Subtle Control of Carbon Chain Length in Polyunsaturated Fatty Acid Synthases

Some marine bacteria synthesize docosahexaenoic acid (DHA; C-22) and eicosapentaenoic acid (EPA; C-20) by enzyme complexes composed of four subunits (A-D). We recently revealed that beta-ketoacyl synthase (KSC)/chain length factor (CLF)-like domains in the "C" subunit of DHA synthase catalyzed the last elongation step (C-20 to C-22) even though their amino acid sequences are very similar to those of EPA synthase. To investigate the amino acid residues controlling the product chain length, conserved residues in the KSC/CLF-like domains in DHA synthase were replaced with corresponding EPA synthase residues. Among 12 mutants, two CLF-like domain-mutated genes completely lost DHA productivity and produced trace amounts of EPA when coexpressed with dha-ABD in Escherichia coli, whereas when coexpressed with epa-ABD, they produced the same amounts of EPA as epa-ABCD. These results suggest that the product profiles were subtly controlled by several amino acid residues.