Intersubunit transfer of fatty acyl groups during fatty acid reduction

The impact of LuxF on light intensity in bacterial bioluminescence

The enzymes involved in bacterial bioluminescence are encoded in the lux operon with a conserved gene order of luxCDABEG. Some photobacterial strains carry an additional gene, termed luxF, which produces the LuxF protein, whose function and influence on bacterial bioluminescence is still uncertain. The LuxF protein binds the flavin derivative 6-(3'-(R)-myristyl)-flavin mononucleotide (myrFMN), which is generated as a side product in the luciferase-catalyzed reaction. This study utilized an Escherichia coli (E. coli) based lux operon expression system where the lux operons of Photobacterium leiognathi subsp. mandapamensis 27561 or of Photobacterium leiognathi subsp. leiognathi 25521, namely luxCDAB(F)EG, were cloned into a single expression vector. Exclusion of luxF gene from the lux operon enabled novel insights into the role of LuxF protein in light emission. E. coli cultures harboring and expressing the genes of the lux operon including luxF gene emit more light than without luxF gene. Furthermore, isolation of the tightly bound flavin derivative revealed the presence of at least three different flavin derivatives. Analysis by UV/Vis absorption and NMR spectroscopy as well as mass spectrometry showed that the flavin derivatives bear fatty acids of various chain lengths. This distribution of FMN derivatives is vastly different to what was found in bioluminescent bacteria and indicates that the luciferase is supplied with a range of aldehyde substrates in E. coli.

Molecular Mechanisms of Bacterial Bioluminescence

Bioluminescence refers to the production of light by living organisms. Bioluminescent bacteria with a variety of bioluminescence emission characteristics have been identified in Vibrionaceae, Shewanellaceae and Enterobacteriaceae. Bioluminescent bacteria are mainly found in marine habitats and they are either free-floating, sessile or have specialized to live in symbiosis with other marine organisms. On the molecular level, bioluminescence is enabled by a cascade of chemical reactions catalyzed by enzymes encoded by the lux operon with the gene order luxCDABEG. The luxA and luxB genes encode the α- and β- subunits, respectively, of the enzyme luciferase producing the light emitting species. LuxC, luxD and luxE constitute the fatty acid reductase complex, responsible for the synthesis of the long-chain aldehyde substrate and luxG encodes a flavin reductase. In bacteria, the heterodimeric luciferase catalyzes the monooxygenation of long-chain aliphatic aldehydes to the corresponding acids utilizing reduced FMN and molecular oxygen. The energy released as a photon results from an excited state flavin-4a-hydroxide, emitting light centered around 490 nm. Advances in the mechanistic understanding of bacterial bioluminescence have been spurred by the structural characterization of protein encoded by the lux operon. However, the number of available crystal structures is limited to LuxAB (Vibrio harveyi), LuxD (Vibrio harveyi) and LuxF (Photobacterium leiognathi). Based on the crystal structure of LuxD and homology models of LuxC and LuxE, we provide a hypothetical model of the overall structure of the LuxCDE fatty acid reductase complex that is in line with biochemical observations.

AcpM, the meromycolate extension acyl carrier protein of Mycobacterium tuberculosis, is activated by the 4'-phosphopantetheinyl transferase PptT, a potential target of the multistep mycolic acid biosynthesis

Modification of acyl carrier proteins (ACP) or domains by the covalent binding of a 4'-phosphopantetheine (4'-PP) moiety is a fundamental condition for activation of fatty acid synthases (FASes) and polyketide synthases (PKSes). Binding of 4'-PP is mediated by 4' phosphopantetheinyl transfersases (PPTases). Mycobacterium tuberculosis (Mtb) possesses two essential PPTases: acyl carrier protein synthase (Mtb AcpS), which activates the multidomain fatty acid synthase I (FAS I), and Mtb PptT, an Sfp-type broad spectrum PPTase that activates PKSes. To date, it has not been determined which of the two Mtb PPTases, AcpS or PptT, activates the meromycolate extension ACP, Mtb AcpM, en route to the production of mycolic acids, the main components of the mycobacterial cell wall. In this study, we tested the enzymatic activation of a highly purified Mtb apo-AcpM to Mtb holo-AcpM by either Mtb PptT or Mtb AcpS. By using SDS-PAGE band shift assay and mass spectrometry analysis, we found that Mtb PptT is the PPTase that activates Mtb AcpM. We measured the catalytic activity of Mtb PptT toward CoA, using an activation assay of a blue pigment synthase, BpsA (a nonribosomal peptide synthase, NRPS). BpsA activation by Mtb PptT was inhibited by Mtb apo-AcpM through competition for CoA, in accord with Mtb AcpM activation. A structural model of the putative interaction between Mtb PptT and Mtb AcpM suggests that both hydrophobic and electrostatic interactions stabilize this complex. To conclude, activation of Mtb AcpM by Mtb PptT reveals a potential target of the multistep mycolic acid biosynthesis.

Synthesis of customized petroleum-replica fuel molecules by targeted modification of free fatty acid pools in Escherichia coli

Biofuels are the most immediate, practical solution for mitigating dependence on fossil hydrocarbons, but current biofuels (alcohols and biodiesels) require significant downstream processing and are not fully compatible with modern, mass-market internal combustion engines. Rather, the ideal biofuels are structurally and chemically identical to the fossil fuels they seek to replace (i.e., aliphatic n- and iso-alkanes and -alkenes of various chain lengths). Here we report on production of such petroleum-replica hydrocarbons in Escherichia coli. The activity of the fatty acid (FA) reductase complex from Photorhabdus luminescens was coupled with aldehyde decarbonylase from Nostoc punctiforme to use free FAs as substrates for alkane biosynthesis. This combination of genes enabled rational alterations to hydrocarbon chain length (Cn) and the production of branched alkanes through upstream genetic and exogenous manipulations of the FA pool. Genetic components for targeted manipulation of the FA pool included expression of a thioesterase from Cinnamomum camphora (camphor) to alter alkane Cn and expression of the branched-chain α-keto acid dehydrogenase complex and β-keto acyl-acyl carrier protein synthase III from Bacillus subtilis to synthesize branched (iso-) alkanes. Rather than simply reconstituting existing metabolic routes to alkane production found in nature, these results demonstrate the ability to design and implement artificial molecular pathways for the production of renewable, industrially relevant fuel molecules.

Characterization and function of mitochondrial nitric-oxide synthase

The mitochondrial production of nitric oxide is catalyzed by a nitric-oxide synthase. This enzyme has the same cofactor and substrate requirements as other constitutive nitric-oxide synthases. Its occurrence was demonstrated in various mitochondrial preparations (intact, purified mitochondria, permeabilized mitochondria, mitoplasts, submitochondrial particles) from different organs (liver, heart) and species (rat, pig). Endogenous nitric oxide reversibly inhibits oxygen consumption and ATP synthesis by competitive inhibition of cytochrome oxidase. The increased K(m) of cytochrome oxidase for oxygen and the steady-state reduction of the electron chain carriers provided experimental evidence for the direct interaction of this oxidase with endogenous nitric oxide. The increase in hydrogen peroxide production by nitric oxide-producing mitochondria not accompanied by the full reduction of the respiratory chain components indicated that cytochrome c oxidase utilizes nitric oxide as an alternative substrate. Finally, effectors or modulators of cytochrome oxidase (the irreversible step in oxidative phosphorylation) had been proposed during the last 40 years. Nitric oxide is the first molecule that fulfills this role (it is a competitive inhibitor, produced at a fair rate near the target site) extending the oxygen gradient to tissues.

Biochemistry of mitochondrial nitric-oxide synthase

We reported that the generation of nitric oxide by mitochondria is catalyzed by a constitutive, mitochondrial nitric-oxide synthase (mtNOS). Given that this production may establish the basis for a novel regulatory pathway of energy metabolism, oxygen consumption, and oxygen free radical production, it becomes imperative to identify unequivocally and characterize this enzyme to provide a basis for its regulation. The mitochondrial localization of mtNOS was supported by following the hepatic distribution of mtNOS, immunoblotting submitochondrial fractions, and immunohistochemistry of liver tissues. mtNOS was identified as brain NOS alpha by various methods (mass spectrometry of proteolytic fragments, amino acid analysis, molecular weight, pI, and analysis of PCR fragments), excluding the occurrence of a novel isoform or other splice variants. Distribution of mtNOS transcript indicated its occurrence in liver, brain, heart, muscle, kidney, lung, testis, and spleen. In contrast to brain NOS, mtNOS has two post-translational modifications: acylation with myristic acid and phosphorylation at the C terminus. The former modification is a reversible and post-translational process, which may serve for subcellular targeting or membrane anchoring. The latter modification could be linked to enzymatic regulation. These results are discussed in terms of the role that nitric oxide may have in cellular bioenergetics.

A histidine residue in the catalytic mechanism distinguishes Vibrio harveyi aldehyde dehydrogenase from other members of the aldehyde dehydrogenase superfamily

Aldehyde dehydrogenases (ALDHs) catalyze the transfer to NAD(P) of a hydride ion from a thiohemiacetal derivative of the aldehyde coupled with a cysteine residue in the active site. In Vibrio harveyi aldehyde dehydrogenase (Vh-ALDH), a histidine residue (H450) is in proximity (3.8 A) to the cysteine nucleophile (C289) and is thus capable of increasing its reactivity in sharp contrast to other ALDHs in which more distantly located glutamic acid residues are proposed to act as the general base. Mutation of H450 in Vh-ALDH to Gln and Asn resulted in loss of dehydrogenase, (thio)esterase, and acyl-CoA reductase activities; the residual activity of H450Q was higher than that of the H450N mutant in agreement with the capability of Gln but not Asn to partially replace the epsilon-imino group of H450. Coupled with a change in the rate-limiting step, these results indicate that H450 increases the reactivity of C289. Moreover, for the first time, the acylated enzyme intermediate could be directly monitored after reaction with [(3)H]tetradecanoyl-CoA showing that the H450Q mutant was acylated more rapidly than the H450N mutant. Inactivation of the wild-type enzyme with N-ethylmaleimide was much more rapid than the H450Q mutant which in turn was faster than the H450N mutant, demonstrating directly that the nucleophilicity of C289 was affected by H450. As the glutamic acid residue implicated as the general base in promoting cysteine nucleophilicity in other ALDHs is conserved in Vh-ALDH, elucidation of why a histidine residue has evolved to assist in this function in Vh-ALDH will be important to understand the mechanism of ALDHs in general, as well as help delineate the specific roles of the active site glutamic acid residues.

Hyperactivity and interactions of a chimeric myristoryl-ACP thioesterase from the lux system of luminescent bacteria

A chimeric myristoyl-ACP thioesterase with much higher catalytic efficiency than the parental enzymes has been generated by ligating the N-terminal half of the lux-specific thioesterase (LuxD) from Photobacterium phosphoreum with the C-terminal half of LuxD from Vibrio harveyi. The LuxD chimera had the same rate-limiting step and specificity, but cleaved esters and thioesters over eight times faster than the native enzymes. LuxD, along with acyl-protein synthetase (LuxE) and reductase (LuxC), comprise a multienzyme complex channeling activated fatty acids into the aldehyde substrate for the bacterial bioluminescence reaction. As P. phosphoreum LuxD and LuxE modulate each of their respective activities, the effects of mixing V. harveyi and the chimeric LuxD with P. phosphoreum LuxE were investigated. The chimeric LuxD stimulated acylation of LuxE to the same extent as V. harveyi LuxD, but to a lower level than that caused by P. phosphoreum LuxD. Conversely, P. phosphoreum LuxE stimulated the thioesterase activity of V. harveyi LuxD by 30% and the chimeric LuxD by 20% while the activity of P. phosphoreum LuxD was increased by over 140%. These results show that the stimulatory effects are unrelated to the level of thioesterase activity and indicate that the carboxyl terminal region of LuxD interacts with LuxE and causes a conformational change.

Exogenous myristic acid can be partially degraded prior to activation to form acyl-acyl carrier protein intermediates and lipid A in Vibrio harveyi

To study the involvement of acyl carrier protein (ACP) in the metabolism of exogenous fatty acids in Vibrio harveyi, cultures were incubated in minimal medium with [9,10-3H]myristic acid, and labeled proteins were analyzed by gel electrophoresis. Labeled acyl-ACP was positively identified by immunoprecipitation with anti-V. harveyi ACP serum and comigration with acyl-ACP standards and [3H]beta-alanine-labeled bands on both sodium dodecyl sulfate- and urea-polyacrylamide gels. Surprisingly, most of the acyl-ACP label corresponded to fatty acid chain lengths of less than 14 carbons: C14, C12, C10, and C8 represented 33, 40, 14, and 8% of total [3H]14:0-derived acyl-ACPs, respectively, in a dark mutant (M17) of V. harveyi which lacks myristoyl-ACP esterase activity; however, labeled 14:0-ACP was absent in the wild-type strain. 14:0- and 12:0-ACP were also the predominant species labeled in complex medium. In contrast, short-chain acyl-ACPs (< or = C6) were the major labeled derivatives when V. harveyi was incubated with [3H]acetate, indicating that acyl-ACP labeling with [3H]14:0 in vivo is not due to the total degradation of [3H]14:0 to [3H]acetyl coenzyme A followed by resynthesis. Cerulenin increased the mass of medium- to long-chain acyl-ACPs (> or = C8) labeled with [3H]beta-alanine fivefold, while total incorporation of [3H]14:0 was not affected, although a shift to shorter chain lengths was noted. Additional bands which comigrated with acyl-ACP on sodium dodecyl sulfate gels were identified as lipopolysaccharide by acid hydrolysis and thin-layer chromatography. The levels of incorporation of [3H] 14:0 into acyl-ACP and lipopolysaccharide were 2 and 15%, respectively, of that into phospholipid by 10 min. Our results indicate that in contrast to the situation in Escherichia coli, exogenous fatty acids can be activated to acyl-ACP intermediates after partial degradation in V. harveyi and can effectively label products (i.e., lipid A) that require ACP as an acyl donor.

Identification of the acyl transfer site of fatty acyl-protein synthetase from bioluminescent bacteria

Fatty acid activation, transfer, and reduction by the fatty acid reductase multienzyme complex from Photobacterium phosphoreum to generate fatty aldehydes for the luminescence reaction is regulated by the interaction of the synthetase and reductase subunits of this complex. Identification of the specific site involved in covalent transfer of the fatty acyl group between the sites of activation and reduction on the synthetase and reductase subunits, respectively, is a critical step in understanding how subunit interactions modulate the flow of fatty acyl groups through the fatty acid reductase complex. To accomplish this goal, the nucleotide sequence of the luxE gene coding for the acyl-protein synthetase subunit (373 amino acid residues) was determined and the conserved cysteinyl residues implicated in fatty acyl transfer identified. Using site-specific mutagenesis, each of the five conserved cysteine residues was converted to a serine residue, the mutated synthetases expressed in Escherichia coli, and the properties of the mutant proteins examined. On complementation of four of the mutants with the reductase subunit, the synthetase subunit was acylated and the acyl group could be reversibly transferred between the reductase and synthetase subunits, and fatty acid reductase activity was fully regenerated. As well, sensitivity of the acylated synthetases to hydroxylamine cleavage (under denaturation conditions to remove any conformational effects on reactivity) was retained, showing that a cysteine and not a serine residue was still acylated. However, substitution of a cysteine residue only ten amino acid residues from the carboxyl terminal (C364S) prevented acylation of the synthetase and regeneration of fatty acid reductase activity. Moreover, this mutant protein preserved its ability to activate fatty acid to fatty acyl-AMP but could not accept the acyl group from the reductase subunit, demonstrating that the C364S synthetase had retained its conformation and specifically lost the fatty acylation site. These results provide evidence that the flow of fatty acyl groups in the fatty acid reductase complex is modulated by interaction of the reductase subunit with a cysteine residue very close to the carboxyl terminal of the synthetase, which in turn acts as a flexible arm to transfer acyl groups between the sites of activation and reduction.

Inhibition of Vibrio harveyi bioluminescence by cerulenin: in vivo evidence for covalent modification of the reductase enzyme involved in aldehyde synthesis

Bacterial bioluminescence is very sensitive to cerulenin, a fungal antibiotic which is known to inhibit fatty acid synthesis. When Vibrio harveyi cells pretreated with cerulenin were incubated with [3H]myristic acid in vivo, acylation of the 57-kilodalton reductase subunit of the luminescence-specific fatty acid reductase complex was specifically inhibited. In contrast, in vitro acylation of both the synthetase and transferase subunits, as well as the activities of luciferase, transferase, and aldehyde dehydrogenase, were not adversely affected by cerulenin. Light emission of wild-type V. harveyi was 20-fold less sensitive to cerulenin at low concentrations (10 micrograms/ml) than that of the dark mutant strain M17, which requires exogenous myristic acid for luminescence because of a defective transferase subunit. The sensitivity of myristic acid-stimulated luminescence in the mutant strain M17 exceeded that of phospholipid synthesis from [14C]acetate, whereas uptake and incorporation of exogenous [14C]myristic acid into phospholipids was increased by cerulenin. The reductase subunit could be labeled by incubating M17 cells with [3H]tetrahydrocerulenin; this labeling was prevented by preincubation with either unlabeled cerulenin or myristic acid. Labeling of the reductase subunit with [3H]tetrahydrocerulenin was also noted in an aldehyde-stimulated mutant (A16) but not in wild-type cells or in another aldehyde-stimulated mutant (M42) in which [3H]myristoyl turnover at the reductase subunit was found to be defective. These results indicate that (i) cerulenin specifically and covalently inhibits the reductase component of aldehyde synthesis, (ii) this enzyme is partially protected from cerulenin inhibition in the wild-type strain in vivo, and (iii) two dark mutants which exhibit similar luminescence phenotypes (mutants A16 and M42) are blocked at different stages of fatty acid reduction.

Lux C, D and E genes of the Vibrio fischeri luminescence operon code for the reductase, transferase, and synthetase enzymes involved in aldehyde biosynthesis

The lux C, D, and E genes of the Vibrio fischeri luminescence operon code for three polypeptides of 54, 33, and 42 kDa, respectively, which are required for synthesis of the aldehyde substrate for the luminescent reaction. These polypeptides have been identified in V. fischeri and V. harveyi as well as in recombinant E. coli harboring the cloned genes by specific acylation with [3H]fatty acid, showing that they are components of a fatty acid reductase system with reductase, synthetase and transferase activities. By using glycerol in the assay and/or extraction buffer and decreasing the reducing agent, the levels of the acylation of the 54 and 42 kDa polypeptides have been greatly increased. As a consequence, it was possible to demonstrate that the 54 kDa polypeptide coded by the lux C gene has reductase activity. In a subclone missing the lux E gene, the 42 kDa polypeptide was missing and the 54 kDa polypeptide could not be acylated in vitro with tetradecanoic acid (+ATP) and only to a low level in vivo indicating that the synthetase enzyme, responsible for fatty acid activation, is coded by the lux E gene. In vitro acylation with tetradecanoyl CoA of the 33 kDa polypeptide coupled with the specific cleavage of acyl-ACP only in E. coli extracts transformed with DNA containing the lux D gene, demonstrated that the lux D gene coded for the transferase enzyme.