30 - Antibiotics and Toxins

Question 1

What is a common target for natural toxins?

Answer 1

The ribosome, because of its critical function.

Question 2

How did toxins/antibiotics evolve?

Answer 2

As natural weapons used by organisms to kill or outcompete other organisms

Question 3

What are three uses humans have for antibiotics and toxins?

Answer 3

• To combat pathogens
• To conduct research
• To kill other humans

Question 4

What is the only common feature between all antibiotics or toxins?

Answer 4

That they can impair some biochemical activity

Question 5

True or false? Antibiotics and toxins usually have very precise activity and some biological specificity (eg. prokaryote, type of cell etc.).

Answer 5

True

Question 6

How does puromycin inhibit translocation in translation?

Answer 6

It is an aminoacyl-tRNA analog. Both these have a reactive amino group that acts as a nucleophile when appropriately positioned in the A site for attack on the ester linkage of peptidyl-tRNA in the P site. The puromycin reaction is a dead end reaction leading to chain termination.

Question 7

How does the ricin toxin kill organisms?

Answer 7

It cleaves the purine (depurination) from an A residue of the 28S rRNA in a universally conserved region essential for function.

Interaction with elongation factor eEF2 is abolished and translocation cannot occur

Question 8

What is the most common type of post-translational modification of proteins? What enzymes do this?

Answer 8

Proteolytic cleavage (eg. procollagen to collagen)

Aminopeptidases or carboxypeptidases remove only one or a few amino or carboxy terminal residues (eg. initial Met or fMet is often removed)

Question 9

The formation of S-S disulfide bonds in protein is an example of a _____

Answer 9

Post-translational modification of protein

Question 10

List specific post-translational modifications of certain amino acid side chains (7)

Answer 10

• Hydroxylation of Pro and Lys
• Attachment of sugars to Asn, Ser and Thr (glycosylation)
• Phosphorylation
• Acetylation
• Methylation
• Addition of prosthetic groups (eg. heme to globin)
• Attachment of lipids (ef. farnesyl to Cys residues)

Question 11

What type of proteins are synthesized in free polysomes?

Answer 11

Soluble proteins that remain inside the cell

Question 12

What type of proteins are synthesized in membrane-bound polysomes?

Answer 12

Proteins secreted or directed to various subcellular compartments (organelles)

Question 13

What is the signal hypothesis about ribosomes and the ER?

Answer 13

Signal for ribosomal attachment is provided by a hydrophobic sequence of amino acids at the NH2 terminal end of the newly synthesized polypeptide chain

Question 14

True or false? Some ribosomes are always attached to the ER and some are always free.

Answer 14

False. All ribosomes are free unless a protein contains a signal (ER signal peptide) for attachment to the ER. Once done translating the ribosome dissociates from the ER.

The mRNA, however may stay permanently bound to the ER as part of a polyribosome.

Question 15

What is an amino-terminal signal sequence?

Answer 15

A sequence that is near the N end of a protein and normally indicates where this protein will go

Question 16

What is a signal recognition particle (SRP)?

Answer 16

It binds to the signal sequence and ribosome, immediately halts translation and binds to a receptor protein located in the ER.

• Small cytoplasmic RNA species
• Composed of Small cytoplasmic RNA species

Question 17

What are the 5 steps of co-translational targeting of secretory proteins to the ER?

Answer 17

1. Signal sequence emerges from ribosome, it is recognized and bound by signal recognition particle (SRP)
2. SRP escorts complex to ER membrane, SRP binds to SRP receptor
3. SRP released, ribosome binds to membrane translocation complex of 3 proteins (Sec61, ribosome receptor), signal sequence inserted into membrane channel
4. Translation resumes, growing polypeptide chain translocated across membrane
5. Cleavage of signal sequence by signal peptidase releases polypeptide into ER lumen

Question 18

What happens to mature glycoproteins?

Answer 18

They are sorted and packaged for extracellular secretion or for transport to other intracellular locations

Question 19

What do carbohydrate units do for glycoproteins? (2)

Answer 19

• Help glycoproteins fold correctly in ER
• Final modifications direct them to their ultimate destinations (eg. C terminal KDEL sequence have the retained in ER, or else they go to Golgi)

Question 20

What are the steps of protein secretion by bacteria?

Answer 20

1. The nascent polypeptide complexes with SecB, which prevents complete folding during transport to the membrane
2. At the membrane, an ATPase (SecA) derives translocation of the pro-protein through the membrane with the aid of membrane proteins SecE and SecY, which form a membrane pore
3. The leader sequence is then cleaved off the secreted protein by a membrane peptidase

Question 21

What type of transport do proteins destined for the nucleus take?

Answer 21

Gated transport

Question 22

Transmembrane transport is used to transfer proteins to ____? (4)

Answer 22

• Peroxisomes
• Mitochondria
• Endoplasmic reticulum
• Plastids

Question 23

Vesicular transport is used to transfer proteins to ____? (5)

Answer 23

• Golgi (from ER)
• Lysosome (from golgi or endosome)
• Endosome (from cell surface)
• Cell surface (from golgi or secretory vesicles)
• Secretory vesicles (from golgi)

Question 24

What recognizes the mitochondrial outer membrane receptor (TOM) complex? What does this do?

Answer 24

N-terminal leader sequence

Leads to transport of the protein through both receptor (TIM and TOM) into the matrix, where the matrix targeting signal is cleaved off by a matrix protease.

Question 25

How do proteins get to the mitochondrial intermembrane space?

Answer 25

The TOM complex is recognized by the N-terminal leader sequence and the protein is shuttled through both TIM and TOM receptors into the mitochondrial matrix.

In the matrix, the matrix targeting signal is cleaved off by a matrix protease. Cleavage may expose a new membrane targeting signal that directs the protein back to the inner membrane, the intermembrane space or to the outer membrane.This second targeting signal is then removed by the action of a specific processing peptidase.