2.2.2. Antibiotics II

Question 1

Eukaryotic vesus Prokaryotic Ribosomes

Answer 1

Eukaryotic: 60S + 40S = 80S
Prokaryotic: 50S + 30S = 70S
*Structural differences are one basis for selective toxicity

Question 2

Mitochondria and Selective Toxicity

Answer 2

Mitochondria have their own DNA and their own ribosomes (ribosomes are more similar to bacterial ribosomes than to eukaryotic ones). Therefore, if antibiotic diffuses across all membranes this can be a problem

Question 3

Components of the 30S ribosomal subunit

Answer 3

• one rRNA molecule (16S)

- 21 different proteins (S1-S21)

Question 4

Components of the 50S ribosomal subunit

Answer 4

• two rRNA molecules (5S and 23S)

- 31 different proteins (L1-L31)

Question 5

3 Basic Steps of Protein Synthesis

Answer 5

1. Amino acid activation
2. Formations of Initiation complexes (30S and 70S)
3. Polypeptide Chain Synthesis

Question 6

Amino acid activation

Answer 6

aka “Charging” of tRNA by the enzyme aminoacyl-tRNA synthetase

Question 7

Formation of Initiate Complexes

Answer 7

Step 1: charged tRNA, mRNA and 30S ribosomal subunit (includes IFs and GTP) bind
Step 2: the 50S binds and creates the 70S complex

Question 8

4 Steps of Polypeptide Chain Synthesis

Answer 8

1. Recognition
2. Peptidyl Transfer
3. Translocation
4. Release

Question 9

Antibiotics that target amino acid activation

Answer 9

NONE, unable to achieve selective toxicity

Question 10

Antibiotics that target the initiation complexes

Answer 10

Oxazolidinones prevent the formation of the 70S ribosomal ternary complex by binding to the 50S ribosomal subunit

Question 11

Linezolid (Zyvox)

Answer 11

The first of a new class of antibiotics called oxazolidinones (binds to 50S), used to treat Gram-positive infections

Question 12

Is linezolid bacteriostatic or bactericidal?

Answer 12

BOTH: it is bacteriostatic for staphylococci and enterococci, but is is bactericidal for streptococci (strong/irreversible interaction b/w drug and 50S = bactericidal)

Question 13

Antibiotics that are inhibitors of recognition

Answer 13

Aminoglycosides: prevent codon, anti-codon recognition

Question 14

Aminoglycosides

Answer 14

Targets specific proteins in the 30S ribosomal subunit:
Streptomycin,
Kanamycin,
Tobramycin,
Gentamicin,
Neomycin,
Amikacin,
Paramomycin, etc.

Question 15

3 Mechanisms of Streptomycin

Answer 15

1. Misreading
2. Cyclic Polysomal Blockade
3. Faulty Outer Membrane Proteins

Question 16

Misreading (streptomycin)

Answer 16

A-site becomes distorted (leads to inactive ribosomes or incorrect amino acid insertion)

Question 17

Cyclic Polysomal Blockade (streptomycin)

Answer 17

Although the 70S complex forms, it is unstable and falls apart b/c streptomycin is bound to 30S

Question 18

Faulty Outer Membrane Proteins (streptomycin)

Answer 18

Translational misreading leads to mutant outer membrane proteins that make the bacterial membrane leaky (more drug leaks into the cell and acts at the 30S complex)

Question 19

Benefits of Aminoglycoside therapy

Answer 19

1. Rapid bactericidal effect
2. Broad spectrum
3. Effective against Pseudomonas

Question 20

Drawbacks of Aminoglycoside therapy

Answer 20

1. Resistance
2. Ototoxicity and nephrotoxicity
3. Antagonized by anaerobiasis, low pH, and ions
4. Ineffective against most intracellular bacteria
5. Induce bacterial biofilm formation

Question 21

Ways to develop resistance to Aminoglycosides

Answer 21

1. Altered target in 30S ribosomal subunit
2. Decreased uptake into the cell
3. Enzymatic modifications of the aminoglycoside (transposons or plasmids)

Question 22

Aminocyclitol

Answer 22

Inhibits recognition (e.g., spectinomycin)

Question 23

Spectinomycin

Answer 23

• Inhibits recognition
• Bacteriostatic
• Causes the formation of unstable 70S initiation complexes (BUT DOES NOT CAUSE MISREADING AND DOES NOT INHIBIT POLYSOMAL RIBOSOMES)

Question 24

What does spectinomycin treat?

Answer 24

Exclusively for the treatment of gonorrhea caused by B-lactamase-producing gonococci or to treat gonorrhea in patients allergic to penicillins

Question 25

Tetracyclines

Answer 25

- Inhibitors or recognition - Broad spectrum - Bacteriostatic - Bind to the 30S ribosomal subunit and inhibit binding of aa-tRNA to the A site

Question 26

What does tetracycline treat?

Answer 26

Chlamydia, Mycoplasma, Rickettsia (i.e., intracellular pathogens)

Question 27

Resistance to Tetracyclines

Answer 27

1. Decreased uptake (mutations in the OmpF porin) 2. Efflux from the bacterial cell 3. Elongation factor-like proteins that protect the 30S ribosomal subunit

Question 28

Chloramphenicol

Answer 28

- inhibitors of peptidyl transfer - broad spectrum and bacteriostatic - binds reversible to the 50S ribosomal subunit and alters the tRNA structure blocking peptidyl transfer

Question 29

Lincomycin and Clindamycin

Answer 29

- inhibitors of peptidyl transfer - same action as chloramphenicol - very effective for treatment of G+ bacterial infections

Question 30

What does Clindamycin treat?

Answer 30

staphylococcal and anaerobic G- bacterial infections

Question 31

Resistance to chloramphenicol

Answer 31

plasmid-encoded acetyltransferase that catalyze the acetylation of -OH groups, which prevents 50S binding

Question 32

Resistance to clindamycin

Answer 32

Methylation of 23S ribosomal RNA which prevents drug binding to the 50S ribosomal subunit

Question 33

Macrolides

Answer 33

- inhibitors of translocation - bacteriostatic - can treat intracellular pathogens (Mycoplasma, Legionella, Chlamydia, and Campylobacter)

Question 34

Azithromycin and Clarithromycin

Answer 34

Modified forms of Erythromycin (type of macrolide)

Question 35

Macrolide Action

Answer 35

Isn't clear but is likely: 1. prevent elongation 2. prevent release of empty tRNA 3. blockage of transpeptidation

Question 36

Macrolide Resistance

Answer 36

1. Methylation of the 23S RNA of the 50S subunit 2. Hydrolysis of the lactone ring by an esterase 3. Efflux of the drug

Question 37

Ketolides

Answer 37

- inhibitors of translocation (e.g., Telithromycin) - bind the exit tunnel of the large ribosomal subunit and block the exit of nascent polypeptides - macrolides bind to only one domain of the 50S, these can bind to two

Question 38

Streptogramins

Answer 38

- inhibitor of translocation - Dalfoprisitin (binds to 50S) and Quinuprisitin (premature release of peptide chains) - Synercid = D + Q (alone the drugs are bacteriostatic, together they are bactericidal)