Anti-Bacterials - Protein Synthesis

Question 1

Name some antibacterial agents that target protein synthesis.

Answer 1

Aminoglycosides, tetracyclines, chloramphenicol, macrolides, oxazolidinones, fusidic acid.

Question 2

How many proteins make up the bacterial ribosome?

Answer 2

Over 30.

Question 3

Describe the four phases of bacterial protein synthesis.

Answer 3

Initiation, elongation, termination, ribosome recycling.

Question 4

Which way is the mRNA translated?

Answer 4

5’-3’.

Question 5

What are the ribosome sub-units required in the translation of mRNA?

Answer 5

30S and 50S.

Question 6

What are the three initiation factors required in the translation of mRNA?

Answer 6

JF1, JF2 (GTP), JF3.

Question 7

What is the initiator required in the translation of mRNA?

Answer 7

f-Met-tRNA-f-Met.

Question 8

What does f-Met relate to?

Answer 8

N-formylmethionine.

Question 9

What are the elongation factors required in the translation of mRNA?

Answer 9

EF-Tu (GTP), EF-Ts, EF-G.

Question 10

What phase of bacterial protein synthesis do Oxazolidiones act upon?

Answer 10

Initiation.

Question 11

What does IF3 do in bacterial protein synthesis?

Answer 11

It binds to the 30S subunit in the initiation phase, promoting dissociation of the ribosome into its two subunits and allowing 30S to form an initiation complex.

Question 12

Where does IF1 bind in the initiation phase of bacterial protein synthesis? What effect does this have?

Answer 12

The base of the A-site of the 30S ribosomal subunit. This directs the initiator tRNA to the ribosomal P-site by blocking the A-site.

Question 13

What does IF2 do in bacterial protein synthesis?

Answer 13

IF-2 is a small GTP-binding protein. IF-2-DTP bonds the initiator fMet-tRNAfMet and helps it dock with the small ribosomal subunit.

Question 14

How is the initiation complex completed?

Answer 14

IF-2-GTP hydrolysed and IF3 released and 50S subunit binds.

Question 15

Describe the oxazolidinone nucleus.

Answer 15

A 5-membered ring with a carbonyl group on in the top position, an oxygen to its right, and a nitrogen to its left.

Question 16

How can the properties of oxazolidinones be altered?

Answer 16

By altering the chains on wither side of the nucleus.

Question 17

What classes of drugs act upon the elongation phase of bacterial protein synthesis?

Answer 17

Aminoglycosides, Tetracyclines.

Question 18

What happens when the fMet-tRNA enters the P-site?

Answer 18

It causes a conformational change which opens the A-site for the new aminoacyl-tRNA to bind to the codon.

Question 19

What brings aminoacyl-tRNA into the ribosomal A-site.

Answer 19

EF-Tu.

Question 20

What is transpeptidation?

Answer 20

The amino acid at P-site is transferred to the amino acid at A-sit of the growing peptide chain.

Question 21

How do aminoglycosides work to inhibit bacterial protein synthesis?

Answer 21

Some aminoglycosides change the shape of the 30S subunit, leading to misreading of the mRNA – the detailed mechanism of this process is unknown.

Question 22

Name some aminoglycosides.

Answer 22

Streptomycin, gentamycin.

Question 23

How do tetracyclines work to inhibit bacterial protein synthesis?

Answer 23

Some tetracyclines (and some aminoglycosides) prevent amino acids from entering the ribosome at the 30S subunit. The detailed mechanism of this process is unknown.

Question 24

Name some tetracyclines.

Answer 24

Tetracycline, doxycycline, minocycline.

Question 25

Describe the tetracycline nucleus.

Answer 25

A group of four 6 membered rings.

Question 26

What bacteria is tigecycline effective against? What resistance mechanism is it partially resistant to?

Answer 26

Tigecycline is effective against; MRSA, VRA, many G -ive bacteria and is partially resistant to the efflux mechanism of resistance.

Question 27

When is tigecycline used?

Answer 27

Only sparingly when needed.

Question 28

Why shouldn't tetracycline be given to pregnant mothers or children under 8?

Answer 28

Tetracycline binds very tightly to calcium, becoming deposited in calcifying tissues such as bones and teeth, causing staining and slight weakness.

Question 29

What food/drink should tetracyclines not be given with? Why?

Answer 29

Milk, because they bind tightly to calcium.

Question 30

Describe transpeptidation.

Answer 30

The peptide bond formation involves nucleophilic attack of the amino N of the amino acid that is linked to the 3’ hydroxyl of the terminal adenosine of the tRNA in the A-site on the carbonyl C of the amino acid (with attached nascent polypeptide) in ester linkage to the tRNA in the P-site.

Question 31

What classes of drugs inhibit the transpeptidation phase of bacterial protein synthesis?

Answer 31

Puromycin, Chloramphenicol, Kanamycin (aminoglycoside), kirromycin.

Question 32

How does puromycin act to inhibit the transpeptidation of bacterial protein synthesis?

Answer 32

Puromycin causes premature chain termination. It resembles the aminoacyl-tRNA, it has an ‘NH’ instead of the ‘O’ that joins the amino acid to tRNA. This stronger amide bond is not reactive enough and the new peptide bond forms too slowly. Hence, the ribosome stalls or an incomplete chain is released.

Question 33

What issue causes puromycin to be only used in cell culture?

Answer 33

Unfortunately, this drug has poor selectivity between prokaryotic and eukaryotic cells so is mainly used in cell culture.

Question 34

Which form of chloramphenicol is pharmacologically active?

Answer 34

D-threo.

Question 35

How does chloramphenicol act to inhibit the transpeptidation phase of bacterial protein synthesis?

Answer 35

It has a selective action on peptidyl transferase of the 50S subunit, blocking peptide bond formation.

Question 36

Why is chloramphenicol the drug of choice for treating typhoid and meningococcal meningitis?

Answer 36

Chloramphenicol penetrates mammalian cells and CSF.

Question 37

How does kanamycin act to inhibit the transpeptidation phase of bacterial protein synthesis?

Answer 37

Kanamycin causes a misreading of the code by interfering with the (wobble) base paring. It doesn’t prevent the reaction, it prevents proper binding, causing the wrong tRNA to bind which inactivates the protein.

Question 38

What is the consequence of the production of incorrect proteins?

Answer 38

This production of incorrect proteins means the cell has to destroy these proteins and make new ones, which takes a lot of energy. This eventually kills the cell through energetic depletion.

Question 39

How does kirromycin act to prevent the transpeptidation phase of bacterial protein synthesis?

Answer 39

Kirromycin blocks dissociation of GDP from EF-Tu after hydrolysis. This prevents dissociation of EF-Tu from the ribosome and effectively stalls protein synthesis.