Comparative Study on Acyl Transferases in Fatty Acid and Polyketide Synthases

Fatty acid and polyketide synthases (FASs and PKSs) synthesize physiologically and pharmaceutically important products by condensation of acyl building blocks. In both multidomain enzymes, the acyl transferase (AT) is the key player responsible for the selection of these acyl units for further processing. In this study, the AT domains of different PKS systems are kinetically described in their substrate selectivity, AT-Acyl carrier protein (ACP) domain-domain interaction, and enzymatic kinetic properties. The ATs of modular PKSs, the proteins assembling the most intricate polyketides, turned out to be significantly slower than ATs from iterative FAS and PKS systems, but also more substrate specific. We explain these substantially different properties by the phylogenetically early splitting of species. For the AT of the 6-deoxyerythronolide B synthase (DEBS), the interaction with ACP is analyzed in detail by site-directed mutagenesis of interface residues. Among others, a surface exposed arginine (R850) was replaced by three different residues, leading to mutants with severely different kinetics that cannot be explained by simple effects. Our study enlarges the understanding of ATs in its molecular properties, and is similarly a call for thorough AT-centered PKS engineering strategies.