Abstract: Long-chain polyunsaturated fatty acids (LC-PUFAs) are essential ingredients of the human diet. They are synthesized by LC-PUFA synthases (PFASs) expressed in marine bacteria and other organisms. PFASs are large enzyme complexes that are homologous to mammalian fatty acid synthases and microbial polyketide synthases. One subunit of each PFAS harbors consecutive ketosynthase (KSc) and chain length factor (CLF) domains that collectively catalyze the elongation of a nascent fatty acyl chain via iterative carbon?carbon bond formation. We report the X-ray crystal structure of the KS?CLF didomain from a well-studied PFAS in Moritella marina. Our structure, in combination with biochemical analysis, provides a foundation for understanding the mechanism of substrate recognition and chain length control by the KS?CLF didomain as well as its interaction with a cognate acyl carrier protein partner.

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