Pharmacology – ANTIBIOTICS – DNA, RNA, FOLIC ACID, PROTEIN SYNTHESIS INHIBITORS (MADE EASY)

Pharmacology – ANTIBIOTICS – DNA, RNA, FOLIC ACID, PROTEIN SYNTHESIS INHIBITORS (MADE EASY)


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