**10. Nucleotide deaminases**

Enzymes of the deaminase superfamily catalyze deamination of bases in nucleotides and nucleic acids across in diverse biological contexts. Representatives that act on free nucleoti‐ des or bases are primarily involved in the salvage of pyrimidines and purines, or in their catabolism in bacteria, eukaryotes and phages. Other members of the deaminase superfami‐ ly catalyze the *in situ* deamination of bases in both RNA and DNA. Such modifications play a central role in RNA editing, which is critical for generating the appropriate anti-codon se‐ quences for decoding the genetic code, modification of the sequences of microRNA and oth‐ er transcripts and alteration of the reading frames in mRNAs, defense against viruses via hypermutation-based inactivation, and somatic hypermutation or class switching of antigen receptor genes in vertebrates [Iyer et al., 2011].

organs and sera. One of the approaches for antiviral/anticancer therapy is to design structur‐ al mimics of natural guanine as nucleic acid building blocks, with an anticipation that such analogs would be incorporated into DNA/RNA of virus for cancer cells, interrupting their normal replicative processes. Unfortunately these potent anticancer mimics are believed to be substrates for the enzyme guanine deaminase, which converts them into their respective inactive forms. A potent inhibitor would restore the original potency of these anticancer compounds. Such an activity was determined for azepinomycin [Isshiki et al., 1987] and its

Transition State Analogues of Enzymatic Reaction as Potential Drugs

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

349

analog designed as transition state of this reaction (Fig. 27) [Chakraborty et al., 2011].

Glycoside hydrolases, the enzymes catalyzing hydrolysis of the glycosidic bond in di-, oligoand polysaccharides, and glycoconjugates, are ubiquitous in Nature and fundamental to ex‐ istence. The extreme stability of the glycosidic bond caused that they have evolved into highly proficient catalysts, with an estimated 1017 fold rate enhancement over the uncata‐ lysed reaction. Such rate enhancements mean that enzymes bind the substrate at the transi‐

In the most cases of glycoside hydrolysis, the short-lived transition state possesses substan‐ tial oxocarbenium character (Fig. 27) resembling classical SN1 reaction intermediate. Under

sation predominantly along the bond between the anomeric carbon and endocyclic oxygen and significant relative positive charge accumulation on the pyranose ring [Lee et al., 2004;

The quest for potent and selective inhibitors of glycosidases is extremely active at present. This results from the involvement of glycosidases in lysosomal storage disorders, cancer, vi‐ ral infections, diabetes and many others. Consequently a plethora of glycosidase inhibitors have been already synthesized and evaluated. The number of them is continually growing. It is outside the scope of this chapter to mention all of them in detail. One of the most ap‐ pealing ways to design a transition state analog would be to incorporate both the features of

hybridi‐

these conditions the anomeric carbon possesses trigonal character, which causes sp2

**Figure 27.** Apizenomycin as a template for guanine deaminase inhibitor.

tion state with extraordinary affinity [Gloster & Davies, 2010].

**11. Glycosidases and related enzymes**

Biarnés et al., 2011; Davies at al., 2012].

Adenosine deaminase (ADA) is an enzyme present in all organisms and catalyzes the irre‐ versible deamination of adenosine and deoxyadenosine to inosine and deoxyinosine (Fig. 26). Both adenosine and deoxyadenosine are biologically active purines that can have a deep impact on cellular physiology. For example it plays a vital role in regulating T-cell coactiva‐ tion. Deficiency of this enzyme in humans causes severe combined immunodeficiency. In‐ creased serum activity of this enzyme have been found in many infectious diseases caused by microorganisms infecting the macrophages, in leprosy, brucellosis, HIV infections, viral hepatitis, infectious mononucleosis, liver cirrhosis and tuberculosis. Its extended transition state inhibitor – conformycin was isolated from *Nocardia interforma* and *Streptomyces kanihar‐ aensis*. Analogs of conformycin (Fig. 26) are proposed as an antineoplastic synergists and im‐ munosuppressants [Wolfenden, 2003].

The wide potential of these inhibitors may be illustrated by the fact that deaminoformycin was recently applied to evaluate mechanisms responsible for lethality caused by genetic and herbicide-based activity of adenosine deaminase [Sabina et al., 2007], as well as identifica‐ tion of highly selective inhibitor of purine salvage pathway in malaria parasites [Tyler et al., 2007]. This is because of a unique feature of *Plasmodium falciparum* enzyme that catalyzes the deamination of both adenosine and 5'-methylthioadenosine.

**Figure 26.** Inhibitors of adenosine deaminase.

Guanine deaminase is an enzyme that hydrolyzes guanine to form xanthine that is unsuita‐ ble for DNA/RNA buildup. This enzyme has been found in normal or transformed human organs and sera. One of the approaches for antiviral/anticancer therapy is to design structur‐ al mimics of natural guanine as nucleic acid building blocks, with an anticipation that such analogs would be incorporated into DNA/RNA of virus for cancer cells, interrupting their normal replicative processes. Unfortunately these potent anticancer mimics are believed to be substrates for the enzyme guanine deaminase, which converts them into their respective inactive forms. A potent inhibitor would restore the original potency of these anticancer compounds. Such an activity was determined for azepinomycin [Isshiki et al., 1987] and its analog designed as transition state of this reaction (Fig. 27) [Chakraborty et al., 2011].

**Figure 27.** Apizenomycin as a template for guanine deaminase inhibitor.
