Phosphotriester structure

Recently, several nucleophilic reagents have been used to establish the mode of action of the metabolites of polycyclic aromatic hydrocarbons (PAH). Among them, several phosphodiesters have been examined to clarify the possibility of reaction of PAH epoxides with the phosphate groups(P-alkylation) of nucleic acids (22). In this context we have studied the reaction of 3,4-epoxyprecocene II with dibenzyl phosphate under a variety of conditions. In all cases, instead of the formation of phenol or phosphotriesters observed with PAH epoxides, we obtained predominantly dimer XI. This compound was also the main component of the mixtures obtained by reaction of the above precocene epoxide with other acid catalysts, along with dimers XII and XII. Dimer XII was formed almost exclusively by thermal treatment. The structure and configuration for compound XII has been established by spectral and X-ray diffraction analyses (23). 

Increasing the substrate range of an enzyme could be thought of as improving a promiscuous activity. However, here we will use a more strict definition in which a promiscuous activity involves catalysis of a reaction with a different class of substrate. Many enzymes are promiscuous in the sense that they can catalyze other reactions. This is not surprising considering that the enormous variety of enzymes that exist utilize only a small number of active site chemistries and structural scaffolds. Thus, an almost identical enzyme could catalyze lactone hydrolysis or phosphotriester hydrolysis. Because these activities are often very weak to begin with, directed molecular evolution experiments to improve these activities often result in remarkable improvements. 

The hope that promiscuity is predictable is also supported by the identification of systematic patterns of promiscuity. For example, lactonases, and in particular lactonases that favor hydrophobic lactones, show a consistent tendency to promiscuously catalyze the hydrolysis of phosphotriesters. This pattern has now been seen in lactonases from three different superfamilies PLLs (TIM-barrels from the amidohydrolase superfamily Table 1, entry 11) PONs, or serum paraoxonases (calcium-dependent six-bladded /3-propellers Table 1, entry 13) and AHA (a lactonase from the metallo-/3-lactamase superfamily Table 1, entry 12). That very different scaffolds and active-sites configurations share the same promiscuity pattern suggests that these reactions share a key feature, probably in the geometry of their transition states. This feature must be distinct, also because many of these lactonases do not hydrolyze esters that are much closer to lactones than phosphotriesters, and should thus be amenable to structural analysis and prediction. 

The proposed mechanism for phosphotriester cleavage by PONl and DFPase is shown in Fig. 3.16. According to Bigley and Raushel, these enzymes present two calcium ions located in the core of the structure, however, only one participate in the active site. This is the only mechanism of phosphotriesterases where a single active site metal ion is employed in the catalysis. The pH profiles for both enzymes indi-... 

Figure 30 (a) Structure of phosphotriester GalNAc glycodendrimer. (b) Enzyme-linked... 

Lawrence, D. P, Wenqiao, Ch, Zon, G., Stec, W. J., Uznanski, B., and Broido, M. S. (1987) NMR studies of backbone-alkylated DNA duplex stability, absolute stereochemistry, and chemical shift anomalies of prototypal isopropyl phosphotriester modified octanucleotides, [RpRp]- and [SpSp]- d[GGA (iPr)ATTCC] 2 and -d-[GGAA (iPr)TTCC] 2 7. Biomol, Structure and Dynamics 4, 757-783. 

Asseline, U., Barbier, C, and Thuong, N, T, (1986) Oligothymidylates with alternating alkyl phosphotriester and phosphodiester structure covalently bonded to an intercalating agent. Phosphorus Sulfur 26 63-73. 

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