Acetyl-CoA Carboxylase catalyzes the first committed step in fatty acid synthesis. Escherichia coli acetyl-CoA carboxylase is composed of biotin carboxylase, carboxyltransferase and biotin carboxyl carrier protein functions. The accA and accD genes that code for the α- and β-subunits, respectively, are not in an operon, yet yield an α2β2 carboxyltransferase. Here, we report that carboxyltransferase regulates its own translation by binding the mRNA encoding its subunits. This interaction is mediated by a zinc finger on the β-subunit; mutation of the four cysteines to alanine diminished nucleic acid binding and catalytic activity. Carboxyltransferase binds the coding regions of both subunit mRNAs and inhibits translation, an inhibition that is relieved by the substrate acetyl-CoA. mRNA binding reciprocally inhibits catalytic activity. Preferential binding of carboxyltransferase to RNA in situ was shown using fluorescence resonance energy transfer. We propose an unusual regulatory mechanism by which carboxyltransferase acts as a ‘dimmer switch’ to regulate protein production and catalytic activity, while sensing the metabolic state of the cell through acetyl-CoA concentration.
The suborder Corynebacterianeae comprises bacteria like Mycobacterium tuberculosis and Corynebacterium glutamicum, and these bacteria contain in addition to the linear fatty acids, unique α-branched β-hydroxy fatty acids, called mycolic acids. Whereas acetyl-coenzyme A (CoA) carboxylase activity is required to provide malonyl-CoA for fatty acid synthesis, a new type of carboxylase is apparently additionally present in these bacteria. It activates the α-carbon of a linear fatty acid by carboxylation, thus enabling its decarboxylative condensation with a second fatty acid to afford mycolic acid synthesis. We now show that the acetyl-CoA carboxylase of C. glutamicum consists of the biotinylated α-subunit AccBC, the β-subunit AccD1, and the small peptide AccE of 8.9 kDa, forming an active complex of approximately 812,000 Da. The carboxylase involved in mycolic acid synthesis is made up of the two highly similar β-subunits AccD2 and AccD3 and of AccBC and AccE, the latter two identical to the subunits of the acetyl-CoA carboxylase complex. Since AccD2 and AccD3 orthologues are present in all Corynebacterianeae, these polypeptides are vital for mycolic acid synthesis forming the unique hydrophobic outer layer of these bacteria, and we speculate that the two β-subunits present serve to lend specificity to this unique large multienzyme complex.
The Corynebacterium glutamicum NTA monooxygenase component A protein, which plays the central role in NTA biodegradation, was crystallized. The initial X-ray crystallographic characterization is reported.
Safety and environmental concerns have recently dictated the proper disposal of nitrilotriacetate (NTA). Biodegradation of NTA is initiated by NTA monooxygenase, which is composed of two proteins: component A and component B. The NTA monooxygenase component A protein from Corynebacterium glutamicum was crystallized using the sitting-drop vapour-diffusion method in the presence of ammonium sulfate as the precipitant. X-ray diffraction data were collected to a maximum resolution of 2.5 Å on a synchrotron beamline. The crystal belongs to the monoclinic space group C2, with unit-cell parameters a = 111.04, b = 98.51, c = 171.61 Å, β = 101.94°. The asymmetric unit consists of four molecules, corresponding to a packing density of 2.3 Å3 Da−1. The structure was solved by molecular replacement. Structure refinement is in progress.
nitrilotriacetate; nitrilotriacetate monooxygenase component A
Pyruvate carboxylase (PC) is a conserved metabolic enzyme with important cellular functions. We report here crystallographic and cryoEM studies of S. aureus PC (SaPC) in complex with acetyl-CoA, an allosteric activator, as well as mutagenesis, biochemical and structural studies of the biotin binding site of its carboxyltransferase (CT) domain. The disease-causing A610T mutation abolishes catalytic activity by blocking biotin binding to the CT active site, and Thr908 may play a catalytic role in the CT reaction. The crystal structure of SaPC in complex with CoA reveals a symmetrical tetramer, with one CoA molecule bound to each monomer, and cryoEM studies confirm the symmetrical nature of the tetramer. These observations are in sharp contrast to the highly asymmetrical tetramer of R. etli PC in complex with ethyl-CoA. Our structural information suggests that acetyl-CoA promotes a conformation for the dimer of the biotin carboxylase domain of PC that may be catalytically more competent.
The amino acid-producing Gram-positive Corynebacterium glutamicum is auxotrophic for biotin although biotin ring assembly starting from the precursor pimeloyl-CoA is still functional. It possesses AccBC, the α-subunit of the acyl-carboxylases involved in fatty acid and mycolic acid synthesis, and pyruvate carboxylase as the only biotin-containing proteins. Comparative genome analyses suggested that the putative transport system BioYMN encoded by cg2147, cg2148 and cg2149 might be involved in biotin uptake by C. glutamicum.
By comparison of global gene expression patterns of cells grown with limiting or excess supply of biotin or with dethiobiotin as supplement replacing biotin revealed that expression of genes coding for enzymes of biotin ring assembly and for the putative uptake system was regulated according to biotin availability. RT-PCR and 5'-RACE experiments demonstrated that the genes bioY, bioM, and bioN are transcribed from one promoter as a single transcript. Biochemical analyses revealed that BioYMN catalyzes the effective uptake of biotin with a concentration of 60 nM biotin supporting a half-maximal transport rate. Maximal biotin uptake rates were at least five fold higher in biotin-limited cells as compared to cells grown with excess biotin. Overexpression of bioYMN led to an at least 50 fold higher biotin uptake rate as compared to the empty vector control. Overproduction of BioYMN alleviated biotin limitation and interfered with triggering L-glutamate production by biotin limitation.
The operon bioYMN from C. glutamicum was shown to be induced by biotin limitation. Transport assays with radio-labeled biotin revealed that BioYMN functions as a biotin uptake system. Overexpression of bioYMN affected L-glutamate production triggered by biotin limitation.
Pyruvate carboxylase (PC) is a biotin-containing enzyme that catalyses the HCO3−- and MgATP-dependent carboxylation of pyruvate to form oxaloacetate. This is a very important anaplerotic reaction, replenishing oxaloacetate withdrawn from the Krebs cycle for various pivotal biochemical pathways. PC is therefore considered as an enzyme that is crucial for intermediary metabolism, controlling fuel partitioning toward gluconeogenesis, lipogenesis and insulin secretion. The enzyme was discovered in 1959 and over the last decade there has been much progress in understanding its structure and function. PC from most organisms is a tetrameric protein that is allosterically regulated by acetyl CoA and aspartate. High resolution crystal structures of the holoenzyme with various ligands bound have recently been determined, and have revealed details and the relative positions of the biotin carboxylase, carboxyltransferase and biotin carboxyl carrier domains, and also a unique allosteric effector domain. In the presence of the allosteric effector, acetyl CoA, the biotin moiety transfers the carboxyl group intermediate between the biotin carboxylase domain active site on one polypeptide chain and the carboxyltransferase active site on the adjacent antiparallel polypeptide chain. In addition, the bona fide role of PC in the non-gluconeogenic tissues has been studied using a combination of classical biochemistry and genetic approaches. The first cloning of the promoter of the PC gene in mammals and subsequent transcriptional studies reveal some key cognate transcription factors regulating tissue-specific expression. This review summarizes these advances and also offers some prospects in terms of future directions for the study of this important enzyme.
pyruvate carboxylase; biotin; structure; kinetics; acetyl CoA; role in metabolism; gene expression
The cloning, purification, crystallization and preliminary X-ray characterization of the crystals of biotin acetyl-CoA carboxylase ligase from M. tuberculosis are reported.
The gene encoding biotin acetyl-CoA carboxylase ligase (BirA) from Mycobacterium tuberculosis was cloned and expressed in Escherichia coli with a C-terminal Strep-tag. PEG 4000 as well as PEG 8000 were used as precipitants at pH 7.5 to crystallize the protein using the vapour-diffusion technique. X-ray characterization of crystals at room temperature indicated that the crystals belonged to the orthorhombic space group P212121, with unit-cell parameters a = 79.7, b = 62.8, c = 105.8 Å. Assuming the presence of two BirA molecules in the asymmetric unit, the solvent content of the crystals was 44% (V
M = 2.2 Å3 Da−1). When transferred to a cryoprotectant, crystals grown in the same drop exhibited a difference in one unit-cell parameter, with a = 60.1, b = 64.0, c = 103.6 Å, but belonged to the same P212121 space group. These crystals, with two molecules of BirA present per asymmetric unit, appeared to have a very low solvent content of 28% (V
M = 1.7 Å3 Da−1).
biotin; biotin protein ligase; Mycobacterium tuberculosis
The pseudopeptide pyrrolidinedione antibiotics, such as moiramide B, have recently been discovered to target the multisubunit acetyl coenzyme A (acetyl-CoA) carboxylases of bacteria. In this paper, we describe synthetic variations of each moiety of the modularly composed pyrrolidinediones, providing insight into structure-activity relationships of biochemical target activity, in vitro potency, and in vivo efficacy. The novel derivatives showed highly improved activities against gram-positive bacteria compared to those of previously reported variants. The compounds exhibited a MIC90 value of 0.1 μg/ml against a broad spectrum of Staphylococcus aureus clinical isolates. No cross-resistance to antibiotics currently used in clinical practice was observed. Resistance mutations induced by pyrrolidinediones are exclusively located in the carboxyltransferase subunits of the bacterial acetyl-CoA carboxylase, indicating the identical mechanisms of action of all derivatives tested. Improvement of the physicochemical profile was achieved by salt formation, leading to aqueous solubilities of up to 5 g/liter. For the first time, the in vitro activity of this compound class was compared with its in vivo efficacy, demonstrating a path from compounds weakly active in vivo to agents with significant efficacy. In a murine model of S. aureus sepsis, the 100% effective dose of the best compound reported was 25 mg/kg of body weight, only fourfold higher than that of the comparator molecule linezolid. The obvious improvements achieved by chemical derivatization reflect the potential of this novel antibiotic compound class for future therapy.
The biotin protein ligase from S. aureus has been overexpressed in E. coli, purified, crystallized by the hanging-drop vapour-diffusion method and analysed using X-ray diffraction.
Biotin protein ligase from Staphylococcus aureus catalyses the biotinylation of acetyl-CoA carboxylase and pyruvate carboxylase. Recombinant biotin protein ligase from S. aureus has been cloned, expressed and purified. Crystals were grown using the hanging-drop vapour-diffusion method using PEG 8000 as the precipitant at 295 K. X-ray diffraction data were collected to 2.3 Å resolution from crystals using synchrotron X-ray radiation at 100 K. The diffraction was consistent with the tetragonal space group P42212, with unit-cell parameters a = b = 93.665, c = 131.95.
biotin protein ligase; Staphylococcus aureus
Fatty acid-CoA racemase from M. tuberculosis H37Rv has been overexpressed, purified and crystallized. Diffraction data have been collected to beyond 2.7 Å resolution using a synchrotron-radiation source.
Fatty acid-CoA racemase plays an important role in the β-oxidation of branched-chain fatty acids and fatty-acid derivatives as it catalyzes the conversion of several (2R)-branched-chain fatty acid-CoAs to their (2S)-stereoisomers. Fatty acid-CoA racemase from Mycobacterium tuberculosis H37Rv has been purified to homogeneity and crystallized by the hanging-drop vapour-diffusion method with polyethylene glycol 4000 as precipitant. The crystals belong to the trigonal space group P31 or P32, with unit-cell parameters a = b = 109.56, c = 147.97 Å. The asymmetric unit contains six monomers, corresponding to a V
M value of 2.15 Å3 Da−1. A complete native data set has been collected at 2.7 Å resolution using a synchrotron-radiation source.
fatty acid-CoA racemase; Mycobacterium tuberculosis; α-oxidation; β-oxidation
Pathogenic mycobacteria contain a variety of unique fatty acids that have methyl branches at an even-numbered position at the carboxyl end and a long n-aliphatic chain. One such group of acids, called mycocerosic acids, is found uniquely in the cell wall of pathogenic mycobacteria, and their biosynthesis is essential for growth and pathogenesis. Therefore, the biosynthetic pathway of the unique precursor of such lipids, methylmalonyl coenzyme A (CoA), represents an attractive target for developing new antituberculous drugs. Heterologous protein expression and purification of the individual subunits allowed the successful reconstitution of an essential acyl-CoA carboxylase from Mycobacterium tuberculosis, whose main role appears to be the synthesis of methylmalonyl-CoA. The enzyme complex was reconstituted from the α biotinylated subunit AccA3, the carboxyltransferase β subunit AccD5, and the ɛ subunit AccE5 (Rv3281). The kinetic properties of this enzyme showed a clear substrate preference for propionyl-CoA compared with acetyl-CoA (specificity constant fivefold higher), indicating that the main physiological role of this enzyme complex is to generate methylmalonyl-CoA for the biosynthesis of branched-chain fatty acids. The α and β subunits are capable of forming a stable α6-β6 subcomplex but with very low specific activity. The addition of the ɛ subunit, which binds tightly to the α-β subcomplex, is essential for gaining maximal enzyme activity.
PHBH from Corynebacterium glutamicum was crystallized using the hanging-drop vapour-diffusion method in the presence of NaH2PO4 and K2HPO4 as precipitants. X-ray diffraction data were collected to a maximum resolution of 2.5 Å on a synchrotron beamline.
p-Hydroxybenzoate hydroxylase (PHBH) is an FAD-dependent monooxygenase that catalyzes the hydroxylation of p-hydroxybenzoate (pOHB) to 3,4-dihydroxybenzoate in an NADPH-dependent reaction and plays an important role in the biodegradation of aromatic compounds. PHBH from Corynebacterium glutamicum was crystallized using the hanging-drop vapour-diffusion method in the presence of NaH2PO4 and K2HPO4 as precipitants. X-ray diffraction data were collected to a maximum resolution of 2.5 Å on a synchrotron beamline. The crystal belongs to the hexagonal space group P6322, with unit-cell parameters a = b = 94.72, c = 359.68 Å, γ = 120°. The asymmetric unit contains two molecules, corresponding to a packing density of 2.65 Å3 Da−1. The structure was solved by molecular replacement. Structure refinement is in progress.
p-hydroxybenzoate hydroxylase; Corynebacterium glutamicum; ; FAD-dependent monooxygenases
The enzyme cgHle from C. glutamicum, which has acetyl ester hydrolase activity, was crystallized in four different crystal forms. X-ray diffraction data have been collected to a resolution of 1.2 Å.
CgHle is an enzyme that is encoded by gene cg0961 from Corynebacterium glutamicum. The physiological function of cgHle is so far unclear. Bioinformatic annotations based on sequence homology indicated that cgHle may be an acetyl-CoA:homoserine acetyl transferase and as such may be involved in methionine biosynthesis, but recent evidence has shown that it is an esterase that catalyzes the hydrolysis of acetyl esters. Here, the crystallization of cgHle in two orthorhombic crystal forms, a trigonal crystal form and a monoclinic crystal form is described. The trigonal crystals have a solvent content of 83.7%, which is one of the highest solvent contents ever found for protein crystals. One of the orthorhombic crystals diffracted X-rays to at least 1.2 Å resolution.
CgHle; Corynebacterium glutamicum; homoserine acetyltransferases
A ternary complex of the proteinase inhibitor (BTCI) with trypsin and chymotrypsin was crystallized and its crystal structure was solved by molecular replacement.
A ternary complex of the black-eyed pea trypsin and chymotrypsin inhibitor (BTCI) with trypsin and chymotrypsin was crystallized by the sitting-drop vapour-diffusion method with 0.1 M HEPES pH 7.5, 10%(w/v) polyethylene glycol 6000 and 5%(v/v) 2-methyl-2,4-pentanediol as precipitant. BTCI is a small protein with 83 amino-acid residues isolated from Vigna unguiculata seeds and is able to inhibit trypsin and chymotrypsin simultaneously by forming a stable ternary complex. X-ray data were collected from a single crystal of the trypsin–BTCI–chymotrypsin ternary complex to 2.7 Å resolution under cryogenic conditions. The structure of the ternary complex was solved by molecular replacement using the crystal structures of the BTCI–trypsin binary complex (PDB code 2g81) and chymotrypsin (PDB code 4cha) as search models.
proteinase inhibitors; Bowman–Birk inhibitors
Genes for subunits of acetyl coenzyme A carboxylase (ACC), which is the enzyme that catalyzes the first step in the synthesis of fatty acids in Lactobacillus plantarum L137, were cloned and characterized. We identified six potential open reading frames, namely, manB, fabH, accB, accC, accD, and accA, in that order. Nucleotide sequence analysis suggested that fabH encoded β-ketoacyl-acyl carrier protein synthase III, that the accB, accC, accD, and accA genes encoded biotin carboxyl carrier protein, biotin carboxylase, and the β and α subunits of carboxyltransferase, respectively, and that these genes were clustered. The organization of acc genes was different from that reported for Escherichia coli, for Bacillus subtilis, and for Pseudomonas aeruginosa. E. coli accB and accD mutations were complemented by the L. plantarum accB and accD genes, respectively. The predicted products of all five genes were confirmed by using the T7 expression system in E. coli. The gene product of accB was biotinylated in E. coli. Northern and primer extension analyses demonstrated that the five genes in L. plantarum were regulated polycistronically in an acc operon.
An acyl coenzyme A (CoA) carboxylase, which catalyzes the adenosine triphosphate-dependent fixation of CO2 into acetyl-, propionyl-, and butyryl-CoA, was detected in fractionated cell extracts of Propionibacterium shermanii. Catalytic activity was inhibited by avidin but was unaffected by avidin pretreated with excess biotin. The carboxylase levels detected were relatively small and were related to cellular growth. Maximal carboxylase activity was detected in cells grown for about 96 h. Thereafter, the activity declined rapidly. Optimal CO2 fixation occurred at pH 7.5. Other parameters of the assay system were optimized, and the apparent Km values for substrates were determined. The end product of the reaction (with acetyl-CoA as the substrate) was identified as malonyl-CoA. The stoichiometry of the reaction was such that, for every mole of acetyl-CoA and adenosine triphosphate consumed, 1 mol each of malonyl-CoA, adenosine diphosphate, and orthophosphate was formed. These data provide the first evidence for the presence of another biotin-containing enzyme, an acyl-CoA carboxylase, in these bacteria in addition to the well-characterized methylmalonyl-CoA carboxyltransferase.
Transcarboxylase from Propionibacterium shermanii is a complex biotin-containing enzyme composed of 30 polypeptides of three different types: a hexameric central 12S subunit to which 6 outer 5S subunits are attached through 12 1.3S biotinyl subunits. The enzyme catalyzes a two-step reaction in which methylmalonyl coenzyme A and pyruvate serve as substrates to form propionyl coenzyme A (propionyl-CoA) and oxalacetate, the 12S subunit specifically catalyzing one of the two reactions. We report here the cloning, sequencing, and expression of the 12S subunit. The gene was identified by matching amino acid sequences derived from isolated authentic 12S peptides with the deduced sequence of an open reading frame present in a cloned P. shermanii genomic fragment known to contain the gene encoding the 1.3S biotinyl subunit. The cloned 12S gene encodes a protein of 604 amino acids and of M(r) 65,545. The deduced sequence shows regions of extensive homology with the beta subunit of mammalian propionyl-CoA carboxylase as well as regions of homology with acetyl-CoA carboxylase from several species. Two genomic fragments were subcloned into pUC19 in an orientation such that the 12S open reading frame could be expressed from the lac promoter of the vector. Crude extracts prepared from these cells contained an immunoreactive band on Western blots (immunoblots) which comigrated with authentic 12S. The Escherichia coli-expressed 12S was purified to apparent homogeneity by a three-step procedure and compared with authentic 12S from P. shermanii. Their quaternary structures were identical by electron microscopy, and the E. coli 12S preparation was fully active in the reactions catalyzed by this subunit. We conclude that we have cloned, sequenced, and expressed the 12S subunit which exists in a hexameric active form in E.coli.
Propionyl-coenzyme A carboxylase (PCC), a mitochondrial biotin-dependent enzyme, is essential for the catabolism of the amino acids Thr, Val, Ile and Met, cholesterol, and fatty acids with an odd number of carbon atoms. Deficiencies of PCC activity in humans are linked to the disease propionic acidemia (PA), an autosomal recessive disorder that can be fatal in infants 1–4. The holoenzyme of PCC is an α6β6 dodecamer, with a molecular weight of 750 kD. The α subunit contains the biotin carboxylase (BC) and biotin carboxyl carrier protein (BCCP) domains, while the β subunit supplies the carboxyltransferase (CT) activity. Here we report the crystal structure at 3.2 Å resolution of a bacterial PCC α6β6 holoenzyme as well as cryo-electron microscopy (cryo-EM) reconstructionat 15 Å resolution demonstrating a similar structure for human PCC. The structure defines the overall architecture of PCC and reveals unexpectedly that the α subunits are arranged as monomers in the holoenzyme, decorating a central β6 hexamer. A hitherto unrecognized domain in the α subunit, formed by residues between the BC and BCCP domains, is crucial for interactions with the β subunit. We have named it the BT domain. The structure reveals for the first time the relative positions of the BC and CT active sites in the holoenzyme. They are separated by approximately 55 Å, indicating that the entire BCCP domain must translocate during catalysis. The BCCP domain is located in the active site of the β subunit in the current structure, providing insight for its involvement in the CT reaction. The structural information establishes a molecular basis for understanding the large collection of disease-causing mutations in PCC, and also has important relevance for the holoenzymes of other biotin-dependent carboxylases, including 3-methylcrotonyl-CoA carboxylase (MCC) 5–7 and eukaryotic acetyl-CoA carboxylase (ACC) 8,9.
fatty acid metabolism; amino acid metabolism; propionic acidemia; 3-methylcrotonylglycinuria; protein complex; biotin-dependent carboxylase; acetyl-CoA carboxylase; 3-methylcrotonyl-CoA carboxylase
Biotin carboxylase catalyzes the ATP-dependent carboxylation of biotin, and is one component of the multienzyme complex acetyl-CoA carboxylase that catalyzes the first committed step in fatty acid synthesis in all organisms. Biotin carboxylase from Escherichia coli, whose crystal structures with and without ATP bound have been determined, has served as a model system for this component of the acetyl-CoA carboxylase complex. The two crystal structures revealed a large conformational change of one domain relative to the other domains when ATP is bound. Unfortunately, the crystal structure with ATP bound was obtained with an inactive site-directed mutant of the enzyme. As a consequence the structure with ATP bound lacked key structural information such as for the Mg2+ ions and contained altered conformations of key active site residues. Therefore, nanosecond molecular dynamics studies of the wild-type biotin carboxylase were undertaken to supplant and amend the results of the crystal structures. Specifically, the protein-metal interactionsof the two catalytically critical Mg2+ ions bound in the active site are presented along with a reevaluation of the conformations of active site residues bound to ATP. In addition, the regions of the polypeptide chain that serve as hinges for the large conformational change were identified. The results of the hinge analysis complemented a covariance analysis that identified the individual structural elements of biotin carboxylase that change their conformation in response to ATP binding.
The acyl-CoA carboxylase β subunit (ACCD6) of M. tuberculosis has been crystallized and preliminary X-ray crystallographic analysis has been performed.
Mycobacterium tuberculosis (Mtb) acyl-CoA carboxylase is involved in the biosynthesis of mycolic acids, which are a key component of the bacillus cell wall. The Mtb genome encodes six acyl-CoA carboxylase β subunits (ACCD1–6), three of which (ACCD4–6) are essential for survival of the pathogen on minimal medium. Mtb ACCD6 has been expressed, purified and crystallized. The two forms of Mtb ACCD6 crystals belonged to space groups P41212 and P212121 and diffracted to 2.9 and 2.5 Å resolution, respectively, at a synchrotron-radiation source.
acyl-CoA carboxylase; ACCD6; Rv2247
The crystallization and preliminary X-ray characterization of a shikimate dehydrogenase from C. glutamicum is presented.
The shikimate dehydrogenase from Corynebacterium glutamicum has been cloned into an Escherichia coli expression vector, overexpressed and purified. Native crystals were obtained by the vapour-diffusion technique using 2-methyl-2,4-pentanediol as a precipitant. The crystals belong to the centred monoclinic space group C2, with unit-cell parameters a = 118.77, b = 63.17, c = 35.67 Å, β = 92.26° (at 100 K), and diffract to 1.64 Å on a synchrotron X-ray source. The asymmetric unit is likely to contain one molecule, corresponding to a packing density of 2.08 Å3 Da−1 and a solvent content of about 41%.
shikimate dehydrogenase; Corynebacterium glutamicum
Acyl coenzyme A carboxylase (acyl-CoA carboxylase) was purified from Acidianus brierleyi. The purified enzyme showed a unique subunit structure (three subunits with apparent molecular masses of 62, 59, and 20 kDa) and a molecular mass of approximately 540 kDa, indicating an α4β4γ4 subunit structure. The optimum temperature for the enzyme was 60 to 70°C, and the optimum pH was around 6.4 to 6.9. Interestingly, the purified enzyme also had propionyl-CoA carboxylase activity. The apparent Km for acetyl-CoA was 0.17 ± 0.03 mM, with a Vmax of 43.3 ± 2.8 U mg−1, and the Km for propionyl-CoA was 0.10 ± 0.008 mM, with a Vmax of 40.8 ± 1.0 U mg−1. This result showed that A. brierleyi acyl-CoA carboxylase is a bifunctional enzyme in the modified 3-hydroxypropionate cycle. Both enzymatic activities were inhibited by malonyl-CoA, methymalonyl-CoA, succinyl-CoA, or CoA but not by palmitoyl-CoA. The gene encoding acyl-CoA carboxylase was cloned and characterized. Homology searches of the deduced amino acid sequences of the 62-, 59-, and 20-kDa subunits indicated the presence of functional domains for carboxyltransferase, biotin carboxylase, and biotin carboxyl carrier protein, respectively. Amino acid sequence alignment of acetyl-CoA carboxylases revealed that archaeal acyl-CoA carboxylases are closer to those of Bacteria than to those of Eucarya. The substrate-binding motifs of the enzymes are highly conserved among the three domains. The ATP-binding residues were found in the biotin carboxylase subunit, whereas the conserved biotin-binding site was located on the biotin carboxyl carrier protein. The acyl-CoA-binding site and the carboxybiotin-binding site were found in the carboxyltransferase subunit.
We have cloned a DNA fragment from a genomic library of Myxococcus xanthus using an oligonucleotide probe representing conserved regions of biotin carboxylase subunits of acetyl coenzyme A (acetyl-CoA) carboxylases. The fragment contained two open reading frames (ORF1 and ORF2), designated the accB and accA genes, capable of encoding a 538-amino-acid protein of 58.1 kDa and a 573-amino-acid protein of 61.5 kDa, respectively. The protein (AccA) encoded by the accA gene was strikingly similar to biotin carboxylase subunits of acetyl-CoA and propionyl-CoA carboxylases and of pyruvate carboxylase. The putative motifs for ATP binding, CO2 fixation, and biotin binding were found in AccA. The accB gene was located upstream of the accA gene, and they formed a two-gene operon. The protein (AccB) encoded by the accB gene showed high degrees of sequence similarity with carboxyltransferase subunits of acetyl-CoA and propionyl-CoA carboxylases and of methylmalonyl-CoA decarboxylase. Carboxybiotin-binding and acyl-CoA-binding domains, which are conserved in several carboxyltransferase subunits of acyl-CoA carboxylases, were found in AccB. An accA disruption mutant showed a reduced growth rate and reduced acetyl-CoA carboxylase activity compared with the wild-type strain. Western blot analysis indicated that the product of the accA gene was a biotinylated protein that was expressed during the exponential growth phase. Based on these results, we propose that this M. xanthus acetyl-CoA carboxylase consists of two subunits, which are encoded by the accB and accA genes, and occupies a position between prokaryotic and eukaryotic acetyl-CoA carboxylases in terms of evolution.
The biotin–protein ligase from P. horikoshii OT3 was overexpressed, purified, crystallized and cocrystallized with biotin, ADP and biotinyl-5′-AMP. The crystals belong to space group P21 and diffract to beyond 1.6 Å resolution.
Biotin–protein ligase is an enzyme that catalyzes the ATP-dependent biotinylation of a specific lysine residue in acetyl-CoA carboxylase. The biotin–protein ligase from Pyrococcus horikoshii OT3 has been cloned, overexpressed and purified. Crystallization was performed by the microbatch method or the vapour-diffusion method using PEG 2000 as a precipitant at 295 K. X-ray diffraction data have been collected to 1.6 Å resolution from a native crystal and to 1.55 Å resolution from a selenomethionine-derivative crystal for multiple anomalous dispersion phasing using synchrotron radiation at 100 K. The native crystal belongs to the monoclinic space group P21, with unit-cell parameters a = 38.601, b = 78.264, c = 70.147 Å, β = 101.48°. Assuming a homodimer per asymmetric unit gives a V
M value of 2.14 Å3 Da−1 and a solvent content of 42.5%. Cocrystals with biotin, ADP and biotinyl-5′-AMP were prepared and diffraction data sets were collected to 1.6, 1.6 and 1.45 Å resolution, respectively.
biotin; biotin–protein ligase; Pyrococcus horikoshii OT3; acetyl-CoA carboxylase; fatty-acid biosynthesis
A dcm-1 mutant, obtained by transposon mutagenesis of Myxococcus xanthus, could aggregate and form mounds but was unable to sporulate under nutrient starvation. A sequence analysis of the site of insertion of the transposon showed that the insertion lies within the 3' end of a 1,572-bp open reading frame (ORF) designated the M. xanthus pccB ORF. The wild-type form of the M. xanthus pccB gene, obtained from a lambdaEMBL library of M. xanthus, shows extensive similarity to a beta subunit of propionyl coenzyme A (CoA) carboxylase, an alpha subunit of methylmalonyl-CoA decarboxylase, and a 12S subunit of transcarboxylase. In enzyme assays, extracts of the dcm-1 mutant were deficient in propionyl-CoA carboxylase activity. This enzyme catalyzes the ATP-dependent carboxylation of propionyl-CoA to yield methylmalonyl-CoA. The methylmalonyl-CoA rescued the dcm-1 mutant fruiting body and spore development. During development, the dcm-1 mutant cells also had reduced levels of long-chain fatty acids (C16 to C18) compared to wild-type cells.