Bacterial Genome Mining of Enzymatic Tools for Alkyne Biosynthesis

The alkyne is an important functionality widely used in material science, pharmaceutical science, and chemical biology, but the importance of this functionality is contrasted by the very limited number of enzymes known to be involved in alkyne biosynthesis. We recently reported the first known carrier protein-dependent pathway for terminal alkyne formation, and in silico analysis suggested that this mechanism could be widespread in bacteria. In this paper, we screened additional homologous gene cassettes presumed to be involved in alkyne biosynthesis using both in vitro biochemical study and an E. coli-polyketide synthase (PKS) reporting system for in vivo analysis. We discovered and characterized a new terminal alkyne biosynthetic pathway comprised of TtuA, -B, and -C from Teredinibacter turnerae T7901. While the acyl-CoA ligase homologue (TtuA) demonstrated promiscuity in the activation and loading of medium-chain fatty acids onto the carrier protein (TtuC), the desaturase homologue (TtuB) showed stringent substrate specificity toward C10 fatty acyl moieties. In addition, TtuB was demonstrated to be a bifunctional desaturase/acetylenase that efficiently catalyzed two sequential O2-dependent dehydrogenation reactions. A novel terminal-alkyne bearing polyketide was further produced upon coexpression of ttuABC and a PKS gene in E. coli. The discovery and characterization of TtuA, -B, and -C provides us with a new bifunctional desaturase/acetylenase for mechanistic and structural study and expands the scarce enzyme inventory for the biosynthesis of the alkyne functionality, which has important applications in synthetic and chemical biology.