Thiazide and high ceiling diuretics were recently shown to inhibit all mammalian isoforms
of carbonic anhydrase (CA, EC 4.2.1.1) with a very different profile as compared to
classical inhibitors, such as acetazolamide, methazolamide, and ethoxzolamide. Some
of these structurally related compounds have a very different behavior against the
widespread isozyme CA II, with chlorthalidone, trichloromethiazide, and furosemide
being efficient inhibitors against CA II (K(I)s of 65-138 nM), whereas indapamide
is a much weaker one (K(I) of 2520 nM). Furthermore, some of these diuretics are quite
efficient (low nanomolar) inhibitors of other isoforms, for example, chlorthalidone
against hCA VB, VII, IX, and XIII; indapamide against CA VII, IX, XII, and XIII, trichloromethiazide
against CA VII and IX, and furosemide against CA I and XIV. Examining the four X-ray
crystal structures of their CA II adducts, we observed several (2-3) active site water
molecules interacting with the chlorthalidone, trichloromethiazide, and furosemide
scaffolds which may be responsible for this important difference of activity. Indeed,
indapamide bound to CA II has no interactions with active site water molecules. Chlorthalidone
bound within the CA II active site is in an enolic (lactimic) tautomeric form, with
the enolic OH also participating in two strong hydrogen bonds with Asn67 and a water
molecule. The newly evidenced binding modes of these diuretics may be exploited for
designing better CA II inhibitors as well as compounds with selectivity/affinity for
various isoforms with medicinal chemistry applications.