Incorporation, fate, and turnover of free fatty acids in cyanobacteria

Abstract

Fatty acids are important molecules in bioenergetics and also in industry. The phylum cyanobacteria consists of a group of prokaryotes that typically carry out oxygenic photosynthesis with water as an electron donor and use carbon dioxide as a carbon source to generate a range of biomolecules, including fatty acids. They are also able to import exogenous free fatty acids and direct them to biosynthetic pathways. Here, we review current knowledge on mechanisms and regulation of free fatty acid transport into cyanobacterial cells, their subsequent activation and use in the synthesis of fatty acid-containing biomolecules such as glycolipids and alka(e)nes, as well as recycling of free fatty acids derived from such molecules. This review also covers efforts in the engineering of such cyanobacterial fatty acid-associated pathways en route to optimized biofuel production.

Keywords: acyl–acyl carrier protein; acyl–acyl carrier protein synthetase; biofuel; cyanobacteria; exogenous free fatty acid; lipid.

Figure 1.

General overview of the various cellular processes in cyanobacteria that utilize fatty acids (FAs). FAs are key elements of cyanobacterial membranes (PG—phosphatidylglycerol, MGDG—monogalactosyldiacylglycerol, SQDG—sulfoquinovosyldiacylglycerol, and DGDG—digalactosyldiacylglycerol), are used as energy source, are important for post-translational modifications of specific proteins, are involved in trafficking, signaling, and targeting, can be stored in the form of lipid droplets, and are components of several types of secondary metabolites.

Figure 2.

Schematic representation of the fate and turnover pathways of exogenous free fatty acids (eFFAs) upon import by cyanobacteria. After entering the cyanobacterial cell, eFFAs are initially incorporated (A) by the acyl–(acyl-carrier-protein) synthetase (Aas), which catalyzes their activation to fatty acyl-AMP, and then to fatty acyl-ACP. At this point, activated FFAs can be directed to lipid (B) or alka(e)ne (C, D) biosynthesis pathways, or be further elongated through the fatty acids biosynthesis II (FASII) pathway. In addition, FFAs can be released either by lipases directly from lipids (E) or by the sequential activity of aldehyde-deformylating oxygenase (Ado) and aldehyde dehydrogenase (ALDH) (F). FFAs can later be reactivated through the Aas (G) or be released to the extracellular medium. In the latter case, cell lysis is a major contributor to the release of FFAs (as well as other fatty acid-containing molecules, such as lipids and alka(e)nes) into the environment (I), but FFAs can also be actively secreted through dedicated transporters (H) or extracellular vesicles (J). eFA, exogenous fatty acid; FFA, free fatty acid; Aas, acyl–acyl carrier protein synthetase; FASII, fatty acid synthesis II; Aar, acyl–acyl carrier protein reductase; Ado, aldehyde deformylating oxygenase; Ols, olefin synthase; Lip, Lipase ALDH, aldehyde dehydrogenase; Rnd, resistance nodulation division; ACP, acyl–acyl carrier protein; AMP, adenosine monophosphate; SQDG, sulfoquinovocyldiacylglycerol; DGDG, digalactosyl diacylglycerol MGDG, monogalactosyl diacylglycerol; PG, phosphatidyl glycerol. The localization of most of the proteins and of their products remains unclear. Cyanobacteria appear to possess either the Aar/Ado pathway or the Ols one (not both).

Figure 3.

Pathways of exogenous free fatty acids (eFFAs) incorporation prior to biosynthesis of membrane lipids in prokaryotes. Prokaryotic activation of eFFAs has been reported to be realized by either FACS, FA kinase or Aas. Upon activation, FFAs can be further degraded (β-Ox), incorporated into membrane lipids, or enter the FASII pathway to get elongated. eFA, exogenous fatty acid; FA, fatty acid; ACP, acyl–acyl carrier protein; β-Ox, β-oxidation; FACS, fatty-acyl CoA synthetase; Aas, acyl–acyl carrier protein synthetase; FASII, fatty acid synthesis II.

Figure 4.

Exogenous free fatty acids (eFFAs) incorporation mechanisms are described in cyanobacteria. (A) eFFAs incorporation by the Aas: Aas catalyses the reaction between a FA and the terminal thiol of an ACP, leading to the loading of a FA onto an ACP. (B) eFAs incopration by BrtB. This carboxylate alkylating enzyme catalyzes O–C bond formation between a FA and a secondary alkyl halide in a bartoloside molecule. FA, fatty acid; Aas, acyl–acyl carrier protein synthetase; AMP, adenosine monophosphate; ACP, acyl–acyl carrier protein; Xyl, xylose.

Figure 5.

Cyanobacterial biosynthetic pathways are involved in the production of fatty acid-containing biomolecules. (A) Overview of lipid biosynthesis through the PLS pathway. (B) Alka(e)ne biosynthesis through the Aar/Ado pathway. Aar catalyzes first the reduction of fatty acyl-ACP to the corresponding fatty aldehydes. Ado oxidizes the C_n fatty aldehyde, breaking the C₁–C₂ bond to give a C_n−1 carbon alka(e)ne and C₁-derived formate. (C) Alkene biosynthesis through the Ols pathway. A type I PKS allows the conversion of fatty acyl-ACPs into alkenes through an elongation-decarboxylation mechanism. FA, fatty acid; ACP, acyl–acyl carrier protein; G3P, glycerol-3-phosphate; LPA, lysophosphatidic acid; PA, phosphatidic acid; PAP, phosphatidic acid phosphatase; DAG, diacylglycerol; PG, phosphatidyl glycerol; MGDG, monogalactosyl diacylglycerol DGDG, digalactosyl diacylglycerol SQDG, sulfoquinovocyldiacylglycerol; Aar, acyl-ACP reductase; Ado, aldehyde deformylating oxygenase; FAAL, fatty acyl AMP ligases; Mal-CoA, malonyl CoA; KS, ketosynthase; AT, acyltransferase; KR, ketoreductase; ST, sulfotransferase; TE, thioesterase.

Figure 6.

Fatty acid (FA) turnover in cyanobacteria. (A) Lipases catalyze the hydrolysis of the carboxylic ester bonds to release FAs from the glycerol backbone. (B) The ALDH/Ado pathway: enzymes Ado and ALDH ensure alka(e)nes biodegradation by their oxidation to the corresponding primary fatty alcohols, which are then further oxidized to the corresponding fatty aldehyde, and finally further oxidized into the corresponding FAs. FA, fatty acid; Lip, lipase; MG, monogalactosyl; DG, digalactosyl; SQ, sulfoquinovosyl; DAG, diacylglycerol; MAG, monoacylglycerol; Ado, aldehyde-deformylating oxygenase; ALDH, aldehyde dehydrogenase.