Proteins

Question 1

What is proteomics

Answer 1

The study of sets of proteins and their properties

Question 2

What is the proteome ?

Answer 2

The proteome is the entire set of proteins expressed by a genome

Question 3

Why is the proteome larger than the number of genes ?

Answer 3

The proteome is larger than the number of genes, particularly in eukaryotes, because more than one protein can be produced from a single gene as a result of alternative RNA splicing

Question 4

What are non-coding RNA genes

Answer 4

-Not all genes are expressed as proteins in a particular cell type.

-Genes that do not code for proteins are called non-coding RNA genes and include those that are transcribed to produce tRNA, rRNA, and RNA molecules that control the
expression of other genes.

Question 5

Why can the set of proteins expressed by a given cell type vary ?

Answer 5

-The set of proteins expressed by a given cell type can vary over time and under different conditions.

Some factors affecting the set of proteins expressed by a given cell type are:
-Metabolic activity of the cell
-Cellular stress
-Response to signalling molecules
-Diseased versus healthy cells

Question 6

Why do Eukaryotic cells have a system of internal membranes ?

Answer 6

-Eukaryotic cells have a system of internal membranes, which increases the total area of membrane.

-Because of their size, eukaryotes have a relatively small surface area to volume ratio.
The plasma membrane of eukaryotic cells is therefore too small an area to carry out all the vital functions carried out by membranes

Question 7

What is the endoplasmic reticulum ?

Answer 7

-The endoplasmic reticulum (ER) forms a network of membrane tubules continuous with the nuclear membrane.

-Lipids and proteins are synthesised in the ER

Question 8

What is the Golgi apparatus ?

Answer 8

The Golgi apparatus is a series of flattened membrane discs.

Question 9

What are lysosomes ?

Answer 9

Lysosomes are membrane-bound organelles containing a variety of hydrolases that digest proteins, lipids, nucleic acids and carbohydrates.

Question 10

What do vesicles do ?

Answer 10

Vesicles transport materials between membrane compartments.

Question 11

Where are lipids synthesised ?

Answer 11

Lipids are synthesised in the smooth endoplasmic reticulum (SER) and inserted
into its membrane.

Question 12

What are the types of endoplasmic reticulum ?

Answer 12

Rough ER (RER) has ribosomes on its cytosolic face while smooth ER (SER) lacks
ribosomes.

Question 13

Where does the synthesis of all proteins begin ?

Answer 13

The synthesis of all proteins begins in cytosolic ribosomes

Question 14

Where are cytosolic proteins synthesised ?

Answer 14

The synthesis of cytosolic proteins is completed there (the cytosol), and these proteins remain in the cytosol

Question 15

Describe the synthesis of transmembrane proteins and proteins destined for secretion from the cell (non-cytosolic)

Answer 15

1) Synthesis begins in cytosolic ribosome.

2)Transmembrane proteins carry a signal sequence, which halts translation and directs the ribosome synthesising the protein to dock with the ER, forming RER.

3)A signal sequence is a short stretch of amino acids at one end of the polypeptide that determines the eventual location of a protein in a cell.

4)Translation continues after docking, and the protein is inserted into the membrane of the ER

Question 16

What are proteins produced by ribosomes bound to the ER generally destined for ?

Answer 16

-Insertion into a membrane

-Secretion from the cell

Question 17

Describe the movement of (non-secretory) proteins between membranes

Answer 17

1) Once the proteins are in the ER, they are transported by vesicles that bud off from the ER and fuse with the Golgi apparatus.

2) As proteins move through the Golgi apparatus they undergo post-translational modification.

3) Molecules move through the Golgi discs in vesicles that bud off from one disc and fuse to the next one in the stack. Enzymes catalyse the addition of various sugars in multiple steps to form the carbohydrates - The addition of carbohydrate groups is the
major modification.

4) Vesicles that leave the Golgi apparatus take proteins to the plasma membrane and
lysosomes.

5) Vesicles move along microtubules to other membranes and fuse with them within the cell.

Question 18

What are examples of secreted proteins ?

Answer 18

Peptide hormones and digestive enzymes are examples of secreted proteins.

Question 19

Describe the secretory pathway (movement of secreted proteins after production)

Answer 19

1) Secreted proteins are translated in ribosomes on the RER and enter its lumen.

2) The proteins move through the Golgi apparatus and are then packaged into
secretory vesicles.

3) These vesicles move to and fuse with the plasma membrane, releasing the proteins out of the cell.

Question 20

What type of post-translational modification do many secreted proteins require ?

Answer 20

-Proteolytic cleavage is another type of post-translational modification.

-Many secreted proteins are synthesised as inactive precursors and require proteolytic cleavage to produce active proteins.

-Digestive enzymes are one example of secreted proteins that
require proteolytic cleavage to become active.

Question 21

What is the role of the Golgi apparatus ?

Answer 21

As proteins move through the Golgi apparatus they undergo post-translational modification.

Post-translational modification is that a protein’s structure may become modified by adding chemical groups, such as;
-Carbohydrate groups (major modification)

Enzymes catalyse the addition of various sugars in multiple steps to form the carbohydrates