Heterologous Production of the D-Cycloserine Intermediate O-acetyl-L-serine in Human Lung Cancer Cells

According to the World Health Organization, Tuberculosis (TB) is the second leading cause of death by a single infectious disease behind COVID-19. Despite a century of effort, the current TB vaccine does not effectively prevent pulmonary TB, promote herd immunity, or prevent transmission. Therefore, we seek to develop a genetic prophylaxis for TB. We have determined D-cycloserine to be the optimal target for this approach due to its relatively short six-enzyme biosynthetic pathway. D-CS is a second-line antibiotic for TB that inhibits bacterial cell wall synthesis. The first committed step towards D-CS synthesis is catalyzed by the L-serine-O-acetyltransferase (DcsE) which converts L-serine and acetyl-CoA to O-acetyl-L-serine (L-OAS). To test if the D-CS pathway could be an effective prophylaxis for TB in human cells, we endeavored to express DcsE in human cells and test its functionality. We overexpressed DcsE tagged with FLAG and GFP in A549 lung cancer cells as determined using fluorescence microscopy. We observed that purified DcsE catalyzed the synthesis of L-OAS as observed by HPLC-MS. Therefore, DcsE synthesized in human cells is a functional enzyme capable of converting L-serine and acetyl-CoA to L-OAS demonstrating the first step towards D-CS production in human cells.