Fatty Acid Elongation Is Independent of Acyl-Coenzyme A Synthetase Activities in Leek and Brassica napus

In both animal and plant acyl elongation systems, it has been proposed that fatty acids are first activated to acyl-coenzyme A (CoA) before their elongation, and that the ATP dependence of fatty acid elongation is evidence of acyl-CoA synthetase involvement. However, because CoA is not supplied in standard fatty acid elongation assays, it is not clear if CoA-dependent acyl-CoA synthetase activity can provide levels of acyl-CoAs necessary to support typical rates of fatty acid elongation. Therefore, we examined the role of acyl-CoA synthetase in providing the primer for acyl elongation in leek (Allium porrum L.) epidermal microsomes and Brassica napus L. cv Reston oil bodies. As presented here, fatty acid elongation was independent of CoA and proceeded at maximum rates with CoA-free preparations of malonyl-CoA. We also showed that stearic acid ([1-¹⁴C]18:0)-CoA was synthesized from [1-¹⁴C]18:0 in the presence of CoA-free malonyl-CoA or acetyl-CoA, and that [1-¹⁴C]18:0-CoA synthesis under these conditions was ATP dependent. Furthermore, the appearance of [1-¹⁴C]18:0 in the acyl-CoA fraction was simultaneous with its appearance in phosphatidylcholine. These data, together with the s of a previous study (A. Hlousek-Radojcic, H. Imai, J.G. Jaworski [1995] Plant J 8: 803–809) showing that exogenous [¹⁴C]acyl-CoAs are diluted by a relatively large endogenous pool before they are elongated, strongly indicated that acyl-CoA synthetase did not play a direct role in fatty acid elongation, and that phosphatidylcholine or another glycerolipid was a more likely source of elongation primers than acyl-CoAs.     

Purification and characterization of 3-ketoacyl-acyl carrier protein synthase III from spinach. A condensing enzyme utilizing acetyl-coenzyme A to initiate fatty acid synthesis.

The 3-ketoacyl-acyl carrier protein (ACP) synthase III from spinach was purified to homogeneity by an eight-step procedure that included an ACP-affinity column. The size of the native enzyme was M(r) = 63,000 based on gel filtration, and its subunit size was M(r) = 40,500 based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggesting that 3-ketoacyl-ACP synthase III may be a homodimer. The purified enzyme was highly specific for acetyl-CoA and malonyl-ACP. The Km for acetyl-CoA was 5 microM when assayed in the presence of 10 microM malonyl-CoA. Acetyl-, butyryl-, and hexanoyl-ACP would not substitute for acetyl-CoA as substrates. The specificity for acetyl-CoA suggested that the physiological function of 3-ketoacyl-ACP synthase is to catalyze the initial condensation reaction in fatty acid biosynthesis. The homogeneous 3-ketoacyl-ACP synthase was capable of catalyzing acetyl-CoA:ACP transacylation but at a rate about 90-fold slower than the condensation reaction with malonyl-ACP. The 3-ketoacyl-ACP synthase was inhibited 100% by 5 mM N-ethylmaleimide or 20 mM sodium arsenite.     

Fatty acid chain elongation by microsomal enzymes from the bovine meibomian gland

The composition of meibomian gland lipids suggested that fatty acid chain elongation might play a major role in the synthesis of such lipids. A fatty acid synthase preparation from the bovine meibomian gland catalyzed the formation of C16 acid and the enzyme was immunologically quite similar to that in the mammary gland. The microsomal fraction from the gland, on the other hand, catalyzed elongation of endogenous fatty acids in the presence of ATP and Mg2+ and of exogenous C18-CoA using malonyl-CoA and NADPH as the preferred reductant. The elongated products, ranging up to C28 in chain length, were found mainly as CoA esters and products derived from them. With C18-CoA as the exogenous primer, the elongation rate was linear with incubation time up to 20 min but the rate changed in a sigmoidal manner with increasing protein concentration. The elongation rate was maximal at a pH around 7.0. Typical Michaelis-Menten-type substrate saturation patterns were observed with both malonyl-CoA and NADPH. From linear double-reciprocal plots, the Km values for the two substrates were calculated to be 52 and 11 microM, respectively, with a V of about 340 pmol min-1 mg protein-1 with respect to malonyl-CoA. Exogenous CoA esters of C16 to C22 fatty acids were elongated to give products up to C28 without exhibiting any preference for the primer. The present elongation system could account for the formation of most of the very long chains found in meibomian lipids.     

Nature of the reaction product of [1-14C]stearoyl-CoA elongation by etiolated leek seeding microsomes

The elongation of [1-14C]stearoyl-CoA by microsomes from etiolated leek seedlings, in the presence of malonyl-CoA and NADPH, has been studied at different substrate and enzyme concentrations. The HPTLC analysis of the whole reaction mixture, followed by the analysis of the label in the fatty acid methyl esters of long-chain acyl-CoAs, phosphatidylcholine (PC), and neutral lipids, showed that the acyl-CoA fraction contained most of the labeled very-long-chain fatty acids. The very-long-chain fatty acids were rapidly formed and released from the elongase(s) as acyl-CoAs. The label of long-chain acyl-CoAs increased for 20 min and then decreased, whereas it increased in PC. Labeled very-long-chain fatty acids appeared in the neutral lipid + free fatty acid fraction after a 20-min lag.     

Purification of NADPH-dependent enoyl-CoA reductase involved in the malonyl-CoA dependent fatty acid elongation system of Mycobacterium smegmatis

NADPH-Dependent enoyl-CoA reductase [EC 1.3.1.8] was purified to homogeneity, for the first time, from the crude extract of Mycobacterium smegmatis. The molecular weight of this enzyme was estimated to be around 32,000 using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified enzyme reduced 2-trans-hexadecenoyl-CoA (Km value, 100 microM) and -eicosenoyl-CoA (Km value, 83 microM) almost equally well in the presence of NADPH as a sole electron donor. The Km value for NADPH was 34.5 microM. When NADP3H was incubated with 2-eicosenoyl-CoA and the purified enzyme, the sole tritiated product was arachidate. This enzyme was almost inert to enoyl-CoAs with chains less than 12 carbon atoms long. The purified enzyme still retained FMN, which was detectable by acid ammonium sulfate and was essential for full activity of the enzyme. The enzyme was sensitive to SH-reagents such as N-ethylmaleimide and monoiodoacetamide but was not sensitive to isonicotinamide hydrazide. Anti-NADPH-dependent-enoyl-CoA-reductase rabbit serum was found to inhibit the activities of both the reductase and the malonyl-CoA dependent fatty acid elongation system, supporting the involvement of the reductase in this elongation system.     

Purification and characterization of an unusually large fatty acid synthase from Mycobacterium tuberculosis var. bovis BCG.

Fatty acid synthase was purified from Mycobacterium tuberculosis var. bovis BCG. The method developed gave a 23% yield of the synthase and also yielded purified mycocerosic acid synthase. The fatty acid synthase is of unusually large size and composed of two 500-kDa monomers. The amino acid composition of the two synthases was not identical; the N-terminus of the fatty acid synthase was blocked, whereas that of the mycocerosic acid synthase was not. Western blot analysis of crude mycobacterial extracts with polyclonal antibodies prepared against each synthase showed a single band in each case with no cross-reactivity with the other synthase. Fatty acid synthase required both NADH (Km, 11 microM) and NADPH (Km, 14 microM). The Km for acetyl-CoA and malonyl-CoA were 5 and 6 microM, respectively. Fatty acids were released from the synthase as CoA esters. A bimodal distribution of fatty acids was obtained at around C16 and C26. The primer utilization also reflects the de novo synthesis and elongation capabilities of the enzyme; acetyl-CoA was the preferred primer but CoA esters up to C8 but not C12 and C14 could serve as primers, whereas C16 was readily used as a primer for elongation. Addition of CoA and CoA ester-binding oligosaccharides caused enhanced release of C16. Since this mycobacterial fatty acid synthase is twice as large as other multifunctional fatty acid synthases, it is tempting to suggest that this synthase represents a head to tail fusion of two fatty acid synthase genes coding for a double size protein with one-half producing C16 acid and the other elongating the C16 acid to a C26 acid. The monomer of fatty acid synthase from M. smegmatis was immunologically similar and equal in size to the synthase from M. tuberculosis.     

Fatty acyl-CoA elongation in Blatella germanica integumental microsomes

Insect cuticular hydrocarbons are synthesized de novo in integumental tissue through the concerted action of fatty acid synthases (FASs), fatty acyl-CoA elongases, a reductase, and a decarboxylase to produce hydrocarbons and CO2. Elongation of fatty acyl-CoAs to very long chain fatty acids was studied in the integumental microsomes of the German cockroach, Blatella germanica. Incubation of [1-14C]palmitoyl-CoA, malonyl-CoA, and NADPH resulted in the production of 18-CoA with minor amounts of C20, C22, C24, C30, and C32 labeled acyl-CoA moieties. Similar experiments with [1-14C]stearoyl-CoA rendered C20-CoA as the major product, and lesser amounts of C22 and C24-CoAs were also detected. After solubilization of the microsomal FAS, kinetic parameters were determined radiochemically or by measuring NADPH consumption. The reaction velocity was linear for up to 3 min incubation time, and with a protein concentration up to 0.025 microg/microl. The effect of the chain length on the reaction velocity was compared for palmitoyl-CoA, stearoyl-CoA, and eicosanoyl-CoA. The optimal substrate concentration was 10 microM for C16-CoA, between 8 and 12 microM for C18-CoA, and close to 3 microM for C20-CoA. In vivo hydrocarbon biosynthesis was inhibited from 55.5 to 72.5% in the presence of 1 mM trichloroacetic acid, a known inhibitor of elongation reactions.     

Evidence for multiple condensing enzymes in rat hepatic microsomes catalyzing the condensation of saturated, monounsaturated, and polyunsaturated acyl coenzyme A

The condensation of palmitoyl-CoA with malonyl-CoA by rat hepatic microsomes was competitively inhibited by myristoyl-CoA, whereas it was noncompetitively inhibited by palmitoleoyl and gamma-linolenoyl-CoA. Furthermore, the condensation of palmitoleoyl-CoA with malonyl-CoA was also noncompetitively inhibited by gamma-linolenoyl-CoA. Replacement of normal diet by a fat-free high carbohydrate diet resulted in 8-, 2.5-, and 2.3-fold increases in the condensation rates of both palmitoyl- and myristoyl-CoA, palmitoleoyl-CoA, and gamma-linolenoyl-CoA, respectively. On the other hand, administration of di-(2-ethylhexyl)phthalate (DEHP) resulted in a 2-fold stimulation of the condensation activities with myristoyl- and palmitoyl-CoA, while those with palmitoleoyl- and gamma-linolenoyl-CoA decreased to about 83 and 63%, respectively. Similar results following dietary changes or DEHP administration were obtained for total elongation activities. Finally condensation activities of 16:0, 16:1, and gamma-18:3 CoA were differently affected by the proteolytic enzyme, chymotrypsin. The competitive substrate studies, those of dietary and DEHP administration, and the differential action of chymotrypsin strongly suggest the existence of at least three discrete condensing enzymes catalyzing the condensation of saturated, monounsaturated, and polyunsaturated acyl-CoAs. These studies also indicate that the condensation reaction is the regulating and rate-limiting step of the fatty acid chain elongation system.     

Overproduction of a Functional Fatty Acid Biosynthetic Enzyme Blocks Fatty Acid Synthesis in Escherichia coli

β-Ketoacyl-acyl carrier protein (ACP) synthetase II (KAS II) is one of three Escherichia coli isozymes that catalyze the elongation of growing fatty acid chains by condensation of acyl-ACP with malonyl-ACP. Overexpression of this enzyme has been found to be extremely toxic to E. coli, much more so than overproduction of either of the other KAS isozymes, KAS I or KAS III. The immediate effect of KAS II overproduction is the cessation of phospholipid synthesis, and this inhibition is specifically due to the blockage of fatty acid synthesis. To determine the cause of this inhibition, we examined the intracellular pools of ACP, coenzyme A (CoA), and their acyl thioesters. Although no significant changes were detected in the acyl-ACP pools, the CoA pools were dramatically altered by KAS II overproduction. Malonyl-CoA increased to about 40% of the total cellular CoA pool upon KAS II overproduction from a steady-state level of around 0.5% in the absence of KAS II overproduction. This finding indicated that the conversion of malonyl-CoA to fatty acids had been blocked and could be explained if either the conversion of malonyl-CoA to malonyl-ACP and/or the elongation reactions of fatty acid synthesis had been blocked. Overproduction of malonyl-CoA:ACP transacylase, the enzyme catalyzing the conversion of malonyl-CoA to malonyl-ACP, partially relieved the toxicity of KAS II overproduction, consistent with a model in which high levels of KAS II blocks access of the other KAS isozymes to malonyl-CoA:ACP transacylase.     

Functional differentiation and selective inactivation of multiple <Emphasis Type="Italic"> Saccharomyces cerevisiae </Emphasis>genes involved in very-long-chain fatty acid synthesis

While de novo fatty acid synthesis uses acetyl-CoA, fatty acid elongation uses longer-chain acyl-CoAs as primers. Several mutations that interfere with fatty acid elongation in yeast have already been described, suggesting that there may be different elongases for medium- and long-chain acyl-CoA primers. In the present study, an experimental approach is described that allows differential characterization of the various yeast elongases in vitro. Based on their characteristic primer specificities and product patterns, at least three different yeast elongases are defined. Elongase I extends C12-C16 fatty acyl-CoAs to C16-C18 fatty acids. Elongase II elongates palmitoyl-CoA and stearoyl-CoA up to C22 fatty acids, and elongase III synthesizes 20-26-carbon fatty acids from C18-CoA primers. Elongases I, II and III are specifically inactivated in, respectively, elo1, elo2 and elo3 mutants. Elongases II and III share the same 3-ketoacyl reductase, which is encoded by the YBR159w gene. Inactivation of YBR159w inhibits in vitro fatty acid elongation after the first condensation reaction. Although in vitro elongase activity is absent, the mutant nevertheless contains 10-30% of normal VLCFA levels. On the basis of this finding, an additional elongating activity is inferred to be present in vivo. ybr159Delta cells show synthetic lethality in the presence of cerulenin, which inactivates fatty acid synthase. An involvement of FAS in VLCFA synthesis may account for these findings, but remains to be demonstrated directly. Alternatively, a vital role for C18 and C20 hydroxyacids, which are dramatically overproduced in ybr159Delta cells, may be postulated.     

Elongation of fatty acids by microsomal fractions from the brain of the developing rat.

Elongation of fatty acids by microsomal fractions obtained from rat brain was measured by the incorporation of [2-14C]malonyl-CoA into fatty in the presence of palmitoyl-CoA or stearoyl-CoA. 2. Soluble and microsomal fractions were prepared from 21-day-old rats; density gradient centrifugation demonstrated that the stearoyl-CoA elongation system was localized in the microsomal fraction whereas fatty acid biosynthesis de novo from acetyl-CoA occurred in the soluble fraction. The residual activity de novo in the microsomal fraction was attributed to minor contamination by the soluble fraction. 3. The optimum concentration of [2-14C]malonyl-CoA for elongation of fatty acids was 25 mum for palmitoyl-CoA or stearoyl-CoA, and the corresponding optimum concentrations for the two primer acyl-CoA esters were 8.0 and 7.2 muM respectively. 4. Nadph was the preferred cofactor for fatty acid formation from palmitoyl-CoA or stearoyl-CoA, although NADH could partially replace it. 5. The stearoyl-CoA elongation system required a potassium phosphate buffer concentration of 0.075M for maximum activity; CoA (1 MUM) inhibited this elongation system by approx. 30%. 6. The fatty acids formed from malonyl-CoA and palmitoyl-CoA had a predominant chain length of C18 whereas stearoyl-CoA elongation resulted in an even distribution of fatty acids with chain lengths of C20, C22 and C24. 7. The products of stearoyl-CoA elongation were identified as primarily unesterified fatty acids. 8. The developmental pattern of fatty acid biosynthesis by rat brain microsomal preparations was studied and both the palmitoyl-CoA and stearoyl-CoA elongation systems showed large increases in activity between days 10 and 18 after birth.     

Nature of the Fatty Acid Synthetase Systems in Parenchymal and Epidermal Cells of Allium porrum L. Leaves

Fatty acid synthesis was compared in cell-free extracts of epidermis and parenchyma of Allium porrum L. leaves. Parenchyma extracts had the major fatty acid synthetase (FAS) activity (70-90%) of the whole leaf; palmitic acid was also the major fatty acid synthesized when acetyl-coenzyme A (CoA) was the primer, but when acetyl-acyl carrier protein (ACP) was employed, C_18:0 and C_16:0 were synthesized in equal proportion. With the epidermal FAS system when either acetyl-CoA or acetyl-ACP was tested in the presence of labeled malonyl-CoA, palmitic acid was the only product synthesized. Specific activities of the FAS enzyme activities were determined in both tissue extracts.

The properties of malonyl-CoA:ACP transacylase were examined from the two different tissues. The molecular weights estimated by Sephadex G-200 chromatography were 38,000 for the epidermal enzyme and 45,000 for parenchymal enzyme. The optimal pH was for both enzymes 7.8 to 8.0 and the maximal velocity 0.4 to 0.5 micromoles per milligram protein per minute. These enzymes had different affinities for malonyl-CoA and ACP. For the malonyl-CoA:ACP transacylase of epidermis, the K_m values were 5.6 and 13.7 micromolar for malonyl-CoA and ACP, respectively, and 4.2 and 21.7 micromolar for the parenchymal enzyme. These results suggest that the FAS system in both tissues are nonassociated, that the malonyl-CoA:ACP transacylases are isozymes, and that both in epidermis and in parenchyma tissue two independent FAS system occur. Evidence would suggest that β-ketoacyl-ACP synthase II is present in the parenchymal cells but missing in the epidermal cell.

     

The initiating steps of a type II fatty acid synthase in Plasmodium falciparum are catalyzed by pfACP,pfMCAT, and pfKASIII

Malaria, a disease caused by protozoan parasites of the genus Plasmodium, is one of the most dangerous infectious diseases, claiming millions of lives and infecting hundreds of millions of people annually. The pressing need for new antimalarials has been answered by the discovery of new drug targets from the malaria genome project. One of the early findings was the discovery of two genes encoding Type II fatty acid biosynthesis proteins: ACP (acyl carrier protein) and KASIII (beta-ketoacyl-ACP synthase III). The initiating steps of a Type II system require a third protein: malonyl-coenzyme A:ACP transacylase (MCAT). Here we report the identification of a single gene from P. falciparum encoding pfMCAT and the functional characterization of this enzyme. Pure recombinant pfMCAT catalyzes malonyl transfer from malonyl-coenzyme A (malonyl-CoA) to pfACP. In contrast, pfACP(trans), a construct of pfACP containing an amino-terminal apicoplast transit peptide, was not a substrate for pfMCAT. The product of the pfMCAT reaction, malonyl-pfACP, is a substrate for pfKASIII, which catalyzes the decarboxylative condensation of malonyl-pfACP and various acyl-CoAs. Consistent with a role in de novo fatty acid biosynthesis, pfKASIII exhibited typical KAS (beta-ketoacyl ACP synthase) activity using acetyl-CoA as substrate (k(cat) 230 min(-1), K(M) 17.9 +/- 3.4 microM). The pfKASIII can also catalyze the condensation of malonyl-pfACP and butyryl-CoA (k(cat) 200 min(-1), K(M) 35.7 +/- 4.4 microM) with similar efficiency, whereas isobutyryl-CoA is a poor substrate and displayed 13-fold less activity than that observed for acetyl-CoA. The pfKASIII has little preference for malonyl-pfACP (k(cat)/K(M) 64.9 min(-1)microM(-1)) over E. coli malonyl-ACP (k(cat)/K(M) 44.8 min(-1)microM(-1)). The pfKASIII also catalyzes the acyl-CoA:ACP transacylase (ACAT) reaction typically exhibited by KASIII enzymes, but does so almost 700-fold slower than the KAS reaction. Thiolactomycin did not inhbit pfKASIII (IC(50) > 330 microM), but three structurally similar substituted 1,2-dithiole-3-one compounds did inhibit pfKASIII with IC(50) values between 0.53 microM and 10.4 microM. These compounds also inhibited the growth of P. falciparum in culture.     

Engineering and mechanistic studies of the Arabidopsis FAE1 beta-ketoacyl-CoA synthase, FAE1 KCS.

The Arabidopsis FAE1 beta-ketoacyl-CoA synthase (FAE1 KCS) catalyzes the condensation of malonyl-CoA with long-chain acyl-CoAs. Sequence analysis of FAE1 KCS predicted that this condensing enzyme is anchored to a membrane by two adjacent N-terminal membrane-spanning domains. In order to characterize the FAE1 KCS and analyze its mechanism, FAE1 KCS and its mutants were engineered with a His6-tag at their N-terminus, and expressed in Saccharomyces cerevisiae. The membrane-bound enzyme was then solubilized and purified to near homogeneity on a metal affinity column. Wild-type recombinant FAE1 KCS was active with several acyl-CoA substrates, with highest activity towards saturated and monounsaturated C16 and C18. In the absence of an acyl-CoA substrate, FAE1 KCS was unable to carry out decarboxylation of [3-(14)C]malonyl-CoA, indicating that it requires binding of the acyl-CoA for decarboxylation activity. Site-directed mutagenesis was carried out on the FAE1 KCS to assess if this condensing enzyme was mechanistically related to the well characterized soluble condensing enzymes of fatty acid and flavonoid syntheses. A C223A mutant enzyme lacking the acylation site was unable to carry out decarboxylation of malonyl-CoA even when 18:1-CoA was present. Mutational analyses of the conserved Asn424 and His391 residues indicated the importance of these residues for FAE1-KCS activity. The results presented here provide the initial analysis of the reaction mechanism for a membrane-bound condensing enzyme from any source and provide evidence for a mechanism similar to the soluble condensing enzymes.     

Purification and some properties of chalcone synthase from a carrot suspension culture induced for anthocyanin synthesis and preparation of its specific antiserum

Chalcone synthase was purified to homogeneity by polyacrylamide gel electrophoresis from cell suspension cultures of carrot in which anthocyanin synthesis was induced by transferring the cells from a medium containing 2,4-dichlorophenoxy-acetic acid (2,4-D) to one lacking it. A molecular weight of 80,000-85,000 for the enzyme was determined by gel filtration and disc-gel polyacrylamide electrophoresis, and one of about 40,600 for the subunit by SDS slab-gel electrophoresis. The primary reaction product was chalcone and the pH optimum of the reaction was 8.0. The Km values for 4-coumaroyl-CoA and malonyl-CoA were 5.7 microM and 18 microM, respectively. These properties of carrot chalcone synthase were discussed in comparison to those of that from cell cultures of parsley reported previously. Antiserum against chalcone synthase from carrot was obtained from mice bred under specific pathogen free conditions. Crossreactivity was examined by Western-blotting, and the high specificity of the antiserum against chalcone synthase was demonstrated.     

Characterization of the substrate specificity of PhlD, a type III polyketide synthase from Pseudomonas fluorescens

PhlD, a type III polyketide synthase from Pseudomonas fluorescens, catalyzes the synthesis of phloroglucinol from three molecules of malonyl-CoA. Kinetic analysis by direct measurement of the appearance of the CoASH product (k(cat) = 24 +/- 4 min(-1) and Km = 13 +/- 1 microM) gave a k(cat) value more than an order of magnitude higher than that of any other known type III polyketide synthase. PhlD exhibits broad substrate specificity, accepting C4-C12 aliphatic acyl-CoAs and phenylacetyl-CoA as the starters to form C6-polyoxoalkylated alpha-pyrones from sequential condensation with malonyl-CoA. Interestingly, when primed with long chain acyl-CoAs, PhlD catalyzed extra polyketide elongation to form up to heptaketide products. A homology structural model of PhlD showed the presence of a buried tunnel extending out from the active site to assist the binding of long chain acyl-CoAs. To probe the structural basis for the unusual ability of PhlD to accept long chain acyl-CoAs, both site-directed mutagenesis and saturation mutagenesis were carried out on key residues lining the tunnel. Three mutations, M21I, H24V, and L59M, were found to significantly reduce the reactivity of PhlD with lauroyl-CoA while still retaining its physiological activity to synthesize phloroglucinol. Our homology modeling and mutational studies indicated that even subtle changes in the tunnel volume could affect the ability of PhlD to accept long chain acyl-CoAs. This suggested novel strategies for combinatorial biosynthesis of unnatural pharmaceutically important polyketides.     

C75 activates malonyl-CoA sensitive and insensitive components of the CPT system

Carnitine palmitoyltransferase I (CPT-I) and II (CPT-II) enzymes are components of the carnitine palmitoyltransferase shuttle system which allows entry of long-chain fatty acids into the mitochondrial matrix for subsequent oxidation. This system is tightly regulated by malonyl-CoA levels since this metabolite is a strong reversible inhibitor of the CPT-I enzyme. There are two distinct CPT-I isotypes (CPT-Ialpha and CPT-Ibeta), that exhibit different sensitivity to malonyl-CoA inhibition. Because of its ability to inhibit fatty acid synthase, C75 is able to increase malonyl-CoA intracellular levels. Paradoxically it also activates long-chain fatty acid oxidation. To identify the exact target of C75 within the CPT system, we expressed individually the different components of the system in the yeast Pichia pastoris. We show here that C75 acts on recombinant CPT-Ialpha, but also on the other CPT-I isotype (CPT-Ibeta) and the malonyl-CoA insensitive component of the CPT system, CPT-II.     

Characterization and solubilization of an acyl chain elongation system in microsomes of leek epidermal cells

Microsomes prepared from leek epidermal tissue readily elongate stearoyl-CoA to very long chain fatty acid with malonyl-CoA as the C2 unit. In the absence of stearoyl-CoA, but in the presence of ATP, microsomes elongate endogenous free fatty acids. Endogenous CoA is the source of CoA. Palmitoyl, stearoyl, and higher saturated acyl-CoAs are readily elongated by the microsomal system but oleoyl-CoA is ineffective; however, the higher monounsaturated acyl-CoAs can be elongated. Since the very long chain fatty acids of the leek epidermis are all saturated, it would appear that the reaction controlling the nature of the final acyl product is the inactivity of oleoyl-CoA as a substrate. There is no evidence that acyl carrier protein participates in the elongation reactions. Evidence is also presented suggesting that (a) there may be two elongation systems, one responsible for the conversion of stearoyl-CoA to arachidonyl-CoA and the second involved in the conversion of arachidonyl-CoA to very long chain fatty acids, and that (b) the elongation activities may be associated with a large polypeptide.     

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.     

Fatty acid biosynthesis in Mycobacterium tuberculosis var. bovis Bacillus Calmette-Guérin. Purification and characterization of a novel fatty acid synthase, mycocerosic acid synthase, which elongates n-fatty acyl-CoA with methylmalonyl-CoA

A crude extract from Mycobacterium tuberculosis var. bovis Bacillus Calmette-Guérin was previously shown to incorporate methylmalonyl-CoA into mycocerosic acids, exemplified by 2,4,6,8-tetramethyloctacosanoic acid, and malonyl-CoA into n-fatty acids (Rainwater D. L., and Kolattukudy, P. E. (1983) J. Biol. Chem. 258, 2979-2985). The presence of several fatty acid synthases with differences in substrate preference and product chain length was detected in the crude extract of M. tuberculosis var. bovis. Among them was a mycocerosic acid synthase which was purified to homogeneity using anion-exchange chromatography, gel filtration, affinity chromatography, and hydroxylapatite chromatography. This fatty acid synthase elongated long-chain fatty acyl-CoA primers using methylmalonyl-CoA and NADPH to produce multimethyl-branched mycocerosic acids. The enzyme was specific for methylmalonyl-CoA and would not incorporate malonyl-CoA into fatty acids. It elongated n-C6 to n-C20 CoA esters to generate primarily the corresponding tetramethyl-branched mycocerosic acids. Exogenous [1-14C]acyl-CoA and trideuteromethylmalonyl-CoA were incorporated into the multimethyl-branched fatty acids. Dodecyl sulfate electrophoresis showed that the enzyme had a molecular weight of 238,000, whereas gel filtration showed a native molecular weight of 490,000, indicating that the enzyme is composed of two monomers of identical molecular weight. The enzyme contained an acyl carrier protein-like segment as indicated by incorporation of [1-14C] pantothenate into the 238-kDa protein and production of 1 mol of taurine/mol of the monomer upon hydrolysis of performic acid-oxidized enzyme. It is concluded that the mycocerosic acid synthase is a multifunctional enzyme similar to the well-characterized multifunctional fatty acid synthases except for the substrate specificity.