Diagnóstico de casos de VIH y sida
OBJETIVO 3. ELIMINAR LA TRANSMISIÓN MATERNOINFANTIL DEL VIH PARA EL 2015 Y REDUCIR SUSTANCIALMENTE LAS MUERTES MATERNAS
Cla
(member of 4,6 subclass)
(member of 4,5 subclass)
Figure 6.3. The structures of
both subclasses bind identically; however, be- cause of alternate linkages that exist between rings and the remaining portions of the two subclasses take different trajectories. The ultimate consequence of AG binding remains the same; is no longer able to discriminate
between and when
the tRNA binds to the A-site (57). 5.1 Mechanism of Action
The molecular details by which cause miscoding were recently elucidated Nu- cleotides A1492 and along with
play pivotal roles in discriminating between
cognate interactions and
interactions. When the correct tRNA occupies the A-site, the tRNA and
nucleotides involved in the
interaction form a regular helix confor- mation, and consequently normal minor groove geometry. In the correct presentation a helical minor groove stabilizes the "looped- out" conformation of and the conformation which serves as a sig- nal for bond formation to occur. Incor- rect do not elicit the same response and eventually diffuse away from the A-site with-
B and gentamicin
out the formation of a new bond. How- ever, when either a 4,5 or 4,6 AG binds to
it displaces A1492 and A1493 into the "looped-out" conformation. Therefore, pep- tide bond formation will occur regardless of whether or not the correct tRNA occupies the A-site.
5.2 Structure-Activity Relationship of Rings I
and of Both Subclasses
Although the two subclasses of the A-site binding aminoglycosides are chemically dis- tinct, they do share rings I and in common and up to that point bind in identical ways (60). Therefore, discussion of ring I and ring modifications of both subclasses will be treated at the same time, without regard for whether or not a particular modification was observed in the 4,5 subclass or the 4,6 subclass
(Fig.
For both classes, ring I is the only essential ring. Removal of ring I from all aminoglyco- sides results in inactive compounds; ring I of neomycin alone is minimally active as an an- tibiotic (41). Although this class is grouped by the inclusion of 2-deoxystreptamine, this ring alone is insufficient for antibiotic activity. Sev-
Therapeutic Agents Acting on RNA Targets
ring Stabilizing intramolecular interaction
HO
H
Figure 6.4. Molecular interactions between 4,5 and 4,6 linked aminoglycosides and coli numbering, top) with various modifications tested for activity (bottom). Dashed lines indicate possi- ble hydrogen bonds (some of which are salt bridges when suitably reinforced with favorable electro- static potentials). Arrows point to permissible modifications; arrows with an X point to
sible modifications. (a) Paromomycin ring I. The arrow with a ? points to a modification that was shown to be inactive, but is not readily explained by the crystal structure. Paromomycin ring
Parornomycin ring Paromomycin ring Gentamicin Cla ring
naturally occurring antibiotics contain Some have an NH, at which when only ring I and and neam- acetylated, abolishes antibiotic activity (41).
are examples), but all clinically active Replacement of with an or a agents contain four rings for the 4,5 subclass short-chain alkylamine is tolerated or and three rings for the 4,6 subclass. proves activity. The ring oxygen and
5 Neomycin and Centamicin Type Aminoglycosides
various
water-mediated contacts
Figure 6.4. (Continued.)
(or form a psuedo-trans Watson- that remains active against pro- Crick pair with A1408 (13). In G tozoa despite the fact that in protozoa
substitutes for A at this position, and this tide 1408 is a because at least one the
antibiotics
specificity of With a G in
hydrogen bond is preserved. But an group at (as in neomycin isn't
amino active, position 1408, an analogous pseudo-pairing because neither of the original hydrogen isn't possible. An important observation is bonds can be satisfied. Finally, branching at
Therapeutic Agents Acting on RNA Targets
ring HO
OH 3" 2"
Figure 6.4. (Continued.)
is permissible, at least up to one carbon, because branching a t this position doesn't dis- rupt the pseudo-trans pairing to (41).
All active aminoglycoside compounds have a hydroxyl at are or are unsat- urated at (41); hydroxyl groups at
form a hydrogen bond to the phosphate of The methoxy derivative is not toler- ated at presumably because of steric clash with the phosphate of An amino at the position surprisingly abolishes ac- tivity. Because an amino at this position should make a productive salt bridge, the par- ticular NH, derivative tested may be in- active for other reasons.
The 3' hydroxyl group in paromomycin forms a hydrogen bond to the phosphate of
Phosphorylation at this position abol- ishes all activity and forms a mode of resis- tance (41, 62); however, 3'-deoxy and 3'-epi analogs are not substrates for such resistance enzymes, and they remain active against some bacteria resistant to aminoglycosides (41). Recognition that all 3'-deoxyderivatives lose a productive interaction that presumably low- ers their affinities for prompted the design of 3' ketokanamycin A, in an effort to preserve the hydroxyl phosphate hydrogen bond while eliminating the possibility of inac- tivation by phosphorylating enzymes (63). It is
5 Neomycin and Type Aminoglycosides
epi mannose-0
expected that the 3' keto derivative will in equilibrium a hydrated variant gemdiol analog) and serve as a substrate for phosphorylation. However, the
product would undergo spontaneous elimination of the phosphate moiety, thereby regenerating the 3' ketokanamycin A analog, which retains the same hydrogen bonding to Although the keto analog was substan- tially less active than kanamycin A against E. it was more active than kanamycin A against
E.
harboring the gene fora resistance gene which, when ex- pressed, phosphorylates
Only hydroxyl and amino groups are
at because an intramolecular hydro- gen bond must be satisfied between ring I and of the 4,5 subclass and ring I and ring
4,6 subclass to stabilize the
of the antibiotics in the active state. In subclasses other substitutions at the
would disrupt this hydrogen bond.
con the
by resistance enzy
2'-epi configuration a
mes activity
Ring structurally defines this class of as the 2-deoxystreptamine
Figure 6.4. (Continued.)
cosides and has way to modification more frequently than the other (Fig. Conversion of kanamycin A to include (AHBA) at an aminoglycoside (amikacin) that was effective against many
side resistant bacteria with little reduction in activity against aminoglycoside sensitive bac- teria (41). Success with amikacin prompted an exhaustive search for other derivatives with improved efficacy over parental
glycosides. Several important commercial aminoglycosides emerged from such efforts. It is impossible to catalogue all of the reported modifications, therefore only the most il- lustrative examples will be described here.
N-acylation of is the most explored type of modification at this position. Although only a limited number of highly active deriva- tives were found, a large variety of modifica-
tions are tolerated at Us-
ing AHBA as a basis of comparison for all acyl modifications, clear structure-activity re- lationships of other acylated products are ob- served. Shortening or lengthening the carbon chain more than one carbon unit drastically reduces activity, as does moving the hydroxyl
Therapeutic Agents Acting on RNA Targets
NHR
(R is large)
0
Figure 6.4. (Continued.) group away from the a position or inverting
the stereochemistry of the a carbon. Substi- tuting the a-hydroxyl group with an amino group abolished all activity, whereas a fluorine replacement retains full activity. The a-deoxy derivative retains partial activity. With the ex- ception of unsubstituted amidino and
nidino groups, any change to the terminal generally reduces activity or abolishes activity altogether. Taken together, it is clear
that AHBA makes highly selective, yet known contacts with
The positive qualities of AHBA modifi- cation may be universal to all 2-DOS contain- ing aminoglycosides that bind in the decoding A-site pocket, because the same moiety is found in the naturally occurring
side butirosin and was successfully introduced into gentamicin B, yielding the active com- pound isepamicin (The observation that