6/7 - AB's of the Ribosomes Flashcards
Small Ribosomal Subunit
FUNCTION
&
AB targtts
SELECTION of aminoacyl-tRNA according to mRNA codons
Antibiotics will:
interfere with tRNA binding
or
interfere with the accuracy of protein synthesis
Large Ribosomal Subunit
FUNCTION
&
AB targtts
Polymerization of AA’s -> Poly Peptides
Antibiotics will:
inhibit peptide-bond formation
or
inhibit Growth of the nascent peptide chain
Why is the ribosome evolutionary preferred antibiotic target?
Bacterial rRNA genes are REDUNDANT
several identical genes in genomes of bacteria code for rRNA
MOST CONSERVED
Antibiotic Size in comparison to Ribosome
AB’s are 5000x SMALLER than ribosomes
AB’s inhibit translation by interacting with the:
FUNCTIONAL CENTERS of the ribosome
Drugs target the:
Ribosomal RNA
Why does
Targeting ABs -> the RIBOSOME delays the appearance of resistance traits?
A single spontaneous resistance mutation that occurs in one rRNA gene
DOES NOT CONFER SUFFICIENT LEVEL OF RESISTANCE
because:
majority of the ribosome will STILL carry on unmutated rRNA
23k+ macrolide resistant strains –> only 0.3% developed resistance
What Antibiotic?
MACROLIDE
- *Large Compound –>** can only hit GRAM POS
- too large to get across gram neg*
Macrolactone + 2 Sugars
Erythromycin is unstable @low pH –> converts to inactive ketal
What Antibiotic?
2nd Gen Macrolide
AZITHROMYCIN
Prevent formation of the inactive ketal
(erythromycin, unstable @ low pH)
Better ACID stability & Broader Spectrum
How were MACROLIDES IMPROVED?
2rd Generation
-MYCINS
2nd Gen = Clarithro / Azithro / Roxi
Better:
Acid Stability & Broader Spectrum
things to prevent formation of the inactive ketal (erythromycin)
LOWER MIC
How were MACROLIDES IMPROVED?
3rd Generation KETOLIDES
-MYCINS
3rd Gen = KETOLIDES (Telithromycin)
KETO GROUP –> clandinose sugar
=better activity againstRESISTANT STRAINS
CARBAMATE GROUP
= improves PK & PD
EXTENDED SIDE CHAIN
improves binding to robosome
drawbacks = liver toxicity
Macrolide (-mycin)
MoA
Macrolides bind to the:
- *Large Ribosomal Subunit** @ Nascent peptide Tunnel
- *PARTIALLY OBSTRUCT the EXIT TUNNEL**
Protein-Specific Inhibitors
allow for synthesis of other proteins, DOES NOT ABOLISH ALL
depends on the sequence, some proteins can still be made
- *Alkyl-Aryl Side chain of KETOLIDES**
- -> additional contacts w/ ribosome = ↑drug affinity
- *Desosamine –> A2058**
- target of resistance mechanisms*
Mechanisms of Resistance:
Macrolides
(-mycins)
CHEMICAL MODIFICATION of rRNA
DiMethylation of A2058 (desosamine contact) @ 23S rRNA catalyzed by:
rRNA methyltransferase ERM
INDUCIBLE & activated only in the presence of Macrolide AB’s
- *Ribosome Protection**
- *MsrE –> REMOVES AB from ribosomal tunnel**
Ribosome Modification
rRNA mod // mutations in rRNA & ribosomal protein genes = VERY RARE
- *Drug Efflux**
- *MeF** Macrolide-specific pumps // multi-drug Pumps
What does MsrE do?
To WHAT Antibiotic?
Resistance mechanism for
MACROLIDES & STREPTOGRAMIN B
- *RIBOSOME PROTECTION** For Macrolides:
- *EJECTS AB from RIBOSOMAL TUNNEL**
- *Msr Transporters: Drug Efflux** for
- *Both macrolides & streptogramin B**
What is the MAJOR Mechanism of RESISTANCE for
MACROLIDE AB’s?
(-mycins)
Chemical Modification of rRNA
Di-methylation of A2058 (desosamine target) in the 23S rRNA
catalyzed by:
rRNA methyltransferase ERM
ERM = inducible gene & activated only in presence of Macrolides
Macrolide (-mycin)
Clinical Uses
GRAM POSITIVES
Oral agents for:
Upper & Lower Respiratory Tract Infections
urethritis / skin infections
Which Drug?
OXAZALIDINONES
Linezolid / Tedizolid
complicated skin infxns = MRSA
GRAM POS
mostly + drug resistant S.Areus
Oxazolidinones
linezolid / tedizolid
MoA
Binds to the:
- *Large Ribosomal** subunit in the
- *Catalytic PEPTIDYL TRANSFERASE Center**
Linezolid clashes with the placement of aminoacyl-tRNA
& prevents polymerization of AA’s into proteins
inhibits the ELONGATION STEP of translation
specifically peptide bond formation when:
ALANINE is present @ penultimate position of nascent protein
alanine = very common, needed for ALL PROTEINS in bacteria
Major ADR of
Oxazolidinones
& Why?
(linezolid / tedizolid)
Structure of ribosomal site where Oxazolidonones bind in bacterial ribosome is:
nearly the SAME in human MITOCHONDRIAL RIBOSOMES
allows linezolid to:
inhibit MITO translation in human cells
Reversible Myelosupression
from prolonged treatment
Mechanism of Resistance for:
Oxazolidinones
(linezolid / tedizolid)
RIBOSOMAL MODIFICATION
Mutations in rRNA & ribosomal protein genes = RARE
↑mutated rRNA genes = ↑resistance
+
ACQUIRED rRNA modification
binding of the AB overlaps with the binding site of natural AB’s
aquired CFR gene encoding rRNA methyltransferase
C8 Methylation @ A2503
What is the CFR gene?
often present on transposons & plasmids (facilitates its spread)
&
What AB does it affect?
RIBOSOMAL MODIFICATION** = **Acquired rRNA Modification
Mechanism of Resistance
C8 Methylation @ A2503
CFR gene has overlapping sites in peptidyl transferase center with other naturally occuring AB’s (chlorophenicol)
OXAZOLIDINONES** + **PLEUROMUTILINS
LINCOSAMIDES** + **STREPTOGRAMINS A
+ 16 member ring macrolides
ALL LARGE RIBOSOME TARGETS
except for Streptogramins B + certain macrolides
What Antibiotic?
LINCOSAMIDES
Oral / IV / Topical
ANAEROBIC GRAM POS
+
effect on GUT MICROBIOME –> C.DIFF RISK
(Lincomycin + Clindamycin)
LINCOSAMIDES
(Lincomycin + Clindamycin)
MoA
Binds to:
Large Ribosomal Subunit** on the **Peptidyl Transferase Center
Clashes w/ AminoAcyl tRNA
VV
Inhibits formation of PEPTIDE BONDS
Mechanisms of Resistance
LINCOSAMIDES
(Lincomycin + Clindamycin)
Ribosome Modification
ERM METHYLTRANSFERASE (also for macrolides & streptogramin B)
by dimethylating A2058 = MLSb Resistance
MONOMETHYLATION by some ERM confers resistance to specifically Lincosamides
- *Drug Modification**
- *LNU ENZUMES** –> inactivate clindamycin by adenylation
- *Drug Efflux**
- *LSA(B)** & LSA(A) & VGA (A,C,E)
**What does ERM do?** and to: what antibiotic(s)?
Mechanism of Resistance (rRNA Mutations & Modifcation) for:
MACROLIDES + LINCOSAMIDES + STREPTOGRAMIN B
by:
Dimethylating A2058 = MLSb Resistance
monomethylation is sufficient for LINCOSAMIDES
What Antibiotic?
STREPTOGRAMINS
(A) DalfoPristin + (B) QuinuPristin
Synercid = 30% A + 70% B
Improved solubility –> INJECTABLE
IV ONLY
for life-threatening infxns caused by vanco-resistant Enterococcus faecium & complicated skin infxn by staphylococci
- *GRAM POSITIVE**
- *MRSA + MSSA**
- *STREPTOGRAMIN B**
- *(B) Quinupristin**
BINDING SITE & MoA
QuinuPristin binds in the:
EXIT TUNNEL
&
PREVENTS the EGRESS of newly made protein
- *STREPTOGRAMIN A**
- *(A) DalfoPristin**
BINDING SITE & MoA
Dalfopristin binds in the:
Peptidyl Transferase Center
&
Inhibits AA polymerization
STREPTOGRAMINS
(A) DalfoPristin + (B) QuinuPristin
MoA
SYNERGISTICALLY = BACTERICIDAL
& active @lower concentrations
Dalfopristin REMODELS the rRNA structure
(targets the peptidyl transferase center)
VV
Stimulating BINDING of QuinuPristin
(targets the EXIT tunnel)
STREPTOGRAMINS
(A) DalfoPristin + (B) QuinuPristin
Mechanisms of Resistance
- *Drug Modification**
- VAT AcetylTransferase** –> *inactivates Dalfopristin
- *Vgb Lyase** –> opens the central circle of quinupristin
- *Ribosome Modification**
- *ERM methyltransferase** –> dimethylation of A2058
- *Drug Efflux**
- *VGA & LSA** = streptogrammin A
- *MSR** = macrolides & streptogramin B
What is:
VAT AcetylTransferase
what does it do and to what?
Mechanism of Resistance = Drug Modification
INACTIVATES** & **MODIFIES
DALFOPRISTIN
Streptogramin A
What is:
VGB LYASE
and what does it do to what?
Mechanism of Resistance = Drug Modification
Opens the CENTRAL CIRCLE component of:
QUINUPRISTIN
Streptogrammin B
What drug classes act on the
LARGE RIBOSOMAL SUBUNIT?
MACROLIDES
Erythromycin -> Clarithro/Azithro/Roxithro -> Telithro/Solithro
OXAZOLIDINONES
Linezolid / Tedizolid
LINCOSAMIDES
Lincomycin / Clindamycin
STREPTOGRAMINS
(A) Dalfopristin + (B) QuinuPristin = Syncercid 30-70
PLEUROMUTILINS
PleuroMutilin / RetapaMulin / LefaMulin
What Antibiotic?
PLEUROMUTILINS
PleuroMutilin + RetapaMulin + LefaMulin
Complex DITERPENE Structure
TOPICAL
for
Skin infections = Impetigo / Wounds / Lacerations
Gram-Positive Pathogens
PLEUROMUTILINS
PleuroMutilin + RetapaMulin + LefaMulin
Binding Site & MoA
Bind to the:
- *Peptidyl Transferase Center_ in the _Large Ribosomal Subunit**
- interferes w/ placement of tRNA substrates*
bind only the the EMPTY RIBOSOME
specifically inhibit the INITIATION of TRANSLATION
can NOT bind to ribosome w/ nascent chain
Mechanisms of Resistance
PLEUROMUTILINS
PleuroMutilin + RetapaMulin + LefaMulin
- *Ribosome Modification**
- *CFR** –> rRNA methylation @A2503
- also Lincosamides / oxazolidinones / streptogramin A / macrolides*
Drug Efflux
VgaC & VgaE
confer resistance to Streptogramins & Lincosamides as well
LsaA = intrinsic resistance to just pleuromutilins
What do
VgaC & VgaE
do? and to what AB’s?
EFFLUX PUMPS
mechanisms of resistance for:
Streptogramins** + **Lincosamides** + **Pleuromutilins
What Antibiotic?
4,6 - Substituted AMINOGLYCOSIDE
Kanamycin / Gentamicin / Tobramycin / Plazomicin
TB / Plague / Endocarditis
Aerobic GRAM-NEG
MycoBacteria
Usually in combo with Beta-Lactams
What Antibiotic?
- *4,5 - Substituted AMINOGLYCOSIDE**
- *Neomycin** / Paromycin
TB / Plague / Endocarditis
Aerobic GRAM-NEG
MycoBacteria
Usually in combo with Beta-Lactams
AMINOGLYCOSIDES
Neomycin + Paromomycin // Kanamycin / Gentamicin / Tobramycin
Site of Action
Act on:
- *Small Ribosomal Subunit**
- flexibility of AG’s allow their* adaptation to shape of binding pocket
Binding –> induces conformation of DECODING CENTER
AMINOGLYCOSIDES
Neomycin + Paromomycin // Kanamycin / Gentamicin / Tobramycin
MoA
MISCODING:
Synthesis of Erroneous Proteins
the ribisome accepts INCORRECT aminoacyl-tRNA
incorporates a WRONG AA in the growing protein
SELF-PROMOTED UPTAKE
disrupts membrane structure –> increased AG permeability
BACTERICIDAL
AMINOGLYCOSIDES
Neomycin + Paromomycin // Kanamycin / Gentamicin / Tobramycin
Mechanism for SELECTIVITY
- *Nucleotide 1408** of the Small Subunit is:
- *A in BACTERIA**
- but B in Eukaryotes*
Adenine @ this position is REQUIRED for TIGHT binding of AG’s
AMINOGLYCOSIDES
Neomycin + Paromomycin // Kanamycin / Gentamicin / Tobramycin
Side Effects & WHY?
NEPHRO** & **OTOTOXICITY
due to action on the:
Mitochondrial Ribosomes
the key nucleotide for AG binding is A (like bacteria), not G
AG’s can bind and inhibit translation
Familial mutations in deconding center of mito ribosomes
can predispose patients to OTOTOXIC effects of AG’s
AMINOGLYCOSIDES
Neomycin + Paromomycin // Kanamycin / Gentamicin / Tobramycin
Mechanisms of Resistance
Drug Modification
specific functional side groups in AG could be:
N-Acetylated** / **O-phosphorylated** / **O-adenylated
steric prevention
overcome by eminination of the group targeted by resistance enzymes
- *Ribosome Modification**
- *ArmA/RmtA** –> methylates G1405 in the AG binding site
- *NpmA** –> modifies A1408
What AB?
STREPTOMYCIN
AminoGlycoside
interacts with the ribosome at a site CLOSE but DIFFERENT from other AG’s
Pharmacore = STREPTAMINE
Highly BASIC guanidino groups
facilitates interactions with rRNA
What AntiBiotic?
TETRACYCLINES
2nd Gen = Doxycycline + Minocycline
BOTH
Gram-Pos & Gram-Neg
lyme disease + CAP
Syphilis
What Antibiotic?
- *3rd Gen Tetracycline = GLYCYLCYCLINES**
- *TigeCycline + Eravacycline**
- *IV DRUGS**
Indicated for:
Skin/Ab Infxns + Community Acquired Pneumonia
Bind in conventional tetracycline site in small ribosome, BUT are:
able to withstand the major tetracycline RESISTANCE mechanisms
How do Tetracyclines have GRAM-NEG activity?
- *POLAR NATURE**
- *Basic + Acidic Groups**
allow for it to get through the:
PORIN CHANNELS
of the outer membrane of Gram-Neg Bacteria
Gram-Pos = Diffusion
What is CRITICAL for Tetracycline Activity?
STEREO-ORIENTATION
of the DimethylAmino-Moitety
Epimerization –> reduces activity of TetraCyclines
Ring A can undergo enolization & re-protonization
4-epitetracycline = inactive
TETRACYCLINE
MoA
Tetracycline acts on the:
Small Ribosome and Blocks binding of AminoAcyl-tRNA
- *Tigecycline TAIL** –> forms additional interactions
- *20x affinity** vs tetracycline
TETRACYCLINE
Mechanisms of Resistance
RIBOSOME PROTECTION
primarily in Gram-POS bacteria
TetM** + **TetO = proteins that “CLEAN” the ribosome = Evict Tetracycline
DRUG EFFLUX
primarily in GRAM-NEG bacteria
Transmembrane Efflux Pumps active in 1st/2nd generation
not for Glycylcyclines like tigecycline
What Antibiotic?
RIFAMPICIN
Potent + Broad-Spectrum AB
used in:
Anti-TB combination therapy
RED COLOR
BacteriCIDAL
RIFAMPICIN
Target?
RNA POLYMERASE
Rifampicin binds in the:
Nascent RNA chain EXIT tunnel of the Beta-Subunit of RNA poly
similar to MACROLIDES –> exit tunnel = BUT CLOGS COMPLETELY
VV
INHIBITION of INITIATION of TRANSCRIPTION
inhibits formation of 2nd or 3rd PHOS bonds
Mechanisms of Resistance
RIFAMPICIN
Mutations in GENE ENCODING
on the
beta-subunit of RNA polymerase
What Antibiotic?
SULFONAMIDE
Synthetic BacterioSTATIC AB’s
that mimic / structural analogs of
PABA
an intermediate of the FOLATE biosynthesis Pathway
Sulfonamide
Sulfamethazole
Target / MoA
Sunfonamides mimic & compete with:
PABA
which inhibits the activity of:
DIHYDROPTEROATE SYNTHASE
Which ultimately inhibits the synthesis of:
- *TetrahydroFolate**
- precursor of NUCLEOTIDES & AA’s*
Improvements in SULFONAMIDES
Sulfisoxazole / Sulfamethoxazole
EWG @ the AMIDE GROUP
↑Potency by reducing pKA of Amino group
VV
brings it closer to the pKA of PABA
Mods also:
↑Drug Solubility @ neutral pH
by helping it avoid crystalization of sulfonamides in Kidneys
What Drug?
TRIMETHOPRIM
Target is
- *Sufficiently DIFFERENT in bacterial pathogen** & human host
- *DiHydroFolate Reductase**
Sulfonamide’s target is ONLY in the BACTERIAL PATHOGEN
DiHydropteroate Synthase
not in Human Cells
Trimethoprim
Target / MoA
DIHYDROFOLATE REDUCTASE
another enzyme in Folate Synthesis
found in BOTH HUMANS & Bacteria, but greater affinity for bacteria
BACTRIM
Sulfamethoxazole + Trimethoprim
Acts Synergistically
