New insight into Phaeodactylum tricornutum fatty acid metabolism. Cloning and functional characterization of plastidial and microsomal delta12-fatty acid desaturases

In contrast to 16:3 plants like rapeseed (Brassica napus), which contain alpha-linolenic acid (18:3(Delta9,12,15)) and hexadecatrienoic acid (16:3(Delta7,10,13)) as major polyunsaturated fatty acids in leaves, the silica-less diatom Phaeodactylum tricornutum contains eicosapentaenoic acid (EPA; 20:5(Delta5,8,11,14,17)) and a different isomer of hexadecatrienoic acid (16:3(Delta6,9,12)). In this report, we describe the characterization of two cDNAs having sequence homology to Delta12-fatty acid desaturases from higher plants. These cDNAs were shown to code for a microsomal and a plastidial Delta12-desaturase (PtFAD2 and PtFAD6, respectively) by heterologous expression in yeast (Saccharomyces cerevisiae) and Synechococcus, respectively. Using these systems in the presence of exogenously supplied fatty acids, the substrate specificities of the two desaturases were determined and compared with those of the corresponding rapeseed enzymes (BnFAD2 and BnFAD6). The microsomal desaturases were similarly specific for oleic acid (18:1(Delta9)), suggesting that PtFAD2 is involved in the biosynthesis of EPA. In contrast, the plastidial desaturase from the higher plant and the diatom clearly differed. Although the rapeseed plastidial desaturase showed high activity toward the omega9-fatty acids 18:1(Delta9) and 16:1(Delta7), in line with the fatty acid composition of rapeseed leaves, the enzyme of P. tricornutum was highly specific for 16:1(Delta9). Our results indicate that in contrast to EPA, which is synthesized in the microsomes, the hexadecatrienoic acid isomer found in P. tricornutum (16:3(Delta6,9,12)) is of plastidial origin.

Cloning and functional characterization of Phaeodactylum tricornutum front-end desaturases involved in eicosapentaenoic acid biosynthesis

Phaeodactylum tricornutum is an unicellular silica-less diatom in which eicosapentaenoic acid accumulates up to 30% of the total fatty acids. This marine diatom was used for cloning genes encoding fatty acid desaturases involved in eicosapentaenoic acid biosynthesis. Using a combination of PCR, mass sequencing and library screening, the coding sequences of two desaturases were identified. Both protein sequences contained a cytochrome b5 domain fused to the N-terminus and the three histidine clusters common to all front-end fatty acid desaturases. The full length clones were expressed in Saccharomyces cerevisiae and characterized as Delta5- and Delta6-fatty acid desaturases. The substrate specificity of each enzyme was determined and confirmed their involvement in eicosapentaenoic acid biosynthesis. Using both desaturases in combination with the Delta6-specific elongase from Physcomitrella patens, the biosynthetic pathways of arachidonic and eicosapentaenoic acid were reconstituted in yeast. These reconstitutions indicated that these two desaturases functioned in the omega3- and omega6-pathways, in good agreement with both routes coexisting in Phaeodactylum tricornutum. Interestingly, when the substrate selectivity of each enzyme was determined, both desaturases converted the omega3- and omega6-fatty acids with similar efficiencies, indicating that none of them was specific for either the omega3- or the omega6-pathway. To our knowledge, this is the first report describing the isolation and biochemical characterization of fatty acid desaturases from diatoms.

Production of polyunsaturated fatty acids by polyketide synthases in both prokaryotes and eukaryotes

Polyunsaturated fatty acids (PUFAs) are essential membrane components in higher eukaryotes and are the precursors of many lipid-derived signaling molecules. Here, pathways for PUFA synthesis are described that do not require desaturation and elongation of saturated fatty acids. These pathways are catalyzed by polyketide synthases (PKSs) that are distinct from previously recognized PKSs in both structure and mechanism. Generation of cis double bonds probably involves position-specific isomerases; such enzymes might be useful in the production of new families of antibiotics. It is likely that PUFA synthesis in cold marine ecosystems is accomplished in part by these PKS enzymes.

Biochemical characterization of 3-ketoacyl-acyl carrier protein synthase II from leek epidermis

In order to define its possible involvement in production of stearic acid for wax biosynthesis, the presence of 3-ketoacyl acyl synthase II (KAS II) activity was investigated in different tissues of leek (Allium porrum L.) leaves. KAS II activity was identified in sheath and lamina epidermis, as well as in underlying parenchyma. In all three tissues, activity was inhibited by 50% on addition of 100 microM cerulenin, and showed an absolute requirement for acyl-ACP substrates. More interestingly, the different tissues did not display similar KAS II substrate specificities. Parenchyma and lamina epidermis tissues presented typical KAS II activities, since C(18:0)-ACP was the exclusive product. In contrast, in sheath epidermis, KAS II activity resulted in the synthesis of acyl-chains up to 22 carbons in length, suggesting the existence in this tissue of an unusual KAS II. This activity was sufficient to elongate all of the palmitoyl-ACP produced by the fatty acid synthase, suggesting that C(18:0) is the substrate of the microsomal elongases involved in wax biosynthesis.

Purification of the acyl-CoA elongase complex from developing rapeseed and characterization of the 3-ketoacyl-CoA synthase and the 3-hydroxyacyl-CoA dehydratase

Oleoyl-CoA elongase catalyzes four successive reactions: condensation of malonyl-CoA to oleoyl-CoA, reduction, dehydration, and another reduction. Evidence supporting this mechanism and the multienzymatic nature of the elongation complex are reported. A particulate membrane fraction from rapeseed is able to elongate intermediates (R,S) 3-hydroxy-20:0-CoA and (E) 2,3-20:1-CoA to very long chain fatty acids in the presence of malonyl-CoA. Studies of the 3-ketoacyl-CoA synthase activities showed that maximal activity could be measured by using 15 to 30 microM 18:1-CoA and 30 microM malonyl-CoA, and that 18:0-CoA and 18:1-CoA were the best substrates. Comparison of the condensation and the overall elongation activities indicated that condensation is the rate-limiting step of the elongation process. The 3-hydroxyacyl-CoA dehydratase activity was maximal in the presence of 75 microM Triton X-100 and 25 microg of proteins. Finally, the acyl-CoA elongase complex was solubilized and purified. During the purification process, the 3-hydroxyacyl-CoA dehydratase copurified with the elongase complex, strongly suggesting that this enzyme belongs to the elongase complex. The apparent molecular mass of 700 kDa determined for the elongase complex, and the fact that four different polypeptide bands were detected after sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of the purified fraction, further suggest that the acyl-CoA elongase is a multienzymatic complex.

Antifungal compounds from idioblast cells isolated from avocado fruits

(E,Z,Z)-1-Acetoxy-2-hydroxy-4-oxo-heneicosa-5,12,15-triene was isolated from avocado, Persea americana Mill., idioblast cells. It inhibited spore germination of the fungal pathogen Colletotrichum gloeosporioides. Full characterization is also reported for two additional compounds that have been described and partially characterized previously.

Evidence that oleoyl-CoA and ATP-dependent elongations coexist in rapeseed (Brassica napus L.)

The elongation of different substrates was studied using several subcellular fractions from Brassica napus rapeseed. In the presence of malonyl-CoA, NADH and NADPH, very-long-chain fatty acid (VLCFA) synthesis was observed from either oleoyl-CoA (acyl-CoA elongation) or endogenous primers (ATP-dependent elongation). No activity was detected using oleic acid as precursor. Acyl-CoA and ATP-dependent elongation activities were mainly associated with the 15 000 g/25 min membrane fraction. Reverse-phase TLC analysis showed that the proportions of fatty acids synthesized by these activities were different. Acyl-CoA elongation increased up to 60 microM oleoyl-CoA, and ATP-dependent elongation was maximum at 1 mM ATP. Both activities increased with malonyl-CoA concentration (up to 200 microM). Under all conditions tested, acyl-CoA elongation was higher than ATP-dependent elongation, and, in the presence of both ATP and oleoyl-CoA, the elongation activity was always lower. ATP strongly inhibited acyl-CoA elongation, whereas ATP-dependent elongation was slightly stimulated by low oleoyl-CoA concentrations (up to 15 microM) and decreased in the presence of higher concentrations. CoA (up to 150 microM) had no effect on the ATP-dependent elongation, whereas it inhibited the acyl-CoA elongation. These marked differences strongly support the presence in maturing rapeseed of two different elongating activities differently modulated by ATP and oleoyl-CoA.

Characterization and application of new macroporous membrane ion exchangers

A new ready-to-use unit for high-performance membrane chromatography has been characterized. Its dynamic capacity, resolving power and protein recovery were measured at different flow-rates. The binding capacity was 0.5-2 mg/cm2 with a 95% recovery at 10 ml/min irrespective of the protein concentration up to 10 mg/ml. For very-high flow-rates (50 and 100 ml/min) the recovery was 90% and 70%. At these flow-rates, the maximum back-pressure was about 0.1 MPa and was independent of the filtration area. By increasing the filtration area, a proportional capacity increase was obtained, indicating an easy scale-up. High flow-rates had only a slight effect on resolution. This new adsorber was able to purify IgM from supernatant of cell culture of a human hybridoma in less than 8 min with a high degree of purity (95%).