Intracellular processes

Question 1

What is a protein that is extensively glycosylated called?

Answer 1

Proteoglycans

Question 2

what is glycosylation and its effect?

Answer 2

Addition of a carbohydrate

It alters structure and function of a protein

Question 3

What is phosphorylation?

Answer 3

Addition of a phosphate group

Alters protein function

Question 4

What is acetylation ?

Answer 4

Addition of acetyl group

Regulates gene expression in histone proteins

Question 5

What is farnesylation?

Answer 5

Addition of a farnesyl group

Targets proteins to the cytoplasmic face of the plasma membrane

Question 6

what is ubiquitination?

Answer 6

Ubiquitin chain is added

Marks proteins for degradation

Question 7

What are the 2 mechanisms by which proteins are degraded?

Answer 7

Lysosomal degradation

Proteasomal degradation

Question 8

Briefly describe lysosomal degradation

Answer 8

Takes place in lysosomes
Lysosomal enzymes include lipases, nucleases, protease/proteolytic enzymes

Used to break down proteins that have a long half life (over 20 hours)

Plasma proteins enter the cell via endocytosis (pore forms)
Extracellular proteins enter the cell via receptor-mediated endocytosis
Pathogenic proteins enter via phagocytosis

Question 9

Briefly describe proteasomal degradation

Answer 9

ATP dependant
Takes place in proteasomes

Proteins are tagged with ubiquitin enzyme (to mark them for degradation)
Tagged proteins are taken to proteasome and are recognised, unfolded and translocated in the centre of the proteasome.
Proteins are degraded into peptides.
Peptides are emitted and digested by cytosolic peptidases to form amino acids
Ubiquitin are released for re-use.

Question 10

Differences in proteasomal and lysosomal degradation?

Answer 10

Proteasome: Proteins with short half life, deals with proteins synthesised inside the cell (key metabolic enzymes, defective proteins), takes place in proteasome

Lysosomal: Proteins with a long half life, takes place in lysosomes, deals with proteins from outside the cells (plasma, extracellular)

Question 11

What are the 3 mechanisms by which proteins enter an organelle and what are they used for?

Answer 11

Nuclear pore: for nuclear proteins
Via membrane Translocator : For mitochondrial proteins and the ER
Vesicles : for proteins going beyond the ER

Question 12

What are the 2 pathways for proteins to arrive at specific locations in the cell?

Answer 12

Intracellular (inside the cell)

Extracellular (outside the cell - secreted):

Question 13

What is a signal peptide?

Answer 13

A specific sequence of N-terminal amino acids

Question 14

Describe the extracellular (secretory pathway)

Answer 14

Ribosomes to ER to Golgi Stack to Plasma membrane, lysosome or secreted

Question 15

Describe the intercellular pathway

Answer 15

Ribosomes to Cytosol to Mitochondria, Peroxisomes, Nucleus or STAYS PUT

Question 16

Describe how a protein is targeted to the ER? (draw if helps)

Answer 16

Signal peptide (adress label) binds to Ribosome.
Signal recognition particle (postman) attaches to SP 
Signal Recognition Particle Receptor (name plate on door) on ER surface attaches to SRP.
Polypeptide chain (protein) passes through the translocon (protein channel) and is threaded through the pore into the ER.

Signal peptidase cleaves off the signal peptide and the finished protein ends up in the lumen of the ER