An Inverse Substrate Orientation for the Regioselective Acylation of 3′,5′‐Diaminonucleosides Catalyzed by Candida antarctica lipase B?

Abstract

Abstract. Candida antarctica lipase B (CAL-B

Regioselective Acylation of Nucleosides Catalyzed by Candida Antarctica Lipase B: Enzyme Substrate Recognition

“…It was proposed that there exists a hydrophobic interaction between the propyl group of acyl chain and the hydrophobic side (C 5 -C 6 -R 1 ) of the thymine in the conformation of the 5Ј-butanoylation of β-thymidine. [12] According to the model, the 5Ј-regioselectivity should increase with an enhancement of the hydrophobicity of the R 1 group. However, we found that the 5Ј-selectivity did not increase, but decreased in waved form with enhancing hydrophobicity of the R 1 group (Table 1, Entries 1-5).…”

“…The molecular modeling has revealed that the C2Ј atom lies above the medium-sized pocket in the conformation of the 5Ј-acylation transition state, whereas it fits into the medium-sized pocket in the conformation of the 3Ј-acylation. [12] X-ray crystallographic studies have shown that the medium-sized pocket, into which the sugar moiety binds, has not enough room for a substituent larger than the propyl group. [9,12] As a result, the presence of the R 2 group as a hydroxy group might cause a steric clash and destabilize the conformation of the 3Ј-acylation transition state, thus enhancing the 5Ј-regioselectivity and decreasing the reaction rate.…”

“…[11] Recently, Lavandera et al proposed a molecular basis of this regioselectivity through molecular modeling, and they attributed the preferential acylation of the 5Ј-hydroxy group over the 3Ј-hydroxy one to favorable interactions, due to the appropriate orientation of the 2Ј-deoxynucleoside in the 5Ј-acylation pathway. [12] Nevertheless, they just focused their interest on the enzymatic acylation of 2Ј-deoxynucleosides, such as β-thymidine (1c). In the present work, the substrate recognition of CAL-B in the acylation of nucleosides, including ribonucleosides and 2Ј-deoxynucleosides, was revealed by rational substrate engineering (Scheme 1).…”

Redesigning the mechanism of the lipase-catalysed aminolysis of esters

“…Taking into account the scarcity of computer modelling studies about the aminolysis reaction, 15 we used this tool to rationalise the great differences of reactivity and enantioselectivity shown by CAL-B towards these cycloalkanamines. In recent years, molecular modelling has been successfully used to explain both the mechanism of action and the substrate selectivity of lipases.…”

“…For this reason we modelled a phosphonamidate 15 ( Figure 2A) as an analog of the tetrahedral intermediate TI-2 resulting from the attack of the amine to the carbonyl of the acyl-enzyme. This intermediate is the equivalent to the phosphonate analogs successfully employed in the molecular modelling studies on the transesterification of alcohols.…”