Types of Proteins
Globular Proteins:
- Compact
- Water Soluble - hydrophobic R-Groups are folded inwards away from water, and hydrophilic R-Groups face outwards and this means they can dissolve in blood allowing them to be transported easily.
- Roughly Spherical
- E.g. Insulin
- A hormone which regulates blood glucose levels.
- Transported in the bloodstream so needs to be soluble.
- Its unique shape fits into receptors on cell surface membranes to cause more/less glucose production.
Conjugated Proteins:
- Globular proteins which contain a prosthetic group.
- Prosthetic groups can be lipoproteins, glycoproteins, metal ions or minerals from vitamins.
- E.g. Haemoglobin
- Red oxygen carrying pigment found in erythrocytes.
- Quaternary protein made of two alpha subunits and two beta subunits.
- Each subunit contains a prosthetic haem group, which combine reversibly with oxygen, allowing erythrocytes to transport oxygen around the body.
- E.g. Catalase
- Enzyme which catalyses the breakdown of hydrogen peroxide, which is a harmful byproduct of metabolic processes.
- Quaternary Protein.
- Contains four haem groups.
Fibrous Proteins:
- Insoluble, due to high proportion of hydrophobic R-Groups in the amino acids.
- Strong
- Rigid
- NOT folded into complex 3D shapes.
- E.g. Keratin
- Present in hair, skin and nails.
- Large proportion of sulphur containing amino acid, cysteine, resulting in many strong disulphide bonds.
- More sulphide means the protein is less flexible.
- E.g. Elastin
- Found in blood vessel walls and the alveoli etc.
- Made of elastic fibres, which are like small springs, so it can stretch and return to its original shape.
- Many tropoelastin molecules are crosslinked, providing a stable structure.
- Flexible
- E.g. Collagen
- Connective tissue found in skin, tendons, ligaments and the nervous system.
- Made of three polypeptides wound into a strong, rope like structure.
- Flexible
- Single polypeptide chain forms a triple helix with two other peptides. Every third chain is glycine, a small amino acid which gives the helices staggered ends, allowing them to join end to end. This makes tropocollagen. Tropocollagen the aggregated into larger bundles forming a tear-resistant tissue.