Rational Design of an Efficient Halotolerant Enzymatic System for In Vitro One-Pot Synthesis of Cytidine Diphosphate Choline

Salt accumulation often impedes cytidine diphosphate choline (CDP-choline) in vitro biosynthetic process. In this work a halotolerant in vitro enzymatic system is developed to solve this problem. It applies a halotolerant choline-phosphate cytidylyltransferase (CCT) obtained from rational design instructed by a unique strategy, which refers to one of the features of naturally occurring halophilic enzymes. By increasing acidic residues on protein surface where is most variable with respect to amino acid in the sequence alignment with other CCT, the mutants are obtained. The mutants represent higher catalytic activities and IC50 values (inhibit activity by 50%) at high-salt concentrations. Furthermore, when the halotolerant CCT is applied to in vitro one-pot biosynthesis of CDP-choline, the maximum titer and productivity are 161 ± 3.5 mM and 6.2 ± 0.1 mM L⁻¹ h⁻¹, respectively. When acetate concentration increases, it still keeps relatively high reaction rate and is 2.2-fold higher than process using wild-type CCT (3.87 mM L⁻¹ h⁻¹ comparing with 1.74 mM L⁻¹ h⁻¹). This halotolerant system has great potential for industrial use, and the rational design concept can be applied to modify other enzymes, addressing the salt accumulation problem in in vitro systems, and gives insight into resolving by-product inhibition during reaction.