Antibiotics - Drug Design and Discovery - page 445

Supercoiling is essential for housing inside the cell and strand unwinding is necessary for replication
and transcription.
Topoisomerases are enzymes that convert DNA from one topological form to another. The
enzyme hydrolyses the phosphodiester bond of DNA backbone, making use of tyrosine residues in
the protein, allows the supercoiled DNA to pass into the relaxed form and reseals the strands.
Topoisomerase I cuts a single strand of the double helix, topoisomerase II cuts two strands simul-
taneously. DNA gyrase, discovered by Gellert in 1976, is a topoisomerase II, which occurs only in
prokaryotes. It is able to supercoil a relaxed DNA ring (reaction A). ATP is required as an energy
source in this process. Quinolones inhibit the catalytic activity of DNA gyrase and stabilize the
DNA cut. This means that the higher the topoisomerase activity in a cell, the more active the qui-
nolones will be. DNA gyrase consists of two A subunits and two B subunits and the A subunits are
involved in cleavage and annealing of the DNA strands. The targeting of the enzymes (gyrase A, a
subunit of topoisomerase IV) by l uoroquinolines is organism- and compound-specii c.
25.9 PROTEIN SYNTHESIS
Ribosomes are cellular particles, 200-500 Å in diameter. About 1800 are present per bacterial cell,
where they are bound to the cytoplasmic membrane. All ribosomes can dissociate into a small and a
large subunit. In prokaryotes, the 70 S ribosome (sedimentation coefi cient 70 S) consists of a 30 and
a 50 S subunit. The 30 S subunit contains a 16 S RNA molecule, the 50 S subunit a 23 and a 5 S RNA
molecule. The ribosomes of eukaryotic cells are larger (80 S) with 60 and 40 S subunits. Several
ribosomes simultaneously translate mRNA. The complex of mRNA and several ribosomes is called
as a polyribosome. Several antibiotics interfere with protein synthesis at the level of the ribosome.
The macrolides (for example erythromycin) bind to the 50 S subunit and inhibit the translocation
reaction. The aminoglycoside (for example streptomycin) interferes with the initiation step of pro-
tein synthesis and also induces miscoding during protein synthesis. They interact with the 30 S
subunit. The binding mode of aminoglycosides to the A site of the 16 S ribosomal RNA has been
determined using x-ray diffraction studies and is based on electrostatic interactions and direct and
water-mediated hydrogen bonds. Aminoglycosides stabilize a conformation of the aminoacyl-tRNA
decoding site that normally occurs when the tRNA-mRNA complex is bound. The ribosome now is
unable to discriminate between cognate and noncognate tRNA-mRNA duplex.
Tetracyclines bind to the 30 S subunit and inhibit the binding of aminoacyl-tRNA at the A-site of
the ribosome. Chloramfemicol is an inhibitor of the peptidyl transferase activity of the 50 S subunit
and releases short oligopeptidyl-tRNA. Fusidic acid stabilizes the normally unstable ribosome-
elongation factor G-GTP complex and inhibits translocation.
Puromycin is an antibiotic that has been isolated from S. alboniger in 1953 but that is not used
in the clinic. It has been very useful in the study of the protein synthesis. It resembles the amino-
acyl-adenosine part of aminoacyl-tRNA. It binds to the A-site in the ribosome in the place of an
aminoacyl-tRNA. The amino group of puromycin forms a peptide bond with the peptide, a reaction
catalyzed by peptidyl transferase. The peptidylpuromycin is then released from the ribosome and
thus causes premature chain termination.
25.10 NEW DEVELOPMENTS IN ANTIBACTERIAL RESEARCH
Since the last new structural class of antibiotics has been discovered (quinolons), only few really new
antibiotics have been approved, i.e., linezolid, daptomycin, tigecycline, and retapamulin. Tigecycline
is a glycylcycline that is active against Gram-positive and Gram-negative bacteria. It inhibits protein
synthesis. Daptomycin is used against Gram-positive infections, including resistant pathogens such
as MRSA (methycillin-resistant Staphylococcus aureus ), VRE (vancomycin-resistant enterococci ),
and PRSP (penicillin-resistant Streptococcus pneumoniae ). It is a cyclic lipopeptide that originates
from Streptomyces roseosporus , discovered at Eli Lilly in the early 1980s. It has a unique mode of