Two apparent anomalies in the interactions of GA derivatives
and radicicol (monorden) (107; Scheme 19) with Hsp90
have been under intensive study The first anomaly is that,
despite the fact that both healthy and tumor cells require
Hsp90 for cellular function, they respond differently to these
drugs, and the second is the fact that the affinity of these
drugs for recombinant Hsp90 (rHsp90) is much lower than
the levels required for responses in tumor cell lysates. The
higher binding affinity for Hsp90 in tumor lysates has been
attributed to the existence of other co-chaperones in tumor
cells that are not expressed in healthy cells, and this effect
was demonstrated by the addition of such proteins to
rHsp90. In addition, X-ray crystal studies have demonstrated
that the structure of GA in the unbound form has a
trans-configuration at the amide bond between the benzoquinone
and the rest of the ansa ring, whereas when bound
to Hsp90, GA displays the cis-configuration at this center.
Similarly, Jez et al. reported that the closely related GA
derivative 17-DMAG requires both a macrocyclic ring
conformational change and a trans-cis isomerization of the
amide bond in order to bind to Hsp90. The tumor
selectivity, however, is still a subject of investigation
Two apparent anomalies in the interactions of GA derivatives
and radicicol (monorden) (107; Scheme 19) with Hsp90
have been under intensive study The first anomaly is that,
despite the fact that both healthy and tumor cells require
Hsp90 for cellular function, they respond differently to these
drugs, and the second is the fact that the affinity of these
drugs for recombinant Hsp90 (rHsp90) is much lower than
the levels required for responses in tumor cell lysates. The
higher binding affinity for Hsp90 in tumor lysates has been
attributed to the existence of other co-chaperones in tumor
cells that are not expressed in healthy cells, and this effect
was demonstrated by the addition of such proteins to
rHsp90. In addition, X-ray crystal studies have demonstrated
that the structure of GA in the unbound form has a
trans-configuration at the amide bond between the benzoquinone
and the rest of the ansa ring, whereas when bound
to Hsp90, GA displays the cis-configuration at this center.
Similarly, Jez et al. reported that the closely related GA
derivative 17-DMAG requires both a macrocyclic ring
conformational change and a trans-cis isomerization of the
amide bond in order to bind to Hsp90. The tumor
selectivity, however, is still a subject of investigation
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