## 2 Zinc Binding Groups in Chelation Therapy
### Inhibition of Carbonic Anhydrases  
Carbonic anhydrases (CAs) are zinc metalloenzymes that catalyze the physiological reaction of carbon dioxide hydration to bicarbonate and protons. Five distinct enzyme families of CAs are known: \(\alpha\), \(\beta\), \(\gamma\), \(\delta\), and \(\zeta\). Inhibitors of human \(\alpha\)-CAs (CAi) can be used for several clinical applications, such as antiglaucoma, anticonvulsant, diuretics, and recently as anticancer agents [58]. CAi can be divided into two main classes: those that bind the catalytic zinc ion in the enzyme active site, and those that are bound to the active site but do not interact directly with the metal ion. Among CAi containing a ZBG, four different groups have been studied with crystallography: the ureates/hydroxamates, the mercaptophenols, the metal-complexing anions, and the sulfonamides with their biososteres, such as sulfamates and sulfamides [59]. Primary sulfonamides are the most important class of human CA I and II inhibitors. The pharmacophoric sulfonamide group coordinates Zn\({}^{2+}\) in the enzyme's active site, while the substituents in the benzene ring linked to the sulfonamide group are responsible for the further interactions. [PERSON] et al. designed diaryl sulfonamides containing a 1,3-oxazole moiety (Figure 17, compounds **17a-d**) which can interact with both the lipophilic and hydrophilic sides of the enzyme's  
Figure 16: Catechol. Figure 15: Compounds **15a-e**[55]. active site. Some of these compounds showed inhibitory activity in the picomolar range towards CA I and CA II [60]. It was more recently demonstrated that the introduction of hydroxy and methoxy moieties at the sulfonamide nitrogen can lead to the development of more selective CAIs. Moreover, the effect of the substitution in benzene-sulfonamide (Figure 18, compounds **18a-c**) of sulfonamide moiety with a hydroxamate group (Figure 18, compound **18d**) was also explored, reporting the inhibitory activity of all catalytically active hCA isoforms and two MMPs (MMP-2 and MMP-8). In particular, cytosolic isoforms hCA I and hCA II are weakly inhibited by the hydroxamate 19-d and highly or moderately inhibited by the sulfonamides **18a-c**; meanwhile, **18d** shows a potent inhibition (inhibition constant in the range of 0.94-9.51 uM) of transmembrane isoforms hCA XII and hCA XIV, important potential drug targets for glaucoma and cancer. Lastly, X-ray crystallography of the hydroxamate N-hydroxy-benzamide (**18d**) showed that it binds to the hCA II active site with the CO and OH groups which both coordinate to the zinc ion, forming an energetically favored five-membered chelate complex [61]. Figure 17: General structures of 5-(sulfamoyl! )-aryl-1,3-oxazoles designed by [PERSON] et al. [60] R\({}_{1}\) = alk, R\({}_{2}\) = alk. Figure 18: Compounds **18a-d**[61]. Some groups can be useful as ZBGs of both MMPs and CAs. [PERSON] et al. developed a series of heterocyclic ZBGs as an alternative to the most commonly used hydroxamic acid for the inhibition of MMPs [62]. In particular, four groups were studied as zinc-chelating portions: hydroxypyridinones, pyrones, hydroxypyridinethiones, and thiopyrones. These compounds were evaluated against MMP-1, -2, and -3 in different conditions, and the results suggest that they could be potent, nontoxic, and biocompatible alternatives to the hydroxamic acid chelator available for use in MMPIs. The cyclic structures in MMPIs reveal enhanced bioavailability and bioactivity [62]. Four of these compounds (reported in Figure 19) were also studied as inhibitors of CAs. The crystal structure of CA II in complex with 1,2-HOPTO (Figure 19, compound **19d**) was determined and revealed an interesting bidentate chelating mode with Zn\({}^{2+}\), similar to those reported for compound **18d** (Figure 18, involving His94, His96 and His119). Moreover, 1,2-HOPTO interacts with the Thr199 and Thr200 residues of the active site and it is involved in a hydrogen-bonding network that is translated via water molecules within the entire active site of CA II. [63].