Biomedical Engineering Reference
In-Depth Information
nature of the interactions of MK-499, terfenadine and cisapride with the amino
acids Tyr652 and Phe656. They observed that the mutation of Phe656 to Trp, Tyr,
Met, Leu or Ile altered the block of MK-499, cisapride and terfenadine only
slightly. In contrast, the mutation of Phe656 to the polar amino acids Arg, Glu,
Ser, and Thr, or to small amino acids such as Ala and Gly increased the IC
50
. All
together these results indicate that the most important physicochemical feature of
Phe656 is the hydrophobic volume and not the aromaticity. Mutation of Tyr652 to
Phe or Trp did not influence significantly the sensitivity of hERG to Mk-499,
terfenadine and cisapride, indicating that the hydroxyl group of Tyr652 is not
essential for the interaction with hERG blockers. The IC
50
of MK-499, terfenadine
and cisapride dramatically increased when Tyr652 was mutated to Ile, Val, Ala,
Thr, Gln, and Glu. All together these results highlight that the aromaticity at
position Tyr652 is an important feature for hERG inhibition by these three potent
blockers.
Mitcheson et al. [
6
] found that the hERG block mediated by MK-499 was
affected by mutations of Thr623, Ser624, Val625, Gly648, Tyr652, Phe656, and
Val659. The experiments performed on terfenadine and cisapride supported the
interactions with Tyr652 and Phe656, but not the interaction with Val625. Same
results were obtained by Karczewski et al. [
104
] for MK-499, its enantiomers, and
its analogs obtained replacing the hydroxyl group with a hydrogen or an amine
group. In contrast, the carbonyl analog was insensitive to the Phe656Ala mutation.
This result indicates that the carbonyl oxygen interacts with the hERG channel.
The reduction of sensitivity due to the mutation of Thr623, Ser624, and Val625
to alanine was observed also for clofilium and ibutilide. Sanchez-Chapula et al. [
5
]
determined that the binding site for the weak blocker chloroquine is formed by
Tyr652 and Phe656. In contrast to previous results [
103
], they found that the
mutation Tyr652Phe was detrimental for the affinity, indicating that the hydroxyl
group is essential for the inhibition of the hERG channel mediated by chloroquine.
Mutation of the Phe656 and Tyr652 to alanine reduce the binding affinity of the
potent blockers MK-499, terfenadine, cisapride [
6
,
9
], clofilium, ibutilide [
12
,
105
],
thioridazine [
106
], and of the weak blockers chloroquine [
5
], nifekalant, bepridil
[
8
], maprotiline [
107
], and mianserin [
108
] indicating the importance of the pres-
ence of aromatic amino acids in this position.
In contrast to these results, hERG channel block mediated by some blockers was
almost insensitive to the mutations of Tyr652 and/or Phe656 to alanine. The
inhibition of hERG channels by fluvoxamine was only partially attenuated by the
Phe656Ala mutation [
109
]. Mutation of Phe656 to alanine did not affect the hERG
block by dronedarone and amiodarone [
110
]. Tyr652Ala mutation showed a modest
effect on the block mediated by dronedarone, while it had a more marked effect on
the amiodarone binding. All these data suggest that neither Tyr652 nor Phe656
interact with dronedarone, while Tyr652 might be part of the amiodarone-binding
site. Mutation of Tyr652 to alanine did not affect the hERG block by erythromycin,
and mutation of Phe656 to alanine had only a weak effect on the inhibition [
88
].
An interesting study of Shamovsky et al. [
54
] analyzed the contribution of
the desolvation component of hERG potency and the nondesolvation-related
