Cundliffe 6 antibiotics

Question 1

maximum occupancy of RNA binding sites

Answer 1

3x tRNA binding sites per ribosome => maximum occupancy = 2 E & A never at the same time.
Exit/empty site, Peptide site, Amino acid site.

Question 2

Peptidyl transferase centre:

Answer 2

centrally on larger subunit, exclusively of rRNA, no proteins in immeditate vicinity of catalytic site.

Question 3

Decoding site:

Answer 3

where codon-anticodon paring is monitored on small subunit : ribosome is an RNA enzyme.

Question 4

Ribosomal subunits:

Answer 4

complex particles, containing rRNA + multiple ribosomal proteins. 2x unequal sized ribosomal subunits associate to for intact ribosome (70S Bac, 80S Euk)

Question 5

3 t-RNA-binding sites:

Answer 5

(A,P,E) bridge ribosomal subunit interface & each site includes part of each subunit.

Question 6

Ribosomal active sites:

Answer 6

specific catalytic centers on subunit, peptidyl transferase centre on larger, decoding site on smaller. Ribosomal active sites are much smaller than tRNA-binding sites & consist exclusively (PTC) or almost exclusively (decoding site) of RNA

Question 7

ANTIBIOTICS & RIBOSOMES

Answer 7

Many antibiotics inhibit protein synthesis, mostly bind to ribosomes(active sites) non-covalently, usually 1:1 stoichiometry, many show selective toxicity, 70S vs 80S

Question 8

70S – Specific antibiotics:

Answer 8

• chloramphenicol (meningitis, typhoid),

- erythromycin (penicillin allergic), - streptomycin (TB, methotoxic in high doeses)

Question 9

80S-Specific antibiotics:

Answer 9

cycloheximide (euk not bac), anisomycin (toxic lab use only).

Question 10

antibioticsBind to all ribosomes

Answer 10

tetracycline => derivatives of tetracycline(minocycline, oxytetracycline widely used in human medicine). Not taken up in toxic amounts by mammalian cells.

Question 11

Target site: smaller subunit
30S:
30S/40S

Answer 11

• streptomycin

- tetracycline (respiratory/synocytis

Question 12

Target site: Larger subunit:
50S:
60S:

Answer 12

• chloramphenicol, erythromycin

- anisomycin, cycloheximide.

Question 13

Mode of action:

Tetracycline:

Answer 13

binds to decoding site by H-bonds, ionic bonds & hydrophobic interactions w/ rRNA =>blocks binding of aminoacyl-tRNA to A site.
=> mRNA & tetramycine both bind to 30S/40S

Question 14

Mode of action:

Erythromycin:

Answer 14

binds to rRNA within exit tunnel => blockage, inhibits elongation/ translocation/shift along ribosome by interacting w/ rRNA

Question 15

Mode of action:

Streptomycin:

Answer 15

binds to periphery of decoding site => distorts codon/anticodon interaction => misreading of mRNA codons. Non-cognate tRNA anticodons accepted to decoding sites.
Salt bridges, S12 & 4 H-bonds w/ rRNA => distorts site, possible to take apart & together & function except S12

Question 16

Mode of action:

Chloramphenicol:

Answer 16

blocks peptide bond formation => binds to peptiyl transferase centre, interacts w/ 23S rRNA. hydrophobic interaction. => folded box structure

Question 17

Studies of antibiotic binding sites in ribosomes suggest that

Answer 17

many/all of these drugs interact primarily with ribosomal RNA, typically non-covalent & reversible, involving H-Bonds, salt bridges & hydrophobic interacions. =>ribosome is RNA-Based enzyme.

Question 18

Control of gene expression

Prokaryotes:

Answer 18

subject to transcriptional control, heavy bias to induce/repressible metabolic pathways. Short lives mRNA => adapt rapidly to changes in environment. Involves synthesis of alternative sigma factors => turn on transcription of alternative gene groups.

Question 19

Control of gene expression

Eukaryotes:

Answer 19

: transcriptional control = extremely important => embryogenesis, more emphasis on translational control at polypeptide chain initiation. mRNAs longer lived, some translation IFs subject to de/activation e.g. reversible phosphorylation in response to hormonal signals & IFs specific for sub-groups of cellular mRNAs