Inhibited by coumarins, saccharin, sulfonamide derivatives such as acetazolamide (AZA), benzenesulfonamide and derivatives (4-carboxyethylbenzene-sulfonamide, 4-carboxyethylbenzene-sulfonamide ethyl ester, 4-(acetyl-2-aminoethyl)benzene-sulfonamide, 4-aminoethylbenzene-sulfonamide) and Foscarnet (phosphonoformate trisodium salt).
A sulfonamide derivative of the antihelmintic drug thiabendazole was prepared and investigated for inhibition of the zinc enzyme carbonic anhydrase CA (EC 4.2.1.1). Mammalian isoforms CA I-XIV and the nematode enzyme of Caenorhabditis elegans CAH-4b were included in this study. Thiabendazole-5-sulfonamide was a very effective inhibitor of CAH-4b and CA IX (K(I)s of 6.4-9.5nm) and also inhibited effectively isozymes CA I, II, IV-VII, and XII, with K(I)s in the range of 17.8-73.2nM. The high resolution X-ray crystal structure of its adduct with isozyme II evidenced the structural elements responsible for this potent inhibitory activity.
The cytosolic isoform XIII is a recently discovered member of the human carbonic anhydrase (hCA, EC 4.2.1.1) family. It is selectively expressed among other tissues in the reproductive organs, where it may control pH and ion balance regulation, ensuring thus proper fertilization conditions. The authors report here the X-ray crystallographic structure of this isozyme in the unbound state and in complex with a classical sulfonamide inhibitor, namely acetazolamide. A detailed comparison of the obtained structural data with those already reported for other CA isozymes provides novel insights into the catalytic properties of the members of this protein family. On the basis of the inhibitory properties of acetazolamide against various cytosolic/transmembrane isoforms and the structural differences detected within the active site of the various CA isoforms, further prospects for the design of isozyme-specific CA inhibitors are here proposed.
Foscarnet (phosphonoformate trisodium salt), an antiviral used for the treatment of HIV and herpes virus infections, also acts as an activator or inhibitor of the metalloenzyme carbonic anhydrase (CA, EC 4.2.1.1). Interaction of the drug with 11 CA isozymes has been investigated kinetically, and the X-ray structure of its adduct with isoform I (hCA I-foscarnet complex) has been resolved. The first CA inhibitor possessing a phosphonate zinc-binding group is thus evidenced, together with the factors governing recognition of such small molecules by a metalloenzyme active site. Foscarnet is also a clear-cut example of modulator of an enzyme activity which can act either as an activator or inhibitor of a CA isozyme.
The X-ray crystal structure of the adduct between the zinc metalloenzyme carbonic anhydrase II (CA, EC 4.2.1.1) with the recently discovered natural product coumarin derivative 6-(1S-hydroxy-3-methylbutyl)-7-methoxy-2H-chromen-2-one showed the coumarin hydrolysis product, a cis-2-hydroxy-cinnamic acid derivative, and not the parent coumarin, bound within the enzyme active site. The bound inhibitor exhibits an extended, two-arm conformation that effectively plugs the entrance to the enzyme active site with no interactions with the catalytically crucial zinc ion. The inhibitor is sandwiched between Phe131, with which it makes an edge-to-face stacking, and Asn67/Glu238sym, with which it makes several polar and hydrogen bonding interactions. This unusual binding mode, with no interactions between the inhibitor molecule and the active site metal ion is previously unobserved for this enzyme class and presents a new opportunity for future drug design campaigns to target a mode of inhibition that differs substantially from classical inhibitors such as the clinically used sulfonamides and sulfamates. Several structurally simple coumarin scaffolds were also shown to inhibit all 13 catalytically active mammalian CA isoforms, with inhibition constants ranging from nanomolar to millimolar. The inhibition is time dependent, with maximum inhibition being observed after 6 h.
Despite the similarity in the active site pockets of carbonic anhydrase (CA) isozymes I and II, the binding affinities of benzenesulfonamide inhibitors are invariably higher with CA II as compared to CA I. To explore the structural basis of this molecular recognition phenomenon, we have designed and synthesized simple benzenesulfonamide inhibitors substituted at the para position with positively charged, negatively charged, and neutral functional groups, and we have determined the affinities and X-ray crystal structures of their enzyme complexes. The para-substituents are designed to bind in the midsection of the 15 A deep active site cleft, where interactions with enzyme residues and solvent molecules are possible. We find that a para-substituted positively charged amino group is more poorly tolerated in the active site of CA I compared with CA II. In contrast, a para-substituted negatively charged carboxylate substituent is tolerated equally well in the active sites of both CA isozymes. Notably, enzyme-inhibitor affinity increases upon neutralization of inhibitor charged groups by amidation or esterification. These results inform the design of short molecular linkers connecting the benzenesulfonamide group and a para-substituted tail group in "two-prong" CA inhibitors: an optimal linker segment will be electronically neutral, yet capable of engaging in at least some hydrogen bond interactions with protein residues and/or solvent. Microcalorimetric data reveal that inhibitor binding to CA I is enthalpically less favorable and entropically more favorable than inhibitor binding to CA II. This contrasting behavior may arise in part from differences in active site desolvation and the conformational entropy of inhibitor binding to each isozyme active site.
Enzyme that catalyzes the cleavage of C-C, C-O, C-S, C-N or other bonds by other means than by hydrolysis or oxidation, with two substrates in one reaction direction, and one in the other. In the latter direction, a molecule (of carbon dioxide, water, etc) is eliminated, thus creating a new double bond or a new ring.
A reference proteome is a set of protein sequences derived from a complete proteome which constitutes a defined standard for a particular user community. Reference proteomes are manually defined according to a number of criteria. They cover the proteomes of well- studied model organisms and other proteomes of interest for biomedical and biotechnological research. Reference proteomes have been selected to provide broad coverage of the tree of life, and constitute a representative cross-section of the taxonomic diversity to be found within UniProtKB.
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