alpha and two beta polypeptide chains each containing a haem group. It carries oxygen in the blood as oxygen can bind to the haem (Fe2+) group and oxygen is then released when required.
| What is the test for proteins?
– Biuret test – Solution is placed in a test tube and the sample is mixed with an equal volume of sodium/potassium hydroxide – a few drops of dilute copper (II) sulphates is added and gently mixed If protein is present, the solution turns lilac as an indicator of peptide bonds. In the absence of protein, the solution will remain blue. |