Molecular cloning and sequencing of cDNAs encoding the entire rat fatty acid synthase

Characterization of the inhibitory effect of growth hormone on primary preadipocyte differentiation

GH exerts adipogenic activity in several preadipocyte cell lines, whereas in primary rat preadipocytes, GH has an antiadipogenic activity. To better understand the molecular mechanism involved in adipocyte differentiation, the expression of adipocyte-specific genes was analyzed in differentiating preadipocytes in response to GH. We found that the expression of both adipocyte determination and differentiation factor 1 (ADD1) and peroxisome proliferator activated receptor gamma(PPARgamma) was induced in preadipocytes during differentiation. In the presence of GH, which markedly inhibited triglyceride accumulation, no reduction in the expression level of ADD1 was observed in response to GH, whereas there was a 50% reduction in the expression of PPARgamma. The DNA binding activity of the PPARgamma/retinoid X receptor-alpha(RXRalpha) to the ARE7 element from the aP2 gene was also reduced by approximately 50% in response to GH. GH inhibited the expression of late markers of adipocyte differentiation, fatty acid synthase, aP2, and hormone-sensitive lipase by 70-80%. The antiadipogenic effect of GH was not affected by the mitogen-activated protein (MAP) kinase/ extracellular-regulated protein (ERK) kinase inhibitor PD 98059, indicating that the mitogen-activated protein kinase pathway was not involved in GH inhibition of preadipocyte differentiation. The expression of preadipocyte factor-1/fetal antigen 1 was decreased during differentiation, and GH treatment prevented this down-regulation of Pref1/FA1. A possible role for Pref-1/FA1 in mediating the antiadipogenic effect of GH was indicated by the observation that FA1 inhibited differentiation as effectively as GH. These data suggest that GH exerts its inhibitory activity in adipocyte differentiation at a step after the induction of ADD1 but before the induction of genes required for terminal differentiation.

Differential affinity labeling of the two subunits of the homodimeric animal fatty acid synthase allows isolation of heterodimers consisting of subunits that have been independently modified

To explore the domain interactions that are required for catalytic activity of the multifunctional, homodimeric fatty acid synthase (FAS), we have formulated a strategy that allows isolation of modified dimers containing independently mutated subunits. Either a hexahistidine or a FLAG octapeptide tag was incorporated into the FAS at either the amino terminus, within an internal noncatalytic domain, or at the carboxyl terminus. The presence of the tags had no effect on the activity of the wild-type FAS. His-tagged dimers were mixed with FLAG-tagged dimers, and the subunits were randomized to produce a mixture of His-tagged homodimers, FLAG-tagged homodimers, and doubly tagged heterodimers. The doubly tagged heterodimers could be purified to homogeneity by chromatography on an anti-FLAG immunoaffinity column followed by a metal ion chelating column. This procedure for isolation of FAS heterodimers was utilized to determine whether the two centers for fatty acid synthesis in the FAS dimer can function independently of each other. Doubly tagged heterodimers, consisting of one wild-type subunit and one subunit in which the thioesterase activity had been eliminated, either by mutation or by treatment with phenylmethanesulfonyl fluoride, have 50% of the wild-type thioesterase activity and, in the presence of substrates, accumulate a long chain fatty acyl moiety on the modified subunit, thus blocking further substrate turnover at this center. Nevertheless, the ability of the heterodimer to synthesize fatty acids is also 50% of the wild-type FAS, demonstrating that an individual center for fatty acid synthesis has the same activity when paired with either a functional or nonfunctional catalytic center.

A second type-I PKS gene cluster isolated from Streptomyces hygroscopicus ATCC 29253, a rapamycin-producing strain

Analysis of a 32.8-kb segment of DNA from the rapamycin (Rp) producer, Streptomyces hygroscopicus ATCC 29253, revealed a new type-I polyketide synthase (PKS) cluster consisting of four open reading frames (ORF 1-4), each encoding a single PKS module. The four ORFs are transcribed in the same direction and are flanked by several smaller ORFs (ORF 5-9), which may be related to the PKS cluster. The first PKS-containing ORF has a ligase domain at the N-terminus of the polypeptide. This domain has 55% aa identity to the CoA ligase domain of the Rp PKS (Schwecke et al., 1995. Proc. Natl. Acad. Sci. 92, 7839-7843) which is also encoded in this strain (Lowden et al., 1996. Angew. Chem. Int. Ed. Engl. 35, 2249-2251). ORF5 (340 aa) and ORF6 (924 aa) were found to be homologous to RapK (41% aa identity) and RapH (35% aa identity), which are hypothesized to be a pteridine-dependent dioxygenase and a regulatory protein, respectively (Molnar et al., 1996. Gene 169, 1-7). In addition, ORF7 (391 aa) was found to have up to 42% aa identity to a number of plant 3-deoxy-D-arabino-heptulosonate-7-phosphate synthases (DAHPS) and 47% aa identity to PhzF, a bacterial DAHPS involved in phenazine antibiotic synthesis. The proximity of the DAHPS-encoding gene to the PKS cluster containing a Rp-like ligase domain suggests that a derivative of shikimate may be used as the PKS starter. ORF8 (283 aa) was found to have homology (32% aa identity) to a Synechocystis sp. gene of unknown function. The N-terminal portion of ORF9 was found to be similar to a tetracycline 6-hydroxylase (34% aa identity) from Streptomyces aureofaciens.

Identification and characterization of the niddamycin polyketide synthase genes from Streptomyces caelestis

The genes encoding the polyketide synthase (PKS) portion of the niddamycin biosynthetic pathway were isolated from a library of Streptomyces caelestis NRRL-2821 chromosomal DNA. Analysis of 40 kb of DNA revealed the presence of five large open reading frames (ORFs) encoding the seven modular sets of enzymatic activities required for the synthesis of a 16-membered lactone ring. The enzymatic motifs identified within each module were consistent with those predicted from the structure of niddamycin. Disruption of the second ORF of the PKS coding region eliminated niddamycin production, demonstrating that the cloned genes are involved in the biosynthesis of this compound.

Human fatty acid synthase: assembling recombinant halves of the fatty acid synthase subunit protein reconstitutes enzyme activity

Our model of the native fatty acid synthase (FAS) depicts it as a dimer of two identical multifunctional proteins (Mr approximately 272,000) arranged in an antiparallel configuration so that the active Cys-SH of the beta-ketoacyl synthase of one subunit (where the acyl group is attached) is juxtaposed within 2 A of the pantetheinyl-SH of the second subunit (where the malonyl group is bound). This arrangement generates two active centers for fatty acid synthesis and predicts that if we have two appropriate halves of the monomer, we should be able to reconstitute an active fatty acid-synthesizing site. We cloned, expressed, and purified catalytically active thioredoxin (TRX) fusion proteins of the NH2-terminal half of the human FAS subunit protein (TRX-hFAS-dI; residues 1-1,297; Mr approximately 166) and of the C-terminal half (TRX-hFAS-dII-III; residues 1,296-2,504; Mr approximately 155). Adding equivalent amounts of TRX-hFAS-dI and TRX-hFAS-dII-III to a reaction mixture containing acetyl-CoA, malonyl-CoA, and NADPH resulted in the synthesis of long-chain fatty acids. The rate of synthesis was dependent upon the presence of both recombinant proteins and reached a constant level when they were present in equivalent amounts, indicating that the reconstitution of an active fatty acid-synthesizing site required the presence of every partial activity associated with the subunit protein. Analyses of the product acids revealed myristate to be the most abundant with small amounts of palmitate and stearate, possibly because of the way the fused recombinant proteins interacted with each other so that the thioesterase hydrolyzed the acyl group in its myristoyl state. The successful reconstitution of the human FAS activity from its domain I and domains II and III fully supports our model for the structure-function relationship of FAS in animal tissues.

Characterization of a novel microsomal fatty acid synthetase (FAS) compared to a cytosolic FAS in the housefly, Musca domestica

A novel membrane-bound fatty acid synthetase (FAS) associated with the microsomal fraction from the housefly, Musca domestica, was solubilized and purified to homogeneity. The microsomal FAS was solubilized by 0.75 M KCl in phosphate buffer and was purified to homogeneity by the sequential use of ammonium sulfate precipitation followed by Sepharose CL-6B, DEAE Sephacel and Red Agarose (dye ligand affinity) chromatography. The specific activity of the microsomal FAS was increased 1,440-fold to 6,522 U/mg during purification. The cytosolic FAS from the housefly was also purified by similar methods and the specific activity increased 183-fold to 7,533 U/mg. The relative molecular mass of the microsomal and cytosolic FAS are 419 +/- 22 kDa and 405 +/- 18 kDa, respectively, for the dimers as determined by gel permeation chromatography. The microsomal and the cytosolic FAS yield different tryptic digestion maps and have slightly different amino acid compositions, which demonstrate structural differences between the two FASs. In addition, there are differences between the two FASs in their kinetic characteristics and their ability to incorporate methylmalonylCoA into the growing fatty acyl chain.

Acyltransferase domain substitutions in erythromycin polyketide synthase yield novel erythromycin derivatives

The methylmalonyl coenzyme A (methylmalonyl-CoA)-specific acyltransferase (AT) domains of modules 1 and 2 of the 6-deoxyerythronolide B synthase (DEBS1) of Saccharopolyspora erythraea ER720 were replaced with three heterologous AT domains that are believed, based on sequence comparisons, to be specific for malonyl-CoA. The three substituted AT domains were "Hyg" AT2 from module 2 of a type I polyketide synthase (PKS)-like gene cluster isolated from the rapamycin producer Streptomyces hygroscopicus ATCC 29253, "Ven" AT isolated from a PKS-like gene cluster of the pikromycin producer Streptomyces venezuelae ATCC 15439, and RAPS AT14 from module 14 of the rapamycin PKS gene cluster of S. hygroscopicus ATCC 29253. These changes led to the production of novel erythromycin derivatives by the engineered strains of S. erythraea ER720. Specifically, 12-desmethyl-12-deoxyerythromycin A, which lacks the methyl group at C-12 of the macrolactone ring, was produced by the strains in which the resident AT1 domain was replaced, and 10-desmethylerythromycin A and 10-desmethyl-12-deoxyerythromycin A, both of which lack the methyl group at C-10 of the macrolactone ring, were produced by the recombinant strains in which the resident AT2 domain was replaced. All of the novel erythromycin derivatives exhibited antibiotic activity against Staphylococcus aureus. The production of the erythromycin derivatives through AT replacements confirms the computer predicted substrate specificities of "Hyg" AT2 and "Ven" AT and the substrate specificity of RAPS AT14 deduced from the structure of rapamycin. Moreover, these experiments demonstrate that at least some AT domains of the complete 6-deoxyerythronolide B synthase of S. erythraea can be replaced by functionally related domains from different organisms to make novel, bioactive compounds.

Rat adipose tissue amino acid metabolism in vivo as assessed by microdialysis and arteriovenous techniques

In fed, anesthetized rats, microdialysis demonstrated a net release of glycerol, glutamine, serine, tyrosine, and taurine and a net uptake of glutamate, aspartate, glycine, and arginine across the inguinal adipose depot. However, the results also indicated excessive proteolysis associated with implantation of the microdialysis probe, and a novel arteriovenous difference technique was developed. Arteriovenous difference across the inguinal fat pat demonstrated a net uptake of glucose and a net release of lactate and glycerol. Starvation (48 h) resulted in higher rates of glycerol and lactate release with lower rates of glucose uptake. A net uptake of triacylglycerol was seen in starved-refed animals. Net glutamine, tyrosine, and taurine release was seen in fed and starved animals, but in starved-refed animals taurine and serine were the only amino acids showing significant release. No significant net uptake or release of ammonia, pyruvate, or alanine was observed. These experiments confirm that adipose tissue is a site of glutamine synthesis and suggest that the principal substrates are derived from intracellular proteolysis. The results also demonstrate the viability of an arteriovenous difference technique for the study of adipose tissue in the rat.

Animal fatty acid synthase: functional mapping and cloning and expression of the domain I constituent activities

Animal fatty acid synthase (FAS; EC 2.3.1.85) is a homodimer of a multifunctional subunit protein and catalyzes the synthesis of palmitate from acetyl-CoA, malonyl-CoA, and NADPH. The subunit (Mr approximately 270,000) carries seven distinct component activities and a site for the prosthetic group 4'-phosphopantetheine (acyl carrier protein). Based on proteolytic mapping, the organization of the activity domains along the subunit polypeptide from the N terminus is as follows: beta-ketoacyl synthase, acetyl and malonyl transacylases, beta-hydroxyacyl dehydratase, enoyl reductase, beta-ketoacyl reductase, acyl carrier protein, and thioesterase. By comparing the amino acid sequences of the chicken, rat, and human synthases, we found that kallikrein cleavage sites occur in the least conserved regions of the FAS polypeptide subunit. Determining the amino acid sequences of the N-terminal end of the major kallikrein cleavage peptides helped delineate the most likely boundaries of the component activities in the cDNA-derived amino acid sequence. To confirm this organization, we cloned the chicken FAS cDNA coding for domain I and expressed it in Escherichia coli as a maltose-binding fusion protein. The isolated recombinant protein contained the activities of the acetyl and malonyl transacylases and the beta-hydroxyacyl dehydratase. Based on the boundaries of the acetyl and malonyl transacylases and the beta-hydroxyacyl dehydratase, we also cloned the appropriate cDNA fragments encoding the domains that contain the transacylases and the dehydratase in pET vectors and expressed them in E. coli as thioredoxin-6xHis fusion proteins. The purified recombinant proteins contained, respectively, the activities of the acetyl and malonyl transacylases and the dehydratase. These results not only confirmed the order of the component activities in domain I, but also paved the way for successful expression and characterization of the remaining activities.

p260/270 expressed in embryonic abdominal leg cells of Bombyx mori can transfer palmitate to peptides

During the study on the mechanisms of abdominal leg development in the silkworm Bombyx mori, we found that a high molecular mass protein (p260/270) was expressed specifically in abdominal leg cells during early embryonic stages and disappeared by a late embryonic stage. p260/270 consists of two polypeptides with molecular masses of 260 and 270 kDa. We have established a purification procedure for p260/270 and have raised an antibody against p260/270. Immunoblot analysis of the ECa/ECa (additional crescent) and EN/EN (new additional crescent) mutants, which lack the Bombyx abdominal-A gene and therefore do not express abdominal legs, demonstrated that the two mutants lacked p260/270. Therefore, we speculate that the expression of p260/270 may be regulated by the Bombyx abdominal-A gene. cDNA cloning and sequencing demonstrated that p260 and p270 have structures similar to that of rat fatty-acid synthase, which synthesizes palmitate. Most of the enzymatic domains for palmitate synthesis were well conserved in the amino acid sequences of p260 and p270, while the thioesterase domains of p260 and p270 were less homologous to that of rat fatty-acid synthase. Purified p260/270 can transfer palmitate to cysteine residues of synthetic peptides in vitro. We propose that p260/270 may be involved in protein palmitoylation and may function in abdominal leg development.

Regulatory elements in the first intron of the rat fatty acid synthase gene

Sequence elements have been identified within the 1.2 kb-long first intron of the fatty acid synthase (FAS) gene that mediate both positive and negative effects on transcription. The negative regulatory element, when positioned downstream of either the FAS or simian virus 40 promoter, down-regulates the expression of a coupled reporter gene in an orientation-dependent manner. Sequences mediating this effect have been mapped, by deletion mutagenesis, to two regions approximately within nucleotides +405 to +768 and +924 to +1083. Both regions contain sequence elements that are strongly protected from DNase I digestion by nuclear extracts prepared from liver, but not by those prepared from spleen. The results of run-on assays performed with nuclei derived from tissues that express FAS at either high or low levels indicate that the different rates of transcription of the endogenous FAS gene result from differences in the extent of initiation, so it is unlikely that the negative effect is caused by transcriptional pausing in the first intron. The positive element maps to nt +292 to +297 and corresponds to an authentic binding site for upstream stimulatory factor (USF). This USF-binding element can up-regulate transcription from a heterologous promoter in a position- and orientation-independent manner. However, in the context of the entire FAS first intron, the effect of the USF-binding site is masked unless the effect of the negative elements is ablated by mutagenesis. These results suggest that the dominant negative element of the first intron may play a role in determining the tissue-specific expression of the FAS gene.

Phosphatidylinositol 3-kinase is a requirement for insulin-like growth factor I-induced differentiation, but not for mitogenesis, in fetal brown adipocytes

In the present study we have examined the role of phosphatidylinositol 3-kinase (PI 3-kinase) in the insulin-like growth factor I (IGF-I)-signaling pathways involved in differentiation and in mitogenesis in fetal rat brown adipocytes. Activation of PI 3-kinase in response to IGF-I was markedly inhibited by two PI 3-kinase inhibitors (wortmannin and LY294002) in a dose-dependent manner. IGF-I-stimulated glucose uptake was also inhibited by both compounds. The expression of adipogenic-related genes such as fatty acid synthase, malic enzyme, glycerol 3-phosphate dehydrogenase, and acetylcoenzyme A carboxylase induced by IGF-I was totally prevented in the presence of IGF-I and any of those inhibitors, resulting in a marked decrease of the cytoplasmic lipid content. Moreover, the expression of the thermogenic marker uncoupling protein induced by IGF-I was also down-regulated in the presence of wortmannin/LY294002. IGF-I-induced adipogenic- and thermogenic-related gene expression was only partly inhibited by the p70S6k inhibitor rapamycin. In addition, pretreatment of brown adipocytes with either wortmannin or LY294002, but not with rapamycin, blocked protein kinase C zeta activation by IGF-I. In contrast, IGF-I-induced fetal brown adipocyte proliferation was PI 3-kinase-independent. Our results show for the first time an essential requirement of PI 3-kinase in the IGF-I-signaling pathways leading to fetal brown adipocyte differentiation, but not leading to mitogenesis. In addition, protein kinase C zeta seems to be a signaling molecule also involved in the IGF-I differentiation pathways downstream from PI 3-kinase.

Acute and reversible fatty metamorphosis of cultured rat hepatocytes

Rat hepatocyte cell line M6, B347 and J525, among which only M6 is slightly deviated from diploidy, showed marked microvesicular fatty metamorphosis in response to treatment with Tweens at concentrations of 0.05-0.025% in Eagle's minimum essential medium (MEM). Within 24 h treated cells became fatty at 100% in frequency and filled with small lipid droplets, as revealed by fat staining or at the ultrastructural level. Fatty hepatocytes, however, took again non-fatty morphology 72 h after withdrawal of Tweens from the culture medium. Growth of the cell exhibited mild retardation at the early phase of the treatment but almost similar cell density to that of control cells was achieved 24 h after the treatment. Other detergents without fatty-acid moiety, including NP-40, triton X-100, sodium deoxycholic acid and sodium cholic acid, were ineffective to induce fatty change. Oleate, a fatty-acid moiety of Tween 80 or 85, and linolate caused reversible fatty metamorphosis of the cell lines at concentrations of 1.9 x 10(4) mol/L or more and 3.8 x 10(4) mol/L or more, respectively. Ethanol induced mild steatosis of the cell lines and enhanced fatty change by linolate. Hepatic fatty acid-binding protein was not detected in the cell lines before or after the induction of fatty change. These results indicate that fatty acid itself is directly incorporated in cultured rat hepatocytes and expelled 3 days later without apparent cell degeneration.

New alcohol dehydrogenases for the synthesis of chiral compounds

The enantioselective reduction of carbonyl groups is of interest for the production of various chiral compounds such as hydroxy acids, amino acids, hydroxy esters, or alcohols. Such products have high economic value and are most interesting as additives for food and feed or as building blocks for organic synthesis. Enzymatic reactions or biotransformations with whole cells (growing or resting) for this purpose are described. Although conversions with whole cells are advantageous with respect to saving expensive isolation of the desired enzymes, the products often lack high enantiomeric excess and the process results in low time-space-yield. For the synthesis of chiral alcohols, only lab-scale syntheses with commercially available alcohol dehydrogenases have been described yet. However, most of these enzymes are of limited use for technical applications because they lack substrate specificity, stability (yeast ADH) or enantioselectivity (Thermoanaerobium brockii ADH). Furthermore, all enzymes so far described are forming (S)-alcohols. Quite recently, we found and characterized several new bacterial alcohol dehydrogenases, which are suited for the preparation of chiral alcohols as well as for hydroxy esters in technical scale. Remarkably, of all these novel ADHs the (R)-specific enzymes were found in strains of the genus Lactobacillus. Meanwhile, these new enzymes were characterized extensively. Protein data (amino acid sequence, bound cations) confirm that these catalysts are novel enzymes. (R)-specific as well as (S)-specific ADHs accept a broad variety of ketones and ketoesters as substrates. The applicability of alcohol dehydrogenases for chiral syntheses as an example for the technical use of coenzyme-dependent enzymes is demonstrated and discussed in this contribution. In particular NAD-dependent enzymes coupled with the coenzyme regeneration by formate dehydrogenase proved to be economically feasible for the production of fine chemicals.

Cloning and expression of the multifunctional human fatty acid synthase and its subdomains in Escherichia coli

We engineered a full-length (8.3-kbp) cDNA coding for fatty acid synthase (FAS; EC 2.3.1.85) from the human brain FAS cDNA clones we characterized previously. In the process of accomplishing this task, we developed a novel PCR procedure, recombinant PCR, which is very useful in joining two overlapping DNA fragments that do not have a common or unique restriction site. The full-length cDNA was cloned in pMAL-c2 for heterologous expression in Escherichia coli as a maltose-binding protein fusion. The recombinant protein was purified by using amylose-resin affinity and hydroxylapatite chromatography. As expected from the coding capacity of the cDNA expressed, the chimeric recombinant protein has a molecular weight of 310,000 and reacts with antibodies against both human FAS and maltose-binding protein. The maltose-binding protein-human FAS (MBP-hFAS) catalyzed palmitate synthesis from acetyl-CoA, malonyl-CoA, and NADPH and exhibited all of the partial activities of FAS at levels comparable with those of the native human enzyme purified from HepG2 cells. Like the native HepG2 FAS, the products of MBP-hFAS are mainly palmitic acid (> 90%) and minimal amounts of stearic and arachidic acids. Similarly, a human FAS cDNA encoding domain I (beta-ketoacyl synthase, acetyl-CoA and malonyl-CoA transacylases, and beta-hydroxyacyl dehydratase) was cloned and expressed in E. coli using pMAL-c2. The expressed fusion protein, MBP-hFAS domain I, was purified to apparent homogeneity (M(r) 190,000) and exhibited the activities of the acetyl/malonyl transacylases and the beta-hydroxyacyl dehydratase. In addition, a human FAS cDNA encoding domains II and III (enoyl and beta-ketoacyl reductases, acyl carrier protein, and thioesterase) was cloned in pET-32b(+) and expressed in E. coli as a fusion protein with thioredoxin and six in-frame histidine residues. The recombinant fusion protein, thioredoxin-human FAS domains II and III, that was purified from E. coli had a molecular weight of 159,000 and exhibited the activities of the enoyl and beta-ketoacyl reductases and the thioesterase. Both the MBP and the thioredoxin-His-tags do not appear to interfere with the catalytic activity of human FAS or its partial activities.

Expression in Escherichia coli and refolding of the malonyl-/acetyltransferase domain of the multifunctional animal fatty acid synthase

A cDNA encoding residues 429-815 of the multifunctional rat fatty acid synthase has been expressed in Escherichia coli and the recombinant protein refolded in vitro as a catalytically active malonyl-/acetyltransferase. Kinetic properties of the refolded recombinant enzyme were indistinguishable from those of a transferase preparation derived from the natural fatty acid synthase by limited proteolysis, indicating that the transferase domain is capable of folding correctly as an independent protein. Replacement of the active site Ser-581 (full-length fatty acid synthase numbering) with alanine completely eliminated catalytic activity, whereas replacement with cysteine resulted in retention of about 1% activity. The wild type transferase was extremely susceptible to inhibition by diethyl pyrocarbonate, and protection against inhibition was afforded by both malonyl- and acetyl-CoA. Replacement of the highly conserved residue His-683 with Ala reduced activity by 99.95%, and the residual activity was relatively unaffected by diethyl pyrocarbonate. The rate of acylation of the active site serine residue was also reduced by several orders of magnitude in the His-683 --> Ala mutant. These results indicate that His-683 plays an essential role in catalysis, likely by accepting a proton from the active site serine, thus increasing its nucleophilicity.

Insulin-like growth factor I and insulin induce adipogenic-related gene expression in fetal brown adipocyte primary cultures

Fetal rat brown adipocytes show high-affinity binding sites for both insulin-like growth factor I (IGF-I) and insulin. Cell culture for 24 h in the presence of IGF-I or insulin, independently, up-regulated the mRNA expression of adipogenic-related genes, such as fatty acid synthase (FAS), glycerol-3-phosphate de-hydrogenase and insulin-regulated glucose transporter Glut4, and down-regulated the expression of phosphoenolpyruvate carboxykinase mRNA in a dose-dependent manner. Moreover, both IGF-I and insulin increased the FAS gene transcription rate at 2 h, producing a time-dependent accumulation of FAS mRNA. Furthermore IGF-I or insulin increased glucose uptake and lipid content throughout the 24 h culture period. Our results suggest that both IGF-I and insulin are major signals involved in initiating and/or maintaining the expression of adipogenic-related genes in fetal rat brown adipocytes.

Overproduction of cholesterol and fatty acids causes massive liver enlargement in transgenic mice expressing truncated SREBP-1a

The NH2-terminal domain of sterol-regulatory element binding protein-1a (SREBP-1a) activates transcription of genes encoding enzymes of cholesterol and fatty acid biosynthesis in cultured cells. This domain is synthesized as part of a membrane-bound precursor that is attached to the nuclear envelope and endoplasmic reticulum. In sterol-depleted cells a two-step proteolytic process releases this NH2-terminal domain, which enters the nucleus and activates transcription. Proteolysis is suppressed by sterols, thereby suppressing transcription. In the current experiments we produce transgenic mice that overexpress a truncated version of human SREBP-1a that includes the NH2-terminal domain but lacks the membrane attachment site. This protein enters the nucleus without a requirement for proteolysis, and therefore it cannot be down-regulated. Expression was driven by the phosphoenolpyruvate carboxykinase (PEPCK) promoter, which gives high level expression in liver. When placed on a low carbohydrate/high protein diet to induce the PEPCK promoter, the transgenic mice developed progressive and massive enlargement of the liver, owing to the engorgement of hepatocytes with cholesterol and triglycerides. The mRNAs encoding 3-hydroxy-3-methylglutaryl CoA (HMG CoA) synthase, HMG CoA reductase, squalene synthase, acetyl-CoA carboxylase, fatty acid synthase, and stearoyl-CoA desaturase-1 were all elevated markedly, as was the LDL receptor mRNA. The rates of cholesterol and fatty acid synthesis in liver were elevated 5- and 25-fold, respectively. Remarkably, plasma lipid levels were not elevated. The amount of white adipose tissue decreased progressively as the liver enlarged. These studies indicate that the NH2-terminal domain of SREBP-1a can produce major effects on lipid synthesis and storage in the liver.

Isolation and characterization of a gene affecting fatty acid elongation in Saccharomyces cerevisiae

Fatty acid elongation defective mutants were isolated from Saccharomyces cerevisiae by mutagenizing strains that were defective in fatty acid synthase (FAS) activity. Cells of the fatty acid synthase-defective strains can grow when supplemented with tetradecanoic acid (14:0) due to the presence of membrane bound elongation systems that can extend the 14 carbon fatty acid to longer chain species. After mutagenesis and rescue on medium containing a mixture of 14:0, 16:0 and 18:0, cells were screened for their inability to grow on medium containing only 14:0. From 150,000 colonies, four stable isolates were identified, all of which appear to represent the same complementation group. Gas chromatography of lipid extracts from mutant elo1-1 (designated as elongation defective) cells grown with long or medium chain fatty acids indicates that it fails to efficiently elongate (12, 13, or 14) carbon fatty acids. A gene disrupted fas2Delta::LEU2;elo1Delta::HIS3 mutant incorporates 14-18-carbon fatty acids into membrane lipids, indicating that fatty acid transport is not affected by the mutation. Molecular cloning and sequence analysis of the ELO1 gene suggests that the encoded protein is a membrane bound polypeptide that contains at least five potential membrane spanning regions and a presumptive NADPH binding site. Analysis of the ELO1 mRNA levels indicates that the gene is expressed in cells grown on fatty acid deficient medium. It is rapidly induced in wild type cells that are supplemented with 14:0 and is repressed when cells are supplied with 16- and 18-carbon fatty acids.

[Fatty acid synthases--strategic functions of multienzymes]

Several decades of biochemical research have led to our present detailed knowledge on cellular metabolism, is wealth of individual reactions, enzymes, and pathways, and their organization, interplay, and regulation. Although most metabolic reactions are simultaneous and occur in the same cell, they may also act independently of one other and without causing disturbing interferences. This demonstrates that the cellular interior is organized structurally and functionally and is not simply a "bag of enzymes." This organization ranges from distinct and functionally specialized organelles down to subtle or even hypothetical structures at the molecular level. The lowest level of structurally stable, supramolecular catalytic entities in the cell thus for known is that of the multienzyme complexes. Among the limited number of known multi-enzymes, fatty acid synthase is certainly one of the most complex and also best studied. Various structural and functional variants of this multienzyme system are known. These may be discussed in terms of specific requirements of the respective organisms.

Production of a polyhydroxyalkanoate biopolymer in insect cells with a modified eucaryotic fatty acid synthase

A novel pathway for the synthesis of poly-3-hydroxybutyrate has been engineered by simultaneous delivery of two genes into insect cells (Spodoptera frugiperda) by use of individual baculovirus vectors. This system includes expression of a dehydrase-domain mutant rat fatty acid synthase cDNA and the phbC gene encoding polyhydroxyalkanoate synthase from Alcaligenes eutrophus. The dehydrase-deficient fatty acid synthase provides de novo synthesis of R-(-)-3-hydroxybutyryl-coenzyme A as a premature termination product rather than palmityl-coenzyme A, the normal product of wild-type rat fatty acid synthase. High levels of this mutant multifunctional protein provide a suitable precursor pool of R-(-)-3-hydroxybutyryl-coenzyme A for conversion to poly-3-hydroxybutyrate in insect cells coexpressing the phbC gene product. This strategy for redesigning a poly-3-hydroxybutyrate biosynthetic pathway suggests a new method for generating structurally diverse polyhydroxyalkanoates by metabolic engineering.

Sterol regulation of fatty acid synthase promoter. Coordinate feedback regulation of two major lipid pathways

The gene encoding fatty acid synthase, the essential multi-functional enzyme of fatty acid biosynthesis, is shown to be regulated by cellular sterol levels similar to genes that encode important proteins of cholesterol metabolism. We show that expression of the endogenous FAS gene is repressed when regulatory sterols are included in the culture medium of HepG2 cells and that the FAS promoter is subject to similar regulation when fused to the luciferase reporter gene. Mutational studies demonstrate that sterol regulation is mediated by binding sites for the sterol regulatory element-binding protein (SREBP) and transcription factor Sp1, making it mechanistically similar to sterol regulation of the low density lipoprotein receptor gene. It is also demonstrated that SREBP and Sp1 synergistically activate the FAS promoter in Drosophila tissue culture cells, which lack endogenous Sp1. These experiments provide key molecular evidence that directly links the metabolism of fatty acids and cholesterol together.

Differentiation of rat brown adipocytes during late foetal development: role of insulin-like growth factor I

Rat brown adipocytes at day 22 of foetal development showed greater size, higher mitochondria content and larger amounts of lipids, as determined by flow cytometry, than 20-day foetal cells. Simultaneously, an inhibition on the percentage of brown adipocytes into S+G2/M phases of the cell cycle was observed between days 20 and 22 of foetal development. The expression of several adipogenesis-related genes, such as fatty acid synthase, malic enzyme, glucose-6-phosphate dehydrogenase and insulin-regulated glucose transporter, increased at the end of foetal life in brown adipose tissue. In addition, the lipogenic enzyme activities and the lipogenic flux increased during late foetal development, resulting in mature brown adipocytes showing a multilocular fat droplet phenotype. Concurrently, brown adipocytes induced the expression of the uncoupling protein (UP) mRNA and UP protein, as visualized by immunofluorescence. The three isoforms of CCAAT enhancer-binding proteins (C/EBPs) were expressed at the mRNA level in brown adipose tissue at day 20. C/EBP alpha decreased and C/EBP beta and delta increased their expression between days 20 and 22 of foetal development, respectively. Brown adipose tissue constitutively expressed insulin-like growth factor I (IGF-I) and IGF-I receptor (IGF-IR) mRNAs. Moreover, IGF-IR mRNA content increased between days 20 and 22 in parallel with the occurrence of tissue differentiation.

Human fatty acid synthase: properties and molecular cloning

Fatty acid synthase (FAS; EC 2.3.1.85) was purified to near homogeneity from a human hepatoma cell line, HepG2. The HepG2 FAS has a specific activity of 600 nmol of NADPH oxidized per min per mg, which is about half that of chicken liver FAS. All the partial activities of human FAS are comparable to those of other animal FASs, except for the beta-ketoacyl synthase, whose significantly lower activity is attributable to the low 4'-phosphopantetheine content of HepG2 FAS. We cloned the human brain FAS cDNA. The cDNA sequence has an open reading frame of 7512 bp that encodes 2504 amino acids (M(r), 272,516). The amino acid sequence of the human FAS has 79% and 63% identity, respectively, with the sequences of the rat and chicken enzymes. Northern analysis revealed that human FAS mRNA was about 9.3 kb in size and that its level varied among human tissues, with brain, lung, and liver tissues showing prominent expression. The nucleotide sequence of a segment of the HepG2 FAS cDNA (bases 2327-3964) was identical to that of the cDNA from normal human liver and brain tissues, except for a 53-bp sequence (bases 3892-3944) that does not alter the reading frame. This altered sequence is also present in HepG2 genomic DNA. The origin and significance of this sequence variance in the HepG2 FAS gene are unclear, but the variance apparently does not contribute to the lower activity of HepG2 FAS.

The Aspergillus parasiticus polyketide synthase gene pksA, a homolog of Aspergillus nidulans wA, is required for aflatoxin B1 biosynthesis

Aflatoxins comprise a group of polyketide-derived carcinogenic mycotoxins produced by Aspergillus parasiticus and Aspergillus flavus. By transformation with a disruption construct, pXX, we disrupted the aflatoxin pathway in A. parasiticus SRRC 2043, resulting in the inability of this strain to produce aflatoxin intermediates as well as a major yellow pigment in the transformants. The disruption was attributed to a single-crossover, homologous integration event between pXX and the recipient A. parasiticus genome at a specific locus, designated pksA. Sequence analysis suggest that pksA is a homolog of the Aspergillus nidulans wA gene, a polyketide synthase gene involved in conidial wall pigment biosynthesis. The conserved beta-ketoacyl synthase, acyltransferase and acyl carrier-protein domains were present in the deduced amino acid sequence of the pksA product. No beta-ketoacyl reductase and enoyl reductase domains were found, suggesting that pksA does not encode catalytic activities for processing beta-carbon similar to those required for long chain fatty acid synthesis. The pksA gene is located in the aflatoxin pathway gene cluster and is linked to the nor-1 gene, an aflatoxin pathway gene required for converting norsolorinic acid to averantin. These two genes are divergently transcribed from a 1.5 kb intergenic region. We propose that pksA is a polyketide synthase gene required for the early steps of aflatoxin biosynthesis.

Sterigmatocystin biosynthesis in Aspergillus nidulans requires a novel type I polyketide synthase

A filamentous fungus, Aspergillus nidulans, produces the carcinogenic mycotoxin sterigmatocystin (ST), which is a polyketide-derived secondary metabolite. A gene (pksST) encoding the ST polyketide synthase (PKSst) in A. nidulans was cloned, sequenced, and characterized. Large induced deletion mutants, which did not make ST or any ST intermediates, were used to identify genes associated with ST biosynthesis. Among the transcripts detected within the deletion region, which showed developmental expression with ST production, was a 7.2-kb transcript. Functional inactivation of the gene encoding the 7.2-kb transcript blocked production of ST and all ST intermediate substrates but did not affect transcription of the pathway genes, indicating that this gene was involved in a very early step of ST biosynthesis. These results also indicate that PKSst was not associated with activation of other ST genes. Sequencing of the region spanning this gene revealed that it encoded a polypeptide with a deduced length of 2,181 amino acids that had high levels of similarity to many of the known polyketide synthases and FASs. This gene, pksST, encodes a multifunctional novel type I polyketide synthase which has as active sites a beta-ketoacyl acyl carrier protein synthase, an acyltransferase, duplicated acyl carrier proteins, and a thioesterase, all of these catalytic sites may be multiply used. In addition, a 1.9-kb transcript, which also showed developmental expression, was mapped adjacent to pksST, and the sequence of this gene revealed that it encoded a cytochrome P-450 monooxygenase-like peptide.

A Sorangium cellulosum (myxobacterium) gene cluster for the biosynthesis of the macrolide antibiotic soraphen A: cloning, characterization, and homology to polyketide synthase genes from actinomycetes

A 40-kb region of DNA from Sorangium cellulosum So ce26, which contains polyketide synthase (PKS) genes for synthesis of the antifungal macrolide antibiotic soraphen A, was cloned. These genes were detected by homology to Streptomyces violaceoruber genes encoding components of granaticin PKS, thus extending this powerful technique for the identification of bacterial PKS genes, which has so far been applied only to actinomycetes, to the gram-negative myxobacteria. Functional analysis by gene disruption has indicated that about 32 kb of contiguous DNA of the cloned region contains genes involved in soraphen A biosynthesis. The nucleotide sequence of a 6.4-kb DNA fragment, derived from the region with homology to granaticin PKS genes, was determined. Analysis of this sequence has revealed the presence of a single large open reading frame beginning and ending outside the 6.4-kb fragment. The deduced amino acid sequence indicates the presence of a domain with a high level of similarity to beta-ketoacyl synthases that are involved in polyketide synthesis. Other domains with high levels of similarity to regions of known polyketide biosynthetic functions were identified, including those for acyl transferase, acyl carrier protein, ketoreductase, and dehydratase. We present data which indicate that soraphen A biosynthesis is catalyzed by large, multifunctional enzymes analogous to other bacterial PKSs of type I.

The Escherichia coli malonyl-CoA:acyl carrier protein transacylase at 1.5-A resolution. Crystal structure of a fatty acid synthase component

Endogenous fatty acids are synthesized in all organisms in a pathway catalyzed by the fatty acid synthase complex. In bacteria, where the fatty acids are used primarily for incorporation into components of cell membranes, fatty acid synthase is made up of several independent cytoplasmic enzymes, each catalyzing one specific reaction. The initiation of the elongation step, which extends the length of the growing acyl chain by two carbons, requires the transfer of the malonyl moiety from malonyl-CoA onto the acyl carrier protein. We report here the crystal structure (refined at 1.5-A resolution to an R factor of 0.19) of the malonyl-CoA specific transferase from Escherichia coli. The protein has an alpha/beta type architecture, but its fold is unique. The active site inferred from the location of the catalytic Ser-92 contains a typical nucleophilic elbow as observed in alpha/beta hydrolases. Serine 92 is hydrogen bonded to His-201 in a fashion similar to various serine hydrolases. However, instead of a carboxyl acid typically found in catalytic triads, the main chain carbonyl of Gln-250 serves as a hydrogen bond acceptor in an interaction with His-201. Two other residues, Arg-117 and Glu-11, are also located in the active site, although their function is not clear.

Heterologous expression of an engineered biosynthetic pathway: functional dissection of type II polyketide synthase components in Streptomyces species

Polyketides are an extensive class of secondary metabolites with diverse molecular structures and biological activities. A plasmid-based multicomponent polyketide synthase expression cassette was constructed using a subset of actinorhodin (act) biosynthetic genes (actI-orf1, actI-orf2, actI-orf3, actIII, actVII, and actIV) from Streptomyces coelicolor which specify the construction of the anthraquinone product aloesaponarin II, a molecule derived from acetyl coenzyme A and 7 malonyl coenzyme A extender units. This system was designed as an indicator pathway in Streptomyces parvulus to quantify polyketide product formation and to examine the functional significance of specific polyketide synthase components, including the act beta-ketoacyl synthase (beta-KS; encoded by actI-orf1 and actI-orf2) and the act cyclase/dehydrase (encoded by actVII-orf4). Site-directed mutagenesis of the putative active site Cys (to a Gln) in the actI-orf1 beta-KS product completely abrogated aloesaponarin II production. Changing the putative acyltransferase active-site Ser (to a Leu) located in the actI-orf1 beta-KS product led to significantly reduced but continued production of aloesaponarin II. Replacement of the expression cassette with one containing a mutant form of actI-orf2 gave no production of aloesaponarin II or any other detectable polyketide products. However, an expression cassette containing a mutant form of actVII-orf4 gave primarily mutactin with low-level production of aloesaponarin II.

Transient induction of fatty acid synthase in rat liver after removal of a peroxisome proliferator

Removal of a peroxisome proliferator from the diet triggered the degradation of peroxisomes and induced the transient expression of a 220 kDa soluble protein in rat liver. The 220 kDa protein was purified by conventional methods and analyzed by amino acid sequencing. A total of 99 amino acid residues in 4 lysylendopeptidase-digested peptides completely matched those in rat fatty acid synthase. The transient induction of fatty acid synthase mRNA during peroxisome degradation was confirmed by Northern blotting.

A super-family of medium-chain dehydrogenases/reductases (MDR). Sub-lines including zeta-crystallin, alcohol and polyol dehydrogenases, quinone oxidoreductase enoyl reductases, VAT-1 and other proteins

The protein super-family of medium-chain alcohol dehydrogenases (and glutathione-dependent formaldehyde dehydrogenase), polyol dehydrogenases, threonine dehydrogenase, archaeon glucose dehydrogenase, and eye lens reductase-active zeta-crystallins also includes Escherichia coli quinone oxidoreductase, Torpedo VAT-1 protein, and enoyl reductases of mammalian fatty acid and yeast erythronolide synthases. In addition, two proteins with hitherto unknown function are shown to belong to this super-family of medium-chain dehydrogenases and reductases (MDR). Alignment of zeta-crystallins/quinone oxidoreductases/VAT-1 reveals 38 strictly conserved residues, of which approximately half are glycine residues, including those at several space-restricted turn positions and critical coenzyme-binding positions in the alcohol dehydrogenases. This indicates a conserved three-dimensional structure at the corresponding parts of these distantly related proteins and a conserved binding of a coenzyme in the two proteins with hitherto unknown function, thus ascribing a likely oxidoreductase function to these proteins. When all forms are aligned, including enoyl reductases, a zeta-crystallin homologue from Leishmania and the two proteins with hitherto unknown function, only three residues are strictly conserved among the 106 proteins characterised within the superfamily, and significantly these residues are all glycines, corresponding to Gly66, Gly86 and Gly201 of mammalian class I alcohol dehydrogenase. Notably, these residues are located in different domains. Hence, a distant origin and divergent functions, but related forms and interactions, appear to apply to the entire chains of the many prokaryotic and eukaryotic members. Additionally, in the zeta-crystallins/quinone oxidoreductases, a highly conserved tyrosine residue is found. This residue, in the three-dimensional structure of the homologous alcohol dehydrogenase, is positioned at the subunit cleft that contains the active site and could therefore be involved in catalysis. If so, this residue and its role may resemble the pattern of a conserved tyrosine residue in the different family of short-chain dehydrogenases/reductases (SDR).

Fatty acid synthesis: a potential selective target for antineoplastic therapy

OA-519 is a prognostic molecule found in tumor cells from breast cancer patients with markedly worsened prognosis. We purified OA-519 from human breast carcinoma cells, obtained its peptide sequence, and unambiguously identified it as fatty acid synthase through sequence homology and enzymology. Tumor fatty acid synthase is an approximately 270-kDa polypeptide which specifically abolished immunostaining of human breast cancers by anti-OA-519 antibodies. Tumor fatty acid synthase oxidized NADPH in a malonyl-CoA-dependent fashion and synthesized fatty acids composed of 80% palmitate, 10% myristate, and 10% stearate from acetyl-CoA, malonyl-CoA, and NADPH with a specific activity of 624 nmol of NADPH oxidized per min per mg. Tumor cell lines with elevated fatty acid synthase showed commensurate increases in incorporation of [U-14C]acetate into acylglycerols demonstrating that fatty acid synthase increases occur in the context of overall increases in endogenous fatty acid synthesis. Cerulenin inhibited acylglycerol synthesis in tumor cells and fibroblast controls in a dose-dependent fashion and also caused a growth inhibition which generally paralleled the level of endogenous fatty acid synthesis. Supraphysiologic levels of palmitate, 14 microM in dimethyl sulfoxide, significantly reversed the growth inhibition caused by cerulenin at concentrations of up to 5 micrograms/ml, indicating that cerulenin-mediated growth inhibition was due to fatty acid synthase inhibition.

Analysis of type II polyketide beta-ketoacyl synthase specificity in Streptomyces coelicolor A3(2) by trans complementation of actinorhodin synthase mutants

Complementation of defined actinorhodin beta-ketoacyl synthase (KS) mutants by various other KS genes suggested that the ORF1-encoded KS may be relatively generalized in function, whereas the ORF2-encoded KS component may provide specificity in polyketide chain construction. Evidence for differential temporal-spatial expression of the actinorhodin and spore pigment KSs in Streptomyces coelicolor was obtained.

Construction of a cDNA encoding the multifunctional animal fatty acid synthase and expression in Spodoptera frugiperda cells using baculoviral vectors

A cDNA encoding the 2505-residue multifunctional rat fatty acid synthase has been constructed and expressed as a catalytically active protein in Spodoptera frugiperda (Sf9) cells using Autographa californica nuclear polyhedrosis virus (baculovirus). The 7.5 kb cDNA was engineered by the amplification and sequential splicing together of seven fragments contained in overlapping cDNAs that collectively spanned the entire rat fatty acid synthase coding sequence. The full-length cDNA was cloned into a baculoviral transfer vector and used together with linearized baculoviral DNA to co-transfect Sf9 cells. Recombinant viral clones were purified and identified by Western blotting. The recombinant fatty acid synthase was expressed maximally 2 days after infection of the Sf9 cells, constituting up to 20% of the soluble cytoplasm, and could be conveniently separated from the insect host fatty acid synthase by high-performance anion-exchange chromatography. The catalytic properties of the purified recombinant fatty acid synthase are indistinguishable from those of the best preparations of the natural protein obtained from rat liver. These results indicate that, in the insect cell host, all seven catalytic components of the 2505-residue recombinant fatty acid synthase fold correctly, the acyl-carrier-protein domain is appropriately phosphopantetheinylated post-translationally, and the multifunctional polypeptide forms catalytically competent dimers. Thus the baculoviral system appears to be well suited for the expression of specific fatty acid synthase mutants that can be used to explore the mechanism by which the seven domains of this multifunctional homodimer co-operate in the biosynthesis of fatty acids.

Isolation and characterization of two Brassica napus embryo acyl-ACP thioesterase cDNA clones

Acyl-ACP thioesterases are involved in regulating chain termination of fatty acid biosynthesis in plant systems. Previously, acyl-ACP thioesterase purified from Brassica napus seed tissue has been shown to have a high preference for hydrolysing oleoyl-ACP. Here, oligonucleotides derived from B. napus oleoyl-ACP thioesterase protein sequence data have been used to isolate two acyl-ACP thioesterase clones from a B. napus embryo cDNA library. The two clones, pNL2 and pNL3, contain 1642 bp and 1523 bp respectively and differ in the length of their 3' non-coding regions. Both cDNAs contain open reading frames of 366 amino acids which encode for 42 kDa polypeptides. Mature rape thioesterase has an apparent molecular weight of 38 kDa on SDS-PAGE and these cDNAs therefore encode for precursor forms of the enzyme. This latter finding is consistent with the expected plastidial location of fatty acid synthase enzymes. Northern blot analysis shows thioesterase mRNA size to be ca. 1.6 kb and for the thioesterase genes to be highly expressed in seed tissue coincident with the most active phase of storage lipid synthesis. There is some sequence heterogeneity between the two cDNA clones, but overall they are highly homologous sharing 95.7% identity at the DNA level and 98.4% identity at the amino acid level. Some sequence heterogeneity was also observed between the deduced and directly determined thioesterase protein sequences. Consistent with the observed sequence heterogeneity was Southern blot data showing B. napus thioesterase to be encoded by a small multi-gene family.

The presence of a novel type of surface polysaccharide in Rhizobium meliloti requires a new fatty acid synthase-like gene cluster involved in symbiotic nodule development

Bacterial exopolysaccharide (EPS) and lipopolysaccharide (LPS) molecules have been shown to play important roles in plant-bacterium interactions. Here we have demonstrated that the fix-23 loci, which compensate for exo mutations during symbiotic nodule development, are involved in the production of a novel polysaccharide that is rich in 3-deoxy-D-manno-2-octulosonic acid (Kdo) but is not the classical LPS. This molecule is likely to be a surface antigen since antiserum to whole Rhizobium meliloti cells reacts strongly with it, and since mutations in fix-23 result in an inability to produce this polysaccharide and to bind bacteriophage 16-3. It is likely that this Kdo-rich polysaccharide is analogous to certain Escherichia coli K-antigens which are anchored to the membrane via a phospholipid moiety. DNA sequence analysis of one gene cluster of this region revealed that the predicted protein products of six genes exhibit a high degree of homology and similar organization to those of the rat fatty acid synthase multifunctional enzyme domains.

Characterization of fatty acid synthase monomers restrained from reassociating by immobilization to a solid support

The controversial question as to whether the ketoreductase activity of the animal fatty acid synthase is lost on dissociation of the homodimer has been addressed by using immobilized subunits which cannot reassociate under the conditions of assay. Ketoreductase activity, assessed with the model substrate S-acetoacetyl-N-acetylcysteamine, was identical in immobilized monomers and dimers, exhibiting normal Michaelis-Menten kinetics with Km values in the millimolar range. When acetoacetyl-CoA was used as a substrate, however, biphasic kinetics were observed in the case of the dimer, with estimated Km values in the micro- and milli-molar ranges, but only the high-Km reaction was observed with the monomer. Thus when the ketoreductase activities of the monomer and dimer are assessed with acetoacetyl-CoA at concentrations sufficient to saturate only the low-Km reaction, it appears that the ketoreductase activity towards acetoacetyl-CoA is lost upon dissociation. Reduction of acetoacetyl-CoA via the low-Km pathway is CoA-dependent, indicating that acetoacetyl-CoA can react with the dimer by two mechanisms: a high-Km pathway analogous to that utilized by model substrates and a low-Km pathway in which substrate and product are transferred between acyl-CoA and acyl-enzyme forms. The results indicate that the ketoreductase activity per se is unaffected by subunit dissociation and are consistent with a model in which the transfer of substrate from CoA ester to the acyl-carrier-protein domain necessitates juxtaposition of the transferase active-site serine residue of one subunit and the phosphopantetheine moiety of the adjacent subunit.

Bacterial cholesterol oxidases are able to act as flavoprotein-linked ketosteroid monooxygenases that catalyse the hydroxylation of cholesterol to 4-cholesten-6-ol-3-one

A new metabolite of cholesterol was found in reaction mixtures containing cholesterol or 4-cholesten-3-one as a substrate and extra- or intracellular protein extracts from recombinant Streptomyces lividans and Escherichia coli strains carrying cloned DNA fragments of Streptomyces sp. SA-COO, the producer of Streptomyces cholesterol oxidase. The new metabolite was identified as 4-cholesten-6-ol-3-one based on comparisons of its high-performance liquid chromatography, gas chromatography/mass spectrometry, infrared and proton-nuclear magnetic resonance spectra with those of an authentic standard. Genetic analyses showed that the enzyme responsible for the production of 4-cholesten-6-ol-3-one is cholesterol oxidase encoded by the choA gene. Commercially purified cholesterol oxidase (EC 1.1.3.6.) of a Streptomyces sp., as well as of Brevibacterium sterolicum and a Pseudomonas sp., and a highly purified recombinant Streptomyces cholesterol oxidase were also able to catalyse the 6-hydroxylation reaction. Hydrogen peroxide accumulating in the reaction mixtures as a consequence of the 3 beta-hydroxysteroid oxidase activity of the enzyme was shown to have no role in the formation of the 6-hydroxylated derivative. We propose a possible scheme of a branched reaction pathway for the concurrent formation of 4-cholesten-3-one and 4-cholesten-6-ol-3-one by cholesterol oxidase, and the observed differences in the rate of formation of the 6-hydroxy-ketosteroid by the enzymes of different bacterial sources are also discussed.