**6. Suicide substrates yielding transition state analogues**

Peptide aldehydes and boronic acids are inhibitors of serine and threonine enzymes forming both, hydrogen and covalent bonds in the enzyme active site. Tetrahedral adduct generated from these compounds upon their action on enzymes bear a closer relationship to the struc‐ ture of the true intermediate and they may be considered as suicidal substrates. There is, however, an important difference between these two types of inhibitors. The boronic acid derivative possesses a negative charge, whereas the hemiacetal adduct is neutral (Fig 17).

**Figure 17.** Suicidal substrates yielding transition state analogs

get for antihypertensive drugs. Aliskiren (Fig. 15), a promising drug lowering blood pres‐ sure in sodium-depleted marmosets and hypertensive human patients, was developed using a combination of crystal structure analysis of renin–inhibitor complexes and computa‐ tional methods [Wood et al., 2003]. The therapy was introduced under the names Takturna

Another possibility arose from use of fluoroketone derivatives. α-monofluoroketones are approximately 50% hydrated, whereas the α,α-difluoroketones are 100% hydrated in aque‐ ous solutions (Fig. 16). The latter ones are obviously of choice because of their striking simi‐ larity to phosphinic inhibitors (two hydroxyls placed at terahedral atom). This approach is applied very rarely but gave good inhibitors of fungal endothiapepsin [Tuan et al., 2007]

and Rasilez.

340 Drug Discovery

**Figure 15.** Aliskiren (left hand side) and its more potent analog.

and matrix metalloprotease [Reiter et al., 2000] (Fig. 16).

**Figure 16.** Difluoroketones as transition state inhibitor.

**6. Suicide substrates yielding transition state analogues**

Peptide aldehydes and boronic acids are inhibitors of serine and threonine enzymes forming both, hydrogen and covalent bonds in the enzyme active site. Tetrahedral adduct generated from these compounds upon their action on enzymes bear a closer relationship to the struc‐ ture of the true intermediate and they may be considered as suicidal substrates. There is,

Hence, the peptidyl boronic acid adduct is a better transition state analog than the hemiace‐ tal formed from peptidic aldehyde [Polgár, 2005]. Aldehydes typically have a low preva‐ lence in drugs and drug candidates because of their potential chemical reactivity and susceptibility to be engaged in a reduction/oxidation pathways in vivo. Therefore peptidyl boronic acids are considered as far better drug candidates. Additionally slight changes in pH can result in release of the inhibitor from the active site, which is profitous.

In 2003, bortezomib (Fig. 18), a first-in-class therapeutic, gained approval from the US Feder‐ al Drug Administration for the treatment of relapsed multiple myeloma and mantle cell lymphoma. Approval in the UK, for multiple myeloma, followed in 2006. It possesses a unique mode of action. Bortezomib acts as inhibitor of the 26S proteasome, the key regulator of intracellular protein degradation, found in the nucleus and cytosol of all eukaryotic cells, and forming part of the critical ubiquitin-proteasome system. This inhibition results in dis‐ ruption of homeostatic mechanisms within the cell that can lead to cell death.

**Figure 18.** Structure of bortezomib.

This finding initiated intensive researches on boronic inhibitors of serine and threonine pro‐ teases [Trippier & McGuigan, 2010]. For, example recently inhibitors of Lon proteases (bac‐ terial ATP-dependent protease conferring bacterial virulence) afforded interesting antibacterial agents [Frase and Lee, 2007], inhibitors of prostate-specific antigen for prostate cancer imaging and therapy [LeBeau et al., 2008], antifungal inhibitors of kexin (regulatory proteins from *Candida*) [Holyoak et al., 2004; Wheatley & Holyoak, 2007], inhibitor of HCV NS3 protease as potential drug against hepatitis [Zhang et al., 2003; Venkatraman et al., 2009], and anticancer and antibacterial inhibitors of proteasome [Hu et al., 2006]. Represen‐ tative examples of these inhibitors are shown in Figure 19.

**7. Other hydrolases**

The data considering transition state analogue inhibitors of other hydrolases are practically

Transition State Analogues of Enzymatic Reaction as Potential Drugs

http://dx.doi.org/10.5772/52504

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Antibiotic resistance, especially to widely prescribed β-lactam antibiotics, is a serious threat to public health and is responsible for the increase in morbidity, mortality, and health care costs related to the treatment of bacterial infections. In most cases emergence of antibioticresistant bacteria is primarily driven by overuse of β-lactam antibiotics in food and agricul‐ tural products. The most prominent resistance mechanism is related to the expression of βlactamases, which hydrolyze β–lactam fragment of the drug molecule. In nature, four classes of these enzymes exist. Three of them are serine-based, whereas fourth is zinc-de‐ pendent-hydrolase. To counteract β-lactamases, mechanism-based inhibitors were devel‐ oped to be administered in concert with β-lactam antibiotics. Presently, there are three commercially available β-lactamase inhibitors (clavulanate, sulbactam, and tazobactam). The new approach to obtain such inhibitors is combination of structure of potent β-lactam antibiotics with a boronic [Thomson, et al., 2007; Eidam et al., 2010: Ke, et al., 2011; Chan, et al., 2012] or phosphonic [Nukaga, et al., 2004] acid moieties with the goal of mimicking the transition state and creating a high-affinity, reversible inhibitor that cannot be inactivated by

Arginase is a binuclear manganese metalloenzyme that serves as a therapeutic target for the treatment of asthma, erectile dysfunction, and atherosclerosis. The hydrolysis of *L*-arginine

limited to inhibitors of β–lactamases, arginase and urease.

β-lactamases since they do not bear hydrolyzable β-lactam ring.

**Figure 20.** Transition state inhibitors of lactamases.

**Figure 19.** Boronic acid based inhibitors.
