Positional specificity of cyclopropane ring formation from cis-octadecenoic acid isomers in Escherichia coli

Advances in the Structural Biology, Mechanism, and Physiology of Cyclopropane Fatty Acid Modifications of Bacterial Membranes

Cyclopropane fatty acid (CFA) synthase catalyzes a remarkable reaction. The cis double bonds of unsaturated fatty acyl chains of phospholipid bilayers are converted to cyclopropane rings by transfer of a methylene moiety from S-adenosyl-L-methionine (SAM).

Exotic biomodification of fatty acids

Many biotransformations of mid- to long chain fatty acyl derivatives are intrinsically interesting because of their high selectivity and novel mechanisms. These include one carbon transfer, hydration, isomerization, hydrogenation, ladderane and hydrocarbon formation, thiolation and various oxidative transformations such as epoxidation, hydroxylation and desaturation. In addition, hydroperoxidation of polyunsaturated fatty acids leads to a diverse array of bioactive compounds. The bioorganic aspects of selected reactions will be highlighted in this review; 210 references are cited.

Configurational analysis of cyclopropyl fatty acids isolated from Escherichia coli

[reaction: see text] The absolute configuration of methyl lactobacillate and its 9,10 homologue, both isolated from Escherichia coli B-ATCC 11303, was found to be 11R,12S and 9R,10S, respectively.

Characterization of cyclopropane fatty-acid synthase from Sterculia foetida

Cyclopropane synthase from Sterculia foetida developing seeds catalyzes the addition of a methylene group from S-adenosylmethionine to the cis double bond of oleic acid (Bao, X., Katz, S., Pollard, M., and Ohlrogge, J. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 7172-7177). To understand this enzyme better, differential expression in leaf and seed tissues, protein properties, and substrate preferences of plant cyclopropane synthase were investigated. Immunoblot analysis with antibodies raised to recombinant S. foetida cyclopropane synthase (SfCPA-FAS) revealed that SfCPA-FAS is expressed in S. foetida seeds, but not in leaves, and is a membrane protein localized to microsomal fractions. Transformed tobacco cells expressing SfCPA-FAS were labeled in vivo with L-[methyl-(14)C]methionine and assayed in vitro with S-adenosyl-L-[methyl-(14)C]methionine. These kinetic experiments demonstrated that dihydrosterculate was synthesized from oleic acid esterified at the sn-1 position of phosphatidylcholine (PC). Furthermore, analysis of acyl chains at sn-1 and sn-2 positions that accumulated in PC from S. foetida developing seeds and from tobacco cells expressing SfCPA-FAS also demonstrated that greater than 90% of dihydrosterculate was esterified to the sn-1 position. Thus, we conclude that SfCPA-FAS is a microsomal localized membrane protein that catalyzes the addition of methylene groups derived from S-adenosyl-L-methionine across the double bond of oleic acid esterified to the sn-1 position of PC. A survey of plant and bacterial genomes for sequences related to SfCPA-FAS indicated that a peptide domain with a putative flavin-binding site is either fused to the methyltransferase domain of the plant protein or is often found encoded by a gene adjacent to a bacterial cyclopropane synthase gene.

cis-9,10-Methylenehexadecanoic acid inhibits contractility and actomyosin ATPase activity of guinea pig myocardium

Superfusion with a cyclopropane fatty acid, cis-9, 10-methylenehexadecanoic acid (10-300 microM), reduced the contractility of papillary muscle isolated from guinea pigs in a dose-dependent manner. cis-9,10-Methylenehexadecanoic acid also inhibited the Mg(2+)-ATPase activity of guinea pig papillary myocardium by about 40% at 400 microM. Since cis-9, 10-methylenehexadecanoic acid 4 microM inhibited the K(+)-EDTA-ATPase activity inherent in myosin's catalytic activity by about 25%, the fatty acid was thought to interact with the catalytic center of the myosin molecule.

Identification of cis-9,10-methylenehexadecanoic acid in submitochondrial particles of bovine heart

Submitochondrial particles of bovine heart were hydrolyzed by phospholipase A2 and the products were analyzed by liquid chromatography electrospray ionization-mass spectrometry. We found a fatty acid with a molecular mass of 268 Da and a retention time longer than that of linoleic acid. Next, we synthesized organically cis-9,10-methylenehexadecanoic acid, which has a molecular mass similar to that of the extracted fatty acid, and characterized its high performance liquid chromatography and gas chromatography-mass spectrometry profiles. Using these data we were able to identify endogenous cis-9,10-methylenehexadecanoic acid in rat and human heart and liver tissues that had been hydrolyzed by phospholipase A2. This fatty acid was not detected in tissue extracts that had not been hydrolyzed by phospholipase A2. Similar amounts of cis-9, 10-methylenehexadecanoic acid were measured in tissue extracts after total hydrolysis. These results suggest that cis-9, 10-methylenehexadecanoic acid is a fatty acid component, in the sn-2 position, of phospholipids in some mammalian tissue.

Cyclopropane ring formation in membrane lipids of bacteria

It has been known for several decades that cyclopropane fatty acids (CFAs) occur in the phospholipids of many species of bacteria. CFAs are formed by the addition of a methylene group, derived from the methyl group of S-adenosylmethionine, across the carbon-carbon double bond of unsaturated fatty acids (UFAs). The C1 transfer does not involve free fatty acids or intermediates of phospholipid biosynthesis but, rather, mature phospholipid molecules already incorporated into membrane bilayers. Furthermore, CFAs are typically produced at the onset of the stationary phase in bacterial cultures. CFA formation can thus be considered a conditional, postsynthetic modification of bacterial membrane lipid bilayers. This modification is noteworthy in several respects. It is catalyzed by a soluble enzyme, although one of the substrates, the UFA double bond, is normally sequestered deep within the hydrophobic interior of the phospholipid bilayer. The enzyme, CFA synthase, discriminates between phospholipid vesicles containing only saturated fatty acids and those containing UFAs; it exhibits no affinity for vesicles of the former composition. These and other properties imply that topologically novel protein-lipid interactions occur in the biosynthesis of CFAs. The timing and extent of the UFA-to-CFA conversion in batch cultures and the widespread distribution of CFA synthesis among bacteria would seem to suggest an important physiological role for this phenomenon, yet its rationale remains unclear despite experimental tests of a variety of hypotheses. Manipulation of the CFA synthase of Escherichia coli by genetic methods has nevertheless provided valuable insight into the physiology of CFA formation. It has identified the CFA synthase gene as one of several rpoS-regulated genes of E. coli and has provided for the construction of strains in which proposed cellular functions of CFAs can be properly evaluated. Cloning and manipulation of the CFA synthase structural gene have also enabled this novel but extremely unstable enzyme to be purified and analyzed in molecular terms and have led to the identification of mechanistically related enzymes in clinically important bacterial pathogens.

Effects of thiastearic acids on growth and on dihydrosterculic acid and other phospholipid fatty acyl groups of Leishmania promastigotes

Thiastearic acid positional isomers (8, 9, 10, 11) were examined for their ability to inhibit population growth and the biosynthesis of a phosphatidylethanolamine cyclopropane fatty acyl group, cis-9,10-methyleneoctadecanoic acid (dihydrosterculic acid), by promastigotes of Leishmania species. Thiastearic acids are candidate chemotherapeutic agents, since cyclopropane fatty acids are not formed by vertebrate cells. 8- and 10-thiastearic acids strongly inhibited the growth of strains containing the most dihydrosterculic acid (Leishmania tropica and Leishmania donovani; 25-35% phosphatidylethanolamine fatty acyl groups) and less strongly inhibited strains containing no dihydrosterculic acid (Leishmania major). The 11-thiastearic acid was less effective and 9-thiastearic acid ineffective. Strains containing 1-15% dihydrosterculic acid (L. donovani, Leishmania braziliensis, Leishmania aethiopica and Leishmania mexicana mexicana) were with few exceptions not inhibited by any of the isomers. All the thiastearic acid isomers caused a dose-dependent loss of dihydrosterculic acid. This was accompanied by a loss of phosphatidylethanolamine in the case of dihydrosterculic acid-rich leishmanial strains exposed to the 8- and 10-isomers. The 8- and 10-thiastearic acids also caused a loss of C18 unsaturated fatty acyl groups and increases in palmitic and stearic acids in the phosphatidylethanolamine and phosphatidylcholine of the dihydrosterculic acid-rich and dihydrosterculic acid-free leishmanial strains. 11-Thiastearic acid was much less effective and 9-thiastearic acid ineffective. These changes were not evident in those strins which contained 1-15% dihydrosterculic acid and whose growth was not inhibited by the thiastearic acid isomers. It is concluded that thiastearic acid isomers may inhibit both dihydrosterculic acid biosynthesis and fatty acid desaturation, with the 9-isomer having the highest specificity for dihydrosterculic acid biosynthesis. Population growth of promastigotes of Leishmania species in culture is not dependent upon dihydrosterculic acid biosynthesis but is dependent upon fatty acid desaturation.

Methyl sterol and cyclopropane fatty acid composition of Methylococcus capsulatus grown at low oxygen tensions

Methylococcus capsulatus contained extensive intracytoplasmic membranes when grown in fed-batch cultures over a wide range of oxygen tensions (0.1 to 10.6%, vol/vol) and at a constant methane level. Although the biomass decreased as oxygen levels were lowered, consistently high amounts of phospholipid and methyl sterol were synthesized. The greatest amounts of sterol and phospholipid were found in cells grown between 0.5 and 1.1% oxygen (7.2 and 203 mumol/g [dry weight], respectively). While sterol was still synthesized in significant amounts in cells grown at 0.1% oxygen, the major sterol product was the dimethyl form. Analysis by capillary gas chromatography-mass spectrophotometry showed that the phospholipid esterified fatty acids were predominantly 16:0 and 16:1 and that the hexadecenoates consisted of cis delta 9, delta 10, and delta 11 isomers. At low oxygen tensions, the presence of large amounts (25%) of cyclopropane fatty acids (cy 17:0) with the methylene groups at the delta 9, delta 10, and delta 11 positions was detected. Although the delta 9 monoenoic isomer was predominant, growth at low oxygen levels enhanced the synthesis of the delta 10 isomers of 16:1 and cy 17:0. As the oxygen level was increased, the amount of cyclopropanes decreased, such that only a trace of cy 17:0 could be detected in cells grown at 10.6% oxygen. Although M. capsulatus grew at very low oxygen tensions, this growth was accompanied by changes in the membrane lipids.

Biosynthesis of the novel fatty acid, 17-methyl-cis-9,10-methyleneoctadecanoic acid, by the parasitic protozoan, Herpetomonas megaseliae

Herpetomonas megaseliae, a flagellate protozoan parasite of the gut of a dipteran, Megaselia scalaris, is shown by chromatographic, spectrometric and radiotracer methods to synthesize de novo an iso-branched chain cyclopropane fatty acid, 17-methyl-cis-9,10-methyleneoctadecanoic acid.

Toxicity of activated oxygen: lack of dependence on membrane unsaturated fatty acid composition

Membrane unsaturated fatty acid oxidation has been suggested as a mechanism of toxicity for a variety of activated oxygen species. We have tested this hypothesis by manipulating the fatty acid composition of an Escherichia coli mutant that is unable to synthesize unsaturated fatty acids. To provide a wide range of susceptibility to membrane oxidation we have replaced the naturally occurring monoenoic acyl chains with cyclopropanes to greatly reduce the unsaturation level and with linoleate to increase the membrane unsaturation. These cultures were treated with ozone, hydrogen peroxide, singlet oxygen and paraquat. In no case was there substantial protection from toxicity afforded by cyclopropanes nor was there enhancement of toxicity to cells with the polyunsaturated membranes. We suggest, therefore, that oxidation of membrane unsaturated fatty acids is not an essential component of the toxicity to E. coli of active oxygen species.

The cyclopropane fatty acid of trypanosomatids

Cis-9,10-Methyleneoctadecanoic acid, one of a group of cyclopropane fatty acids commonly found in bacteria but not in eukaryotic cells, has been identified in the phosphatidylethanolamines of 27 isolates representing 5 genera of trypanosomatid flagellates (Crithidia, Leptomonas, Herpetomonas, Phytomonas, Leishmania). Its presence did not appear to be associated with endosymbiotic or other microbiol associates. It was absent from 12 isolates of the genera Blastocrithidia, Endotrypanum and Trypanosoma.