The Enantioselectivity of Enzymes Involved in Current Antiviral Therapy Using Nucleoside Analogues: A New Strategy?

Abstract

This review is primarily intended for synthetic bio-organic chemists and enzymologists who are interested in new strategies in the design of virus inhibitors. It is an attempt to assess the importance of the enzymatic properties of L-nucleosides and their analogues, particularly those that are active against viruses such as human immunodeficiency virus (HIV), hepatitis B virus (HBV), herpes simplex virus (HSV), etc. Only data obtained with purified enzymes have been considered and discussed. The examined enzymes include nucleoside- or nucleotide-phosphorylating enzymes, catabolic enzymes, viral target enzymes and cellular polymerases. The enantioselectivities of these enzymes were determined from existing data and are significant only when a sufficient number of enantiomeric pairs of substrates could be examined. The reported data emphasize the weak enantioselectivities of cellular or viral nucleoside kinases and some viral DNA polymerases. Thus, cellular deoxycytidine kinase has a considerably relaxed enantioselectivity with respect to a large number of nucleosides or their analogues, and it occupies a strategic position in the intracellular activation of the compounds. Similarly, HIV-1 reverse transcriptase often has a relatively weak enantioselectivity and can be inhibited by the 5-triphosphates of a large series of L-nucleosides and analogues. In contrast, degradation enzymes, such as adenosine or cytidine deaminases, generally demonstrate strict enantioselectivities favouring D-enantiomers and are used by chemists in asymmetric syntheses. The weak enantioselectivities of some enzymes involved in nucleoside metabolism are more or less pronounced, and one enantiomer or the other is favoured depending on the substrate. This suggests that the low enantioselectivity is fortuitous and does not result from evolutionary pressure, since these enzymes do not create or modify asymmetric centres in substrates. The combined enantioselectivities of the enzymes examined in this review strongly suggest that the field of L-nucleosides and their analogues should be systematically explored in the search for new virus inhibitors.

Structure and function of cellular deoxyribonucleoside kinases

“…both L-dT and L-dC are efficiently phosphorylated. Several other L-nucleosides such as L-BVDU, L-FMAU (2¢-fluoro-5-methyl-b-L-arabinofuranosyluracil) and L-5-iodo-dU were also substrates for TK2 [78,82,88,92]. The role of TK2 in mtDNA synthesis was recently clarified since it was shown that two substitutions (Ile181 to Asn and His90 to Asn) in the TK2 gene lead to partial enzyme deficiency and to mtDNA depletion myopath [14].…”

The Role of Thymidine Kinases in the Activation of Pyrimidine Nucleoside Analogues

“…The structures of compounds described in Table (2). activate FIAU may be a contributing factor for the severe mitochondrial toxicity observed following treatment with this analogue [47]. Unexpectedly, the L-enantiomer of FMAU was found to be a relatively good substrate both for TK1 and TK2 (Table 1) [48], thus being the only reported Lnucleoside with TK1 substrate characteristics [49]. In contrast to TK1, both TK2 and HSV1-TK show relaxed enantioselectivity since they efficiently phosphorylate L-Thd as well as other L-nucleosides, such as L-BVDU, L-FMAU, and L-5-iodo-dUrd (26, 27, The elevated expression levels of TK1 in rapidly proliferating tumors, in contrast to quiescent cells, encouraged the synthesis of boronated nucleoside for boron neutron capture therapy (BNCT) [52].…”

Enantio-Selectivity of Human Nucleoside Monophosphate Kinases

“…[3] In vivo, activation of L-nucleosides necessitates phosphorylation to the triphosphate. [4] While the first phosphorylation step has been extensively studied, less attention has been drawn to the second step. [4] This step, i.e., the phosphorylation of nucleosides monophosphate and their analogues into their diphosphate derivatives, involves nucleoside monophosphate kinases in both the de novo and salvage pathways.…”

“…[4] While the first phosphorylation step has been extensively studied, less attention has been drawn to the second step. [4] This step, i.e., the phosphorylation of nucleosides monophosphate and their analogues into their diphosphate derivatives, involves nucleoside monophosphate kinases in both the de novo and salvage pathways. [4,5] Human cells contain a thymidylate kinase (TMPK) and a uridine monophosphatecytidine monophosphate kinase (UMP-CMPK) that are only 21% identical in sequence but that adopt the same fold characterised by a P loop, a NMP binding domain and a LID domain.…”

“…[4] This step, i.e., the phosphorylation of nucleosides monophosphate and their analogues into their diphosphate derivatives, involves nucleoside monophosphate kinases in both the de novo and salvage pathways. [4,5] Human cells contain a thymidylate kinase (TMPK) and a uridine monophosphatecytidine monophosphate kinase (UMP-CMPK) that are only 21% identical in sequence but that adopt the same fold characterised by a P loop, a NMP binding domain and a LID domain. [6] The recent interest in L-nucleosides of therapeutic potential has raised the need to assess the enantio-selectivity of NMP kinases.…”