Proteins and their functions

Question 1

Levels of protein structure

Answer 1

1) primary- sequence of amino acids
2) secondary- initial folding of polypeptide chain into alpha helix or beta pleated sheet
3) tertiary- overall 3D conformation of protein

Question 2

Primary structure

Answer 2

A single chain of amino acids joined by peptide bonds, forming a polypeptide. ‘R’ groups determine the properties of amino acids eg. polar/non-polar, hydrophobic/hydrophilic, positive/negative charge.

Question 3

Secondary structure

Answer 3

Alpha-helices or beta-pleated sheets are stabilised by hydrogen bonds and van der Waal forces between amino acids along the polypeptide chain.

Question 4

Tertiary structure

Answer 4

The 3D conformation of a protein that requires the lowest energy, maximising internal binding between amino acids.

Question 5

Quarternary structure

Answer 5

A complex of multiple polypeptide chains and/or prosthetic groups.

Question 6

Types of protein

Answer 6

Globular proteins are ball-like in shape, and are functional eg. enzymes. Fibrous proteins are long and elongated, and are structural eg. collagen. Multi-peptide complexes may be homomers or heteromers eg. haemoglobin.

Question 7

Functions of proteins

Answer 7

• binding (ligands, receptors)
• catalysis (enzymes)
• switching (cell signalling)
• structural roles (cytoskeletal elements)

Question 8

Regulation of protein function

Answer 8

1. Synthesis- correct protein made at the right time
2. Localisation- correct place within the cell/body
3. Modification- active or inactive
4. Degradation- broken down and recycled

Question 9

Control of protein synthesis

Answer 9

Extracellular signals stimulate the up-regulation of gene expression when a particular protein is required. Not all proteins are produced by all cells.

Question 10

Localisation

Answer 10

Proteins are transported where they need to go in order for the cell to function. After synthesis and post-translational modification on the rough ER, they are sorted and move from the cytosol into organelles via membrane transporters.

Question 11

Secretory pathway

Answer 11

Proteins for export are packaged into vesicles, modified in the golgi complex, and are secreted out of the membrane.

Question 12

Types of vesicles

Answer 12

Transport vesicles- move proteins from one organelle to another.
Secretory vesicles- can store hormones/enzymes for future use, require an extracellular signal to fuse with the membrane and release their contents.

Question 13

Modification

Answer 13

Post-translational modification, such as phosphorylation, glycolisation and disulphide bridge formation, occurs in the rough ER. Further modification occurs on the golgi complex.

Question 14

Tyrosine kinase

Answer 14

Tyrosine kinase covalently attaches a single phosphate group to tyrosine, which induces a conformational change to inhibit or activate the protein. Phosphatase is involved in dephosphorylation of the amino acid.

Question 15

Growth factor signalling pathway

Answer 15

Outside the cell: a growth factor activates two membrane receptors, causing them to dimerise and be phosphorylated.
In the cytoplasm: an adaptor protein binds to the phosphorylated receptor, a G-protein (Ras) binds to the adaptor.

Question 16

Mutation

Answer 16

Alteration of a DNA sequence via insertion, deletion, substitution or translocation, which may be inherited or acquired. The change to the amino acid sequence may affect the structure of the protein coded for by a gene, and therefore alter its function.

Question 17

Unfolded protein response

Answer 17

Maintains the balance of protein folding within the ER. An imbalance results in an excess of unfolded proteins, which can lead to ER stress, inhibition of translation and cell death.