(9c) Rational Improvement of Simvastatin Synthase Solubility In Escherichia Coli Leads to Higher Whole-Cell Biocatalytic Activity

Simvastatin is the active pharmaceutical ingredient of the blockbuster cholesterol lowering drug Zocor. We have previously developed an Escherichia coli based whole-cell biocatalytic platform towards the synthesis of simvastatin acid starting from the precursor monacolin J sodium salt (MJSS). The centerpiece of the biocatalytic approach is the simvastatin synthase LovD, which is highly prone to misfolding and aggregation when overexpressed from E. coli. Increasing the solubility of LovD without decreasing its catalytic activity can therefore elevate the performance of the whole-cell biocatalyst. Using a combination of homology structural prediction and site-directed mutagenesis, we identified two cysteine residues in LovD that are responsible for nonspecific intermolecular crosslinking, which leads to oligomer formation and protein aggregation. Replacement of Cys40 and Cys60 with alanine residues resulted in marked gain in both protein solubility and whole-cell biocatalytic activities. Further mutagenesis of these two residues with natural amino acids with similar size and polarity to cysteines showed that C40A and C60N are the most beneficial mutations at each position, affording 27% and 26% increase in whole cell activities, respectively. The double mutant C40A/C60N combines the individual improvements and displayed ~50% increases in protein solubility and whole-cell activity. Optimized fed-batch high-cell-density fermentation using YT2/C40A/C60N quantitatively (>99%) converted 45 mM MJSS to SS within 18 hours, which represents a significant improvement over the performance of wild type LovD under identical conditions. The high efficiency of the improved whole-cell platform renders the biocatalytic synthesis of simvastatin an attractive substitute over the existing semisynthetic routes.