Control Mechanism for cis Double‐Bond Formation by Polyunsaturated Fatty‐Acid Synthases

Abstract

Polyunsaturated fatty acids (PUFAs) such as docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (ARA) are essential fatty acids for humans. Some microorganisms biosynthesize these PUFAs through PUFA synthases composed of four subunits with multiple catalytic domains. These PUFA synthases each create a specific PUFA without undesirable byproducts, even though the multiple catalytic domains in each large subunit are very similar. However, the detailed biosynthetic pathways and mechanisms for controlling final‐product profiles are still obscure. In this study, the FabA‐type dehydratase domain (DHFabA) in the C‐subunit and the polyketide synthase‐type dehydratase domain (DHPKS) in the B‐subunit of ARA synthase were revealed to be essential for ARA biosynthesis by in vivo gene exchange assays. Furthermore, in vitro analysis with truncated recombinant enzymes and C4‐ to C8‐acyl ACP substrates showed that ARA and EPA synthases utilized two types of DH domains, DHPKS and DHFabA, depending on the carbon‐chain length, to introduce either saturation or cis double bonds to growing acyl chains.

The lipid metabolism in thraustochytrids

“…In thraustochytrids it also controls the final yield of PUFA production [104]. Two types of DH domains are likely required in PUFA synthase to introduce either saturation or cis double bonds on growing acyl chains [105]. The first DH domain in subunit A is similar to the one found in PKS and only catalyses the dehydration reaction.…”

“…These remain important and intriguing questions since the final design of the PUFA will depend on how these reactions are ordered. As recently shown [105], the length of the growing acyl chain is possibly a key factor to determine the type of DH that needs to be involved. To summarize, PUFA synthase, like FAS, catalyzes a series of iterative and sequential reactions that add two carbons per cycle, but the way these two carbons are processed is not yet fully understood.…”

“…Putative scheme illustrating the reactions catalyzed by the PUFA synthase. This scheme relies on previous reports describing the functions of the different domains [103][104][105]. Compared with the reactions catalyzed by the classical FAS I system, the main differences would be at the level of the DH/I and ER steps, allowing or not the formation of a cis-double bond [92,105].…”

“…This scheme relies on previous reports describing the functions of the different domains [103][104][105]. Compared with the reactions catalyzed by the classical FAS I system, the main differences would be at the level of the DH/I and ER steps, allowing or not the formation of a cis-double bond [92,105]. Also, the potential role of the CLF domain to terminate the elongation process [102][103][104], and how the FA is released from the synthesizing complex, remain open questions.…”

Distinct functions of two FabA‐like dehydratase domains of polyunsaturated fatty acid synthase in the biosynthesis of very long‐chain polyunsaturated fatty acids

“…Double bonds in VLCPUFAs are introduced during the synthetic process by two types of DH domains. A PKS‐like DH domain functions as a typical DH to introduce a 2‐trans double bond in an enoyl‐ACP which is subsequently reduced to a saturated acyl‐ACP, while two tandem FabA‐like DH domains introduce 2‐cis and 3‐cis double bonds which are retained in an acyl chain (Hayashi et al ., 2019b). However, the function of the individual FabA‐like DH domain of a PUFA synthase still remains unknown.…”