**12. Transition state analogues of nucleic acids metabolism**

*N*-Ribosyltransferases are a general class of enzymes that catalyze nucleophilic displacement reactions by migration of the cationic ribooxacarbenium carbon from the fixed purine to phosphate and water nucleophiles, respectively. Two major classes of these enzymes are hy‐ drolases and phosphorylases. Hydrolases, which release the heterocycle to generate a free sugar ribosyl unit, include enzymes for DNA repair, RNA depurinations by plant toxins, and purine and pyrimidine nucleoside and nucleotide metabolism. Phosphorylases, which transfer ribosyl groups to phosphate, are also involved in the pathway for nucleoside sal‐ vage. Genetic defects in this pathway prevent normal purine catabolism in humans.

The focus on transition states for a family of *N*-ribosyltransferases roots from physiologic importance of these enzymes. Similarly as in the case of glycosidases, most sugar transferas‐ es form transition states with cationic charge at the anomeric carbon. The geometry is al‐ tered at this center from sp3 (tetrahedral geometry) in the reactant sugar to sp2 (trigonal planargeometry) at the transition state (Fig. 32) [Schramm, 2002; Murkin et al., 2007; Silva et al., 2011].

**Figure 32.** Course of reaction catalyzed by *N*-ribosyltransferases.

but failed to prevent disease in flu-infected animals. In order to produce a neuraminidase inhibitor, which was orally bioavailable and which was taken orally in capsules or as a sus‐ pension, Tamiflu (oseltavimir) was developed in 1997 [Kim et al.]. Third drug, which has been authorized for the emergency use of treatment of certain hospitalized patients with known or suspected 2009 H1N1 influenza, is permavir [Chand, et al., 2005]. Structures of

All three drugs soon became lead structures for the design and preparation of new, presum‐ ably more effective ones. Syntheses and eveluation of phosphinic analogs and significantly simplified analogs of permavir (Fig. 31) have been recently described [Kati et al., 2001; Bian‐

Modified phosphonic analogs of oseltamivir were used to functionalize gold nanoparticles and were found to bind strongly and selectively to all seasonal and pandemic influenza vi‐ rus strains, and thus could serve as prototypes for novel virus sensors. This may be helpul in

*N*-Ribosyltransferases are a general class of enzymes that catalyze nucleophilic displacement reactions by migration of the cationic ribooxacarbenium carbon from the fixed purine to phosphate and water nucleophiles, respectively. Two major classes of these enzymes are hy‐ drolases and phosphorylases. Hydrolases, which release the heterocycle to generate a free sugar ribosyl unit, include enzymes for DNA repair, RNA depurinations by plant toxins, and purine and pyrimidine nucleoside and nucleotide metabolism. Phosphorylases, which transfer ribosyl groups to phosphate, are also involved in the pathway for nucleoside sal‐

vage. Genetic defects in this pathway prevent normal purine catabolism in humans.

**12. Transition state analogues of nucleic acids metabolism**

these drugs are presented in Figure 30.

352 Drug Discovery

co et al., 2005; Shie et al., 2007; Udommaneethanakit et al., 2009].

**Figure 31.** Second generation of influenza neuraminidase inhibitors.

fast influenza diagnosis [Stanley et al., 2012].

Newborns with a genetic deficiency of purine nucleoside phosphorylase are normal, but ex‐ hibit a specific T-cell immunodeficiency during the first years of development. All other cell and organ systems remain functional. Human purine nucleoside phosphorylase degrades deoxyguanosine, and apoptosis of T-cells occurs as a consequence of the accumulation of deoxyguanosine in the circulation. Thus, control of T-cell proliferation is desirable in T-cell cancers, autoimmune diseases, and tissue transplant rejection. The search for powerful in‐ hibitors of these enzymes as anti-T-cell agents has culminated in the discovery of immucil‐ lins. The atomic replacements between inosine and immunocilin H make an insignificant change in atomic size, but a dramatic change in the molecular electrostatic potential surface (Fig. 33). Thus, analysis of the molecular electrostatic potential surface similarity between transition state and immucilin confirmed utility of this simple approach in helping to design effective inhibitor [Schramm, 2002; Schramm, 2007].

Evolution of immucilin structure, performed using standard structural analogy techniques, enabled to obtain new inhibitors of purine nucleotide phosphorylase of nano- to picomolar affinities to the enzyme (Fig. 34) [Evans et al., 2008; Edwards et al., 2009; Ho et al. 2010; Rej‐ man et al., 2012].

*Plasmodium* parasites (causative agents of malaria) are purine auxotrophs and require pre‐ formed purine bases for synthesis of nucleotides, cofactors, and nucleic acids. The purine phosphoribosyltransferases catalyze transfer the 5-phosphoribosyl group from 5-phosphoα-*D*-ribofuranosyl-1-pyrophosphate to salvage hypoxanthine, guanine, or xanthine to form intracellular nucleosides. Purine salvage in *Plasmodium falciparum* uses hypoxanthine formed in erythrocytes or in parasites by the sequential actions of adenosine deaminase and purine nucleoside phosphorylase

purine nucleoside phosphorylase, being dual inhibitor of the process. Especially the efficacy, oral availability, chemical stability, unique mechanism of action and low toxicity of BCX4945 demonstrate potential for combination therapies with this novel antimalarial agent. Similar studies have been also carried out for acyclic nucleoside phosphonates [Keough et al., 2009].

Transition State Analogues of Enzymatic Reaction as Potential Drugs

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

355

This work is dedicated to Professor Henri-Jean Cristau from Ecole Nationale Superieure de

Department of Bioorganic Chemistry, Faculty of Chemistry, Wroclaw University of Technol‐

[1] Abbenante, G.; Fairlie, D. P. (2005) Protease Inhibitors in the Clinic. Medicinal Chem‐

[2] Adachi, M.; Ohhara, T.; Kurihara, K.; Tamada, T.; Honjo, E.; Okazaki, N.; Arai, S.; Shoyama, Y.; Kimura, K.; Matsumura, H.; Sugiyama, S.; Adachi, H.; Takano, K.; Mori, Y.; Hidaka, K.; Kimura, T.; Hayashi, Y.; Kiso, Y. & Kuroki, R. (2009) Structure of HIV-1 Protease in Complex with Potent Inhibitor KNI-272 Determined by High-Resolution X-ray and Neutron Crystallography. *Proc. Natl. Acad. Sci. U.S.A.* Vol. 106

Authors thank Polish Ministry of Science and Higher Education for financial support.

**Figure 35.** Antimalarial agents of dual inhibitory action.

Chimie in Mointpellier on the occasion of his 70th birthday.

**Acknowledgements**

**Author details**

**References**

Karolina Gluza and Pawel Kafarski

istry Vol. 1 (No. 1): 71-104

(No. 12): 4641-4646

ogy, Wybrzeze Wyspianskiego, Wroclaw, Poland

**Figure 33.** Molecular electrostatic potential surfaces for inosine, the transition state of purine nucleoside phosphory‐ lase and immucilin H.

**Figure 34.** Effective inhibitors of purine nucleotide phosphorylase.

Therefore, effective inhibitors of both enzymes influence the life cycle of P. falciparum and these pathways have been targets for antimalarials since the discovery that *Plasmodium* para‐ sites are purine auxotrophs.

Immunocillins HP and GP [Shi et al., 1999] and BCX4945 [Cassera et al., 2011] (Fig. 35) ap‐ pear to be effective inhibitors of phosphoribosyltransferases and are also able to influence purine nucleoside phosphorylase, being dual inhibitor of the process. Especially the efficacy, oral availability, chemical stability, unique mechanism of action and low toxicity of BCX4945 demonstrate potential for combination therapies with this novel antimalarial agent. Similar studies have been also carried out for acyclic nucleoside phosphonates [Keough et al., 2009].

**Figure 35.** Antimalarial agents of dual inhibitory action.
