in this second part of the lecture covering pharmacology of antibiotics we
are going to cover nucleic acid synthesis inhibitors protein synthesis
inhibitors and metabolic pathway inhibitors
now let's discuss these one by one in detail starting with nucleic acid
synthesis inhibitors so bacteria just like all other living organisms store
their genetic information in the form of DNA the cell uses DNA like an
instruction manual reading it to find out what parts it needs to make to
survive and replicate but first things first in order to carry out their life
processes bacteria must convert their genetic information into functional
molecules this is done by using DNA as a template for the synthesis of RNA
molecule a process known as transcription depending on its structure
RNA can then perform tasks directly or act as a blueprint for the synthesis of
functional proteins in a process known as translation now it should be easy to
see that drugs capable of inhibiting DNA or RNA synthesis would render the
bacteria unable to create proteins and replicate but wouldn't that be toxic to
our own body's cells well luckily for us bacterial enzymes that carry out
synthesis of DNA and RNA happen to be different from ours thus allowing
development of selectively toxic antibiotics the antimicrobial agents
that primarily target nucleic acid synthesis include Metronidazole
Quinolones and Rifamycins so now let's take a closer
look at their mechanism of action our first antibiotic Metronidazole is a
prodrug that requires reductive activation of
its nitro group upon diffusing across the cell membrane of anaerobic bacteria
Metronidazole is converted by the bacterial enzyme pyruvate:ferredoxin
oxidoreductase into nitro radical anion this highly reactive radical is thought
to be the main toxic agent that attacks DNA causing strand breaks and mutations
that lead to bacterial cell death now let's move on to Quinolones unlike
Metronidazole the mechanism of action of Quinolones involves interactions with
topoisomerases which are the enzymes responsible for the unwinding and
unlinking of DNA so in order for the bacterial cells to replicate tightly
coiled bacterial chromosome must unwind so that the DNA code can be accessed and
copied the two principal topoisomerases that perform this task are DNA gyrase
which unwinds and relaxes supercoiled DNA and topoisomerase IV which
facilitates separation of the linked daughter DNA molecules after replication
is complete both DNA gyrase and topoisomerase IV are the primary
target of Quinolones which bind to these enzymes and
effectively inhibit their function this in turn blocks DNA synthesis and
cell growth ultimately leading to bacterial cell death now the vast
majority of Quinolones in clinical use are so called Fluoroquinolones which
have a fluorine atom attached to the central ring system that increases their
antimicrobial activity examples of Fluoroquinolones are Ciprofloxacin
Levofloxacin Moxifloxacin Norfloxacin and Ofloxacin now let's move on to
Rifamycins so in contrast to Quinolones that inhibit DNA replication
Rifamycins interfere with transcription of bacterial DNA into RNA
specifically Rifamycins target enzyme responsible for translating DNA into RNA
called RNA polymerase by combining with a protein of this bacterial
DNA-dependent RNA polymerase Rifamycins bring all synthesis of RNA to a halt
without RNA the bacteria cannot make proteins that are essential for survival
so the cell death eventually results examples of Rifamycins are Rifampin
Rifabutin and Rifapentine now when it comes to side effects Metronidazole
commonly causes headaches nausea and metallic taste in the mouth furthermore
because Metronidazole inhibits metabolism of alcohol in the body
combining it with alcohol causes unpleasant symptoms such as vomiting flushing
of the skin tachycardia and shortness of breath
next Quinolones most commonly cause gastrointestinal side effects such as
nausea vomiting and diarrhea as well as headache and insomnia less common but
more serious adverse effects include prolongation of the QT interval tendon
damage and peripheral neuropathy the mechanism by which Quinolones cause these
rare events is not entirely clear lastly Rifamycins may cause GI
disturbances flu-like symptoms hepatotoxicity and discoloration of body
fluids including urine saliva sweat and tears to red-orange color now let's
switch gears and let's move on to our next class of antibiotics that is
protein synthesis inhibitors so as discussed before bacterial genes are
translated into proteins through RNA the type of RNA that carries message from
the DNA is called messenger RNA or mRNA for short and the protein synthesizing
machine to which the message is carried to is called ribosome the bacterial
ribosome is composed of two subunits the 30S into which mRNA feeds and the 50S which
carries out catalytic functions the ribosome translates messenger RNA into
protein by reading the nucleotide triplets known as codons which specify
amino acids that are required to make up specific proteins transfer RNA or tRNA
for short brings the individual amino acids to the ribosomal aminoacyl site
known as the A-site which then form a peptide bond with a growing chain at the
peptidyl site known as the P-site next the empty tRNA in the P-site is released
and the ribosome moves to the next codon thereby transferring tRNA with new
polypeptide from the A-site to the P-site this process is repeated until
ribosome encounters a stop codon that signals the end of protein
synthesis so as you may have already guessed protein synthesis inhibitors act
at a specific site on the ribosome to inhibit different steps
in the protein synthesis now the antibiotic classes that block bacterial
protein synthesis can be divided into two groups the 30S subunit inhibitors
and the 50S subunit inhibitors classes of antibiotics that bind to the 30S
subunit include Aminoglycosides Tetracyclines and Glycylcyclines
classes of antibiotics that bind to the 50S subunit include Amphenicols
Macrolides Ketolides Lincosamides Streptogramins and
Oxazolidinones now let's take a closer look at their mechanism of action
starting with Aminoglycosides so Aminoglycosides work primarily by binding to
an area adjacent to the decoding site in the 30S subunit of the ribosome where
they interfere with the initiation of protein synthesis and cause misreading of
the genetic code this ultimately leads to either synthesis of nonfunctional
proteins or premature termination of protein synthesis examples of drugs that
belong to this class are Neomycin Amikacin Gentamicin Streptomycin and
Tobramycin when it comes to major side effects use of Aminoglycosides has been
associated with serious toxicities including ototoxicity nephrotoxicity and
in rare instances neuromuscular blockade now let's move on to Tetracyclines and
their derivatives Glycylcyclines so just like Aminoglycosides Tetracyclines
and Glycylcyclines also bind to the 30S ribosomal subunit however their
primary mode of action is by blocking entry of aminoacyl tRNA molecules into
the A-site of the ribosome thus preventing introduction of new amino
acids to the growing peptide chain this action is usually inhibitory and
reversible upon withdrawal of the drug examples of Tetracycline antibiotics
include Doxycycline Minocycline and Tetracycline example of Glycylcycline
antibiotic is Tigecycline when it comes to side effects the use of
Tetracyclines has been associated with GI disturbances photosensitivity and
hepatotoxicity furthermore Tetracyclines have strong affinity for calcium and can
accumulate in developing teeth and bones leading to discoloration of teeth and
inhibition of bone growth now let's move on to our next group of antibiotics
that is Amphenicols so Amphenicols work primarily by
binding to 50S ribosomal subunit where they block the peptidyl transferase
center that catalyzes peptide bond formation this prevents transfer of the
elongating peptide chain to the newly attached aminoacyl tRNA which generally
results in bacteriostatic effect example of Amphenicol antibiotic is
Chloramphenicol when it comes to major side effects intravenous Chloramphenicol
use has been associated with the so-called gray baby syndrome that
results from the inability of an infant's immature liver to metabolize
Chloramphenicol the symptoms include hypotension
abdominal distension and cyanosis which can ultimately lead to death another
rare and sometimes fatal adverse effect that has been reported with the use of
Chloramphenicol is aplastic anemia which can occur weeks or even months after the
treatment now let's move on to Macrolides and their close relatives Ketolides
so Macrolides and their relatives appear to bind primarily to
50S ribosomal subunit near the peptidyl transferase center where they block the
peptide exit tunnel that the newly assembled polypeptides pass through on
their way out of the ribosome this results in inhibition of protein
elongation process and thus bacteriostatic activity against most
organisms examples of Macrolide antibiotics include Azithromycin
Clarithromycin Erythromycin and Fidaxomicin example of
Ketolide antibiotic is Telithromycin when it comes to side effects the most common
ones include nausea vomiting diarrhea and ringing or buzzing in the ears less
common but more serious side effects include QT interval prolongation that
can lead to ventricular arrhythmia Torsades de pointes and cholestatic hepatitis that
is generally associated only with the use of Erythromycin
now the Macrolide binding site in the exit tunnel happens to overlap with the
binding sites of our next two groups of antibiotics Lincosamides and
Streptogramins as a result just like Macrolides Lincosamides and Streptogramins
inhibit primarily the translocation steps of protein synthesis
example of Lincosamide antibiotic is Clindamycin and example of Streptogramin
antibiotic is Quinupristin/Dalfopristin Quinupristin/Dalfopristin is
a combination of two Streptogramin antibiotics which act synergistically
with Dalfopristin enhancing the binding of Quinupristin as well as inhibiting
peptidyl transferase when it comes to side effects Clindamycin is likely to
cause diarrhea nausea vomiting and abdominal cramps furthermore destruction
of natural flora by Clindamycin can lead to overgrowth of Clostridium
difficile that can cause severe diarrhea and inflammation of the colon called
pseudomembranous colitis on the other hand Quinupristin/Dalfopristin may
cause nausea vomiting diarrhea and injection site reactions including pain
burning and irritation now let's move on to our last group of protein synthesis
inhibitors that is Oxazolidinones unlike the other protein synthesis
inhibitors which primarily inhibit the elongation steps
Oxazolidinones inhibit the first step of the synthesis by binding to the a side
on the 50S ribosomal subunit where they prevent the initiation of complex
formation without functional initiation complex bacteria can't synthesize proteins
that are essential for their growth which ultimately results in
bacteriostatic or bactericidal effect depending on the species
examples of Oxazolidinones are Linezolid and Tedizolid the most
common adverse effects associated with these antibiotics are nausea vomiting
diarrhea headache and dizziness less common but potentially more serious
adverse effects include bone marrow suppression optic and peripheral
neuropathy seizures and abnormal liver-function
tests now before we end let's briefly discuss our last class of antibiotics
that is metabolic pathway inhibitors so the primary target of metabolic pathway
inhibitors is the pathway that bacteria use to synthesize folic acid folic acid
is an important vitamin that bacteria as well as humans need in order to make
nucleotides and some amino acids so as you can imagine without folic acid DNA
replication and cellular growth would be disrupted now unlike us humans who
obtain folic acid from the diet bacteria must make folic acid on their own
bacteria synthesize folic acid by taking para-aminobenzoic acid PABA for short
and adding a compound called pteridine in the presence of enzyme dihydropteroate synthase
to form dihydropteroic acid then they add glutamate to make
dihydrofolic acid and use an enzyme called dihydrofolate reductase to make
tetrahydrofolic acid tetrahydrofolic acid is the metabolically active
form of folic acid which acts as a coenzyme for a number of key biochemical
reactions now metabolic pathway inhibitors work by interfering with
bacterial synthesis of tetrahydrofolic acid
they include family of drugs called Sulfonamides or Sulfa drugs and drug
called Trimethoprim so now let's take a closer look at their mechanism of action
first the Sulfonamides act through competitive inhibition of dihydropteroate synthase
this is due to their structural resemblance to para-aminobenzoic acid
the enzyme that normally converts PABA to the precursor of folic
acid combines with the Sulfonamide instead the combination prevents
tetrahydrofolic acid synthesis and thereby stops the growth of the bacteria
examples of Sulfonamide antibiotics that inhibit folic acid synthesis
include Sulfamethoxazole and Sulfacetamide now let's move on to our second
agent Trimethoprim so unlike Sulfonamides Trimethoprim
targets the second key enzyme in the folic acid synthesis pathway that is
dihydrofolate reductase as an analog of dihydrofolic acid
Trimethoprim competitively inhibits this enzyme thus
effectively disrupting production of tetrahydrofolic acid when it comes
to side effects Sulfonamides commonly cause GI disturbances such as nausea
vomiting and diarrhea as well as photosensitivity less common but more
serious side effects include renal stones hepatotoxicity bone marrow
suppression and allergic reactions ranging from rash or hives to
anaphylaxis and Stevens-Johnson syndrome on the other hand side effects of
Trimethoprim are little more modest and generally limited to upset stomach
nausea vomiting and skin rashes and with that I wanted to thank you for watching
I hope you enjoyed this video and as always stay tuned for more