Lec 4- Target Protein synthesis Flashcards
1
Q
Antibiotics- Antibacterial agents
A
- Naturally occurring
- Semi-synthetic: Slight alterations to naturally occurring agents
- Synthetics: Synthesized in the laboratory
- Antibiotics should demonstrate Selective toxcitiy (the drug should be toxic to pathogen not the host)
- Differences in structure or metabolism between the pathogen and the host is used in the selective toxicity
2
Q
Think of any difference between a human host and a bacterial pathogen that would be a target for antibacterial agents
A
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3
Q
Why are antibacterial drug much more abundant than antifungal, antiprotozoal and antihelmintic (small worms) drugs
A
4
Q
What was one of the first inhibitors of bacterial protein synthesis
A
- Streptomycin
- 1944 developed from fungi that killed bacteria
- This could treat penicillin resistant bacteria
- 1st that targeted bacterial protein sysnthesis
5
Q
Antibacterial agents- Ribosomes
A
- Aminoglycosides
- tetracyclines
- macrolides
- Oxazolidinones
- Fusidic acid
- we are expected to recognise parts of the molecules as opposed to the entire molecule
6
Q
Protein synthesis
A
- DNA is transcribed into mRNA within the nucleus
- mRNA diffuses out of the nucleus into the cytoplasm
- mRNA is then translated into protein by ribosomes (this is the step these antimicrobials work at)
- Prokaryotic protein
- 2 main structures make up the ribosome
- Small section (30S)- 16S rRNA + 21 protein
- The large section (the 50S)- 5S/23S rRNA’s + 31 proteins
- They come together to translate mRNA
- NB 30S= moves more quickly through viscous solution so is smaller than 50S
- NB- A large number of protein= a large number of targets for antimicrobials
7
Q
Eukaryotic and prokaryotic ribosomes differ in size and structure
A
- Prokaryotic 70S ribosome
- 23S and 5S rRNA’s (34 proteins) Large section
- 16S + 21 proteins- small section
- Eukarytoic 80S ribosomes
- 28S,5.8S and 5S rRNA’s + 45 proteins Large section
- 18S rRNA + 30 proteins Small section
- Eukaryotic and prokaryotic ribosomes differ relatively greatly, we can use target these differences to gain Selective toxicity and produce an effective antimicrobial
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8
Q
Bacterial protein synthesis
A
-
4 phases
- Initiation
- Elongation
- Termination
- Ribosome recycling
- mRNA is translated 5’=> 3’
- Proteins are synthesized from the N-terminus to the C-terminus
- Requires- Due to complexity good target for antimicrobial because as soon as 1 is inhibited the bacteria will die
- 2 ribosome sub-units- 30S and 50S
- 3 Initiation Factors- IF1, IF2 (GTP) and IF3
- The initiator Met-tRNA-Met (Met= N-formylmethionine)
- Elongation factors EF-Tu(GTP), EF-Ts, EF-G
- mRNA
- Very complicated hence why it is a good target, lots of oppertunities to disrupt
9
Q
Initiation
A
- Binding of IF-3 to the 30S ribosomal sub-unit promotes dissociation of the ribosome into its 2 subunits and allows 30S to form the initiation complex
- Initiation factor IF-1 binds specifically to 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
- IF-2 is a small GTP-binding protein. IF-2-GTP binds the initiator ‘Met-tRNA-Met and helps it to dock with the small ribosome sub-unit
- IF-2-GTP is hydrolysed and IF-3 released and 50S sub-unit binds to complete the initiation complex
- Oxazolidinones: 5 membered rings, N, O, O (double bond)
10
Q
Elongation- the elongation phase of protein synthesis consists of a cycle process
A
- Met-tRNA enters the P-site
- This causes a conformational change which opens the A site for the new aminoacyl-tRNA to bind to codon
- EF-Tu brings aminoacyl-tRNA to ribosome A-site- GTP slowly hydrolysed and aminoacyl enters A site
- The amino acid at P-site is transferred to the amino acid at A site of the growing polypeptide chain- Transpeptidase
- The whole assembly moves on one codon along the ribosome, with the tRNA’s staying put- shift left one pocket
11
Q
Elongation
Aminoglycosides and tetracyclines
A
- Some aminoglycosides change the shape of 30S subunit- misreading mRNA
- Leading to incorrect amino acids and so non-function
- e.g. streptomycin and gentomycin
- Some tetracyclines (and some aminoglycosides) prevent amino acids from entering the ribosome at 30S subunit
- e.g. tetracycline, minicycline and doxycycline
12
Q
Tetracycline structures
A
- How to recognise structure
- 4 Six-membered rings in a row is a tetracycline
13
Q
Tetracycline biosynthesis
A
- Natural or semi-synthetics
14
Q
3rd generation, semi-synthetic tetracyclines
A
- 3rd generation glycylcycline tetracycline= Tigecycline
- Active against
- MRSA
- VRS
- VRE
- many G-
- partly resistant to efflux mechanisms of resistance
15
Q
Properties and uses of tetracyclines
A
- Tetracyclines are orally absorbed, polyketide antibiotics used to treat chest infection, acne, periodontal disease (and other non-infection based conditions)
- They have broad-spectrum activity: G+, G- bacteria, anthrax, Yersinia, rickettsia, mycoplasma, spirochaetes, Neisseria meningitidis
- Doxycycline used in treating malaria
- Deposit in calcifying tissue (bone and teeth) causing staining, should not be used in children U8 or in pregnancy
- Good at binding metal Ions- Ca, Zn, Fe, Al- inactivates it
- Other effects- anti-inflammatory
18
Q
Inhibitors of Transpeptidase- chloramphenicol
A
- Chloramphenicol: isolated in 1947 as a natural product of Streptomyces venezuelae, now produced synthetically
- D-three form is active (how many optical isomers are there)
- Pro-drug forms: palmitate and stearate esters improve taste
- Succinate improves solubility for injections
- Penetrates mammalian cells and CSF, a drug of choice for typhoid and meningococcal meningitis
- Selective action on peptidyl transferase of 50S subunit
19
Q
Inhibitors of transpeptidation
A
- Chloramphenicol- blocks 50S ribosome, preventing peptide bond formation
- Kanamycin (aminoglycoside) causes misreading of the code by interfering with the (wobble) base pairing- cause wrong t-RNA to bind therefore wrong amino acid is put into the protein causing non-functional proteins
- Series of sugars (glyco), with some OH groups replaced with NH (Amino)
20
Q
Inhibitors of Transpeptidation- Kirromycin
A
- Aminoacyl-tRNA transfer is facilitated by 2 elongation factors, EF-Tu and EF-Ts in prokaryotes
- EF-Tu binds GDP (inactive when bound) and GTP (active when bound)
- EF-Tu-GTP binds and delivers an aminoacyl-tRNA to the A site on the ribosome
- As GTP on EF-Tu is hydrolyzed to GDP + Pi
- EF-Tu undergoes a large conformational change and dissociates from the complex
- Kirromycin blocks dissociation of GDP from EF-Tu after hydrolysis
- This prevents dissociation of EF-Tu from the ribosome and effectively stalls protein synthesis
21
Q
Inhibitors of transpeptidase- Fusidic acid- NOT IN EXAM
A
- Rediscovered in 1962 as a product of the fungus, Fusidium Cocciuneum (originally discovered as Cephalosporin P)
- Active only vs a limited number of G+: Staphylococcus (in); Streptococcus and Corynebacterium
- Penetrates bone well, used in staphylococcal osteomyelitis also for some MRSA infections
- Prevents translocation and EFG dependent cleavage of GTP
- Also inhibits 80S EF-2 but doesn’t enter mammalian cells
- Fusidic acid inhibits the multiple turnover EF-G-GTPase
- This stops ribosome movement
22
Q
Inhibitors of Translocation- Macrolide antibiotics
A
- Macrolides bind to 50S ribosome and prevent movement from one codon to the next, halting translation
- Erthromycin
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23
Q
Inhibitors of translocation- Biosynthesis of macrolides
A
