Purification and properties of malonyl-CoA decarboxylase from rat liver mitochondria and its immunological comparison with the enzymes from rat brain, heart, and mammary gland.

Malonyl-CoA decarboxylase (EC 4.1.1.9) was found to be localized in the mitochondria in rat liver. Low ionic strength (10 mm Na phosphate) buffer extracted the bulk (>85%) of the enzyme from the mitochondria. From this extract the enzyme was purified over 2,000-fold using a combination of (NH₄)₂SO₄ precipitation, gel filtration with Sepharose 4B and Sephadex G-150, ion exchange chromatography with QAE-Sephadex and CM-Sephadex, and finally chromatography on NADP-agarose. The purified enzyme, which had a specific activity of about 16 μmol/min/mg, appeared to be electrophoretically homogeneous and had a molecular weight of 160,000. The decarboxylase had a broad pH optimum between 8.5 and 10.0 and showed a typical Michaelis-Menten substrate saturation pattern from which K_m and V were calculated to be 54 μm and 18.8 μmol/min/mg, respectively. This enzyme decarboxylated neither malonic acid nor methylmalonyl-CoA and was severely inhibited by thiol-directed reagents such as p-hydroxymercuribenzoate and N-ethylmaleimide but not by iodoacetamide. Acetyl-CoA, propionyl-CoA, and methylmalonyl-CoA also inhibited the enzyme. The purified decarboxylase was immunogenic in rabbits and Ouchterlony double diffusion analysis revealed a single precipitant line with the purified enzyme. The IgG fraction isolated from the antiserum inhibited the enzyme from not only liver mitochondria but also the mammary gland, heart, and kidney of the rat. However, malonyl-CoA decarboxylase from rat brain mitochondria was not inhibited by the antibody. Malonyl-CoA decarboxylase purified from the uropygial gland of a domestic goose neither cross reacted nor was it inhibited by the antiserum prepared against the rat liver mitochondrial enzyme and the antibody against the goose enzyme neither cross-reacted nor inhibited the enzyme from the rat. It is proposed that a role for mitochondrial malonyl-CoA decarboxylase is to decarboxylate malonyl-CoA generated by propionyl-CoA carboxylase and thus protect mitochondrial enzymes susceptible to inhibition by malonyl-CoA.     

Malonyl-CoA Decarboxylase from Streptomyces erythreus: Purification,properties, and possible role in the production of erythromycin

Malonyl-CoA decarboxylase was purified (800-fold) from an erythromycin-producing strain of Streptomyces erythreus using DEAE-cellulose, Sephadex G-100, SP-Sephadex, and gel filtration with Sephadex G-75. The molecular weight of the native enzyme was 93,000 as determined by gel filtration and the subunit molecular weight was 45,000 as estimated by sodium dodecyl sulfate-polyacrylamide electrophoresis, suggesting an α₂ subunit composition for the native enzyme. Evidence is presented that during the purification procedure and storage a proteolytic cleavage occurred resulting in the formation of 30- and 15-kDa peptides. The enzyme showed a pH optimum of about 5.0 whereas the vertebrate enzyme showed an optimum at alkaline pH. The enzyme decarboxylated malonyl-CoA with a K_m of 143 μm and V of 250 nmol min^?1 mg^?1. For the decarboxylation of methylmalonyl-CoA this enzyme showed the opposite stereospecificity to that shown by vertebrate enzyme; the (R) isomer was decarboxylated at 3% of the rate observed with malonyl-CoA while the (S) isomer was not a substrate. Neither avidin nor biotin affected the rate of malonyl-CoA decarboxylation, suggesting that biotin is not involved in catalysis. Acetyl-CoA and free CoA were found to be competitive inhibitors. Propionyl-CoA, butyryl-CoA, succinyl-CoA, and methylmalonyl-CoA showed little inhibition, and neither thiol-directed reagents nor chelating agents inhibited the enzyme. High ionic strength and sulfate ions caused reversible inhibition of the enzymatic activity. Under two different cultural conditions the time course of appearance of malonyl-CoA decarboxylase was determined by measuring the enzyme activity and the level of the enzyme protein by an immunological method using rabbit antibodies prepared against the enzyme. In both cases the increase and decrease in the decarboxylase correlated with the rate of production of erythromycin, suggesting a possible role for this enzyme in the antibiotic production.     

Isolation and Characterization of Acyl Coenzyme A Carboxylases from Mycobacterium tuberculosis and Mycobacterium bovis, Which Produce Multiple Methyl-Branched Mycocerosic Acids

Mycobacterium tuberculosis H37Ra and M. bovis BCG produce multiple methyl-branched fatty acids called mycocerosic acids, presumably from methyl-malonyl coenzyme A (CoA). An acyl-CoA carboxylase was isolated from these organisms at a 30 to 50% yield by a purification procedure involving ammonium sulfate fractionation, gel filtration, and affinity chromatography with a monomeric avidin–Sepharose 4B-CL gel with d-biotin as the eluant. Sodium dodecyl sulfate electrophoresis and avidin binding indicate that each enzyme is probably composed of two dissimilar subunits with a covalently bound biotin in the larger subunit. The enzyme preparations from H37Ra and BCG had specific activities of 2.1 and 5.5 μmol min⁻¹ mg⁻¹, respectively, when propionyl-CoA was the substrate. The enzymes from the two species displayed striking similarities in their kinetic parameters. They showed maximal activity at pH 8.0 when propionyl-CoA was the substrate, but displayed a relatively broad pH-activity profile when acetyl-CoA was the substrate. With both substrates, potassium phosphate buffer gave maximal activity. Apparent K_m values for propionyl-CoA, ATP, Mg²⁺, and NaHCO₃ were 70 μM, 100 μM, 5.4 mM, and 2.2 mM, respectively. The enzyme also carboxylated acetyl-CoA and butyryl-CoA, and high-performance liquid chromatography showed the expected products of carboxylation. However, with these substrates, the K_m was higher and the V_max was lower than those of propionyl-CoA. The enzyme was shown to be stereospecific, synthesizing exclusively (S)-methylmalonyl-CoA from propionyl-CoA. No other acyl-CoA carboxylase was observed during the purification procedure, indicating that the present carboxylase may provide malonyl-CoA for the synthesis of n-fatty acids as well as methylmalonyl-CoA for the synthesis of mycocerosic acids.     

Fluorometric assays for succinyl-CoA and propionyl-CoA in mitochondrial extracts

Fluorometric assay procedures are described for the quantitative measurements of succinyl-CoA and propionyl-CoA down to concentrations of 0.1 μm in the reaction mixture. The enzymatic assay for succinyl-CoA couples the reaction of 3-ketoacid CoA transferase (succinyl-CoA transferase) to β-OH butyryl-CoA dehydrogenase. A simple purification procedure is described for the isolation of succinyl-CoA transferase from beef heart. Two enzyme assays for propionyl-CoA are described. In the first, CoA, acetyl-CoA and propionyl-CoA are assayed by sequential addition of α-ketoglutarate dehydrogenase, citrate synthase and phosphotransacetylase. The second assay for propionyl-CoA utilized propionyl-CoA carboxylase to convert propionyl-CoA to methylmalonyl-CoA in the presence of ATP and bicarbonate, and the ADP formed was assayed by coupling pyruvate kinase with lactate dehydrogenase. Illustrations are given for the application of these assay procedures to measurements of succinyl-CoA and propionyl-CoA in neutralized perchloric acid extracts prepared from rat heart and liver mitochondria incubated under a variety of conditions.     

Modification of bovine kidney pyruvate dehydrogenase kinase activity by CoA esters and their mechanism of action

The activation of pyruvate dehydrogenasea kinase activity by CoA esters has been further characterized. Half-maximal activation of kinase activity was achieved with about 1.0 microM acetyl-CoA after a 20-s preincubation in the presence of NADH. More than 80% of the acetyl-CoA was consumed during this period in acetylating sites in the pyruvate dehydrogenase complex as a result of the transacetylation reaction proceeding to equilibrium. At 1.0 microM acetyl-CoA, this resulted in more than a 4-fold higher level of CoA than residual acetyl-CoA. Activation of kinase activity could result either from acetylation of specific sites in the complex or tight binding of acetyl-CoA. Removal of CoA enhanced both acetylation and activation, suggesting acetylation mediates activation. For allosteric binding of acetyl-CoA to elicit activation, an activation constant, Ka, less than 50 nM would be required. To further distinguish between those mechanisms, the effects of other CoA esters as well as the reactivity of most of the effective CoA esters were characterized. Several short-chain CoA esters enhanced kinase activity including (in decreasing order of effectiveness) malonyl-CoA, acetoacetyl-CoA, propionyl-CoA, and methylmalonyl-CoA. Butyryl-CoA inhibited kinase activity as did high concentrations of long-chain acyl-CoAs. Inhibition by long-chain acyl-CoAs may result, in part, from detergent-like properties of those esters. Malonyl-CoA, propionyl-CoA, butyryl-CoA, and methylmalonyl-CoA, obtained with radiolabeled acyl groups, were shown to acylate sites in the complex. Propionyl-CoA and butyryl-CoA were tested, in competition with acetyl-CoA or pyruvate, as alternative substrates for acylation of sites in the complex and as competitive effectors of kinase activity. Propionyl-CoA alone rapidly acylated sites in the complex at low concentrations, and low concentrations of propionyl-CoA were effective in activating kinase activity although only a relatively small activation was observed. When an equivalent level (20 microM) of acetyl-CoA and propionyl-CoA was used, marked activation of kinase activity due to a dominant effect of acetyl-CoA was associated with acetylation of a major portion of sites in the complex and with a small portion undergoing acylation with propionyl-CoA. Those results were rapidly achieved in a manner independent of the order of addition of the two CoA esters. That indicates that tight slowly reversible binding of acetyl-CoA is not involved in kinase activation. High levels of propionyl-CoA greatly reduced acetylation by acetyl-CoA and nearly prevented activation of kinase activity by acetyl-CoA.(ABSTRACT TRUNCATED AT 400 WORDS)     

Propionyl-CoA carboxylase of Myxococcus xanthus: catalytic properties and function in developing cells

An acyl-coenzyme A carboxylase that carboxylates acetyl-CoA, butyryl-CoA, propionyl-CoA, and succinyl-CoA was purified from Myxococcus xanthus. Since the enzyme showed maximal rates of carboxylation with propionyl-CoA, the enzyme is thought to be propionyl-CoA carboxylase. The apparent K _m values for acetyl-CoA, butyryl-CoA, propionyl-CoA, and succinyl-CoA were found to be 0.2, 0.2, 0.03, and 1.0 mM, respectively. The native enzyme has a molecular mass of 605–615 kDa and is composed of nonidentical subunits (α and β) with molecular masses of 53 and 56 kDa, respectively. The enzyme showed maximal activity at pH 7.0–7.5 and at 25–30°C, and was affected by variation in concentrations of ATP and Mg²⁺. During development of M. xanthus, the propionyl-CoA carboxylase activity increased gradually, with maximum activity observed during the sporulation stage. Previous work has shown that a propionyl-CoA-carboxylase-deficient mutant of M. xanthus reduces levels of long-chain fatty acids. These results suggest that the propionyl-CoA carboxylase is also responsible for the carboxylation of acetyl-CoA to malonyl-CoA used for the synthesis of long-chain fatty acids during development. Received: 24 February 1998 / Accepted: 25 May 1998     

Synthesis of multimethyl-branched fatty acids by avian and mammalian fatty acid synthetase and its regulation by malonyl-CoA decarboxylase in the uropygial gland

Fatty acid synthetase, partially purified by gel filtration with Sepharose 4B from goose liver, showed the same relative rate of incorporation of methylmalonyl-CoA (compared to malonyl-CoA) as that observed with the purified fatty acid synthetase from the uropygial gland. In the presence of acetyl-CoA, methylmalonyl-CoA was incorporated mainly into 2,4,6,8-tetramethyldecanoic acid and 2,4,6,8,10-pentamethyl-dodecanoic acid by the enzyme from both sources. Methylmalonyl-CoA was a competitive inhibitor with respect to malonyl-CoA for the enzyme from the gland just as previously observed for fatty acid synthetase from other animals. Furthermore, rabbit antiserum prepared against the gland enzyme cross-reacted with the liver enzyme, and Ouchterlony double-diffusion analyses showed complete fusion of the immunoprecipitant lines. The antiserum inhibited both the synthesis of n-fatty acids and branched fatty acids catalyzed by the synthetase from both liver and the uropygial gland. These results suggest that the synthetases from the two tissues are identical and that branched and n-fatty acids are synthesized by the same enzyme. Immunological examination of the 105,000g supernatant prepared from a variety of organs from the goose showed that only the uropygial gland contained a protein which cross-reacted with the antiserum prepared against malonyl-CoA decarboxylase purified from the gland. Thus, it is concluded that the reason for the synthesis of multimethyl-branched fatty acids by the fatty acid synthetase in the gland is that in this organ the tissue-specific and substrate-specific decarboxylase makes only methylmalonyl-CoA available to the synthetase. Fatty acid synthetase, partially purified from the mammary gland and the liver of rats, also catalyzed incorporation of [methyl-¹⁴C]methylmalonyl-CoA into 2,4,6,8-tetramethyldecanoic acid and 2,4,6,8-tetramethylundecanoic acid with acetyl-CoA and propionyl-CoA, respectively, as the primers. Evidence is also presented that fatty acids containing straight and branched regions can be generated by the fatty acid synthetase from the rat and goose, from methylmalonyl-CoA in the presence of malonyl-CoA or other precursors of n-fatty acids. These results provide support for the hypothesis that, under the pathological conditions which result in accumulation of methylmalonyl-CoA, abnormal branched acids can be generated by the fatty acid synthetase.     

Malonyl-CoA decarboxylase from Streptomyces erythreus: purification, properties, and possible role in the production of erythromycin

Malonyl-CoA decarboxylase was purified (800-fold) from an erythromycin-producing strain of Streptomyces erythreus using DEAE-cellulose, Sephadex G-100, SP-Sephadex, and gel filtration with Sephadex G-75. The molecular weight of the native enzyme was 93,000 as determined by gel filtration and the subunit molecular weight was 45,000 as estimated by sodium dodecyl sulfate-polyacrylamide electrophoresis, suggesting an alpha 2 subunit composition for the native enzyme. Evidence is presented that during the purification procedure and storage a proteolytic cleavage occurred resulting in the formation of 30- and 15-kDa peptides. The enzyme showed a pH optimum of about 5.0 whereas the vertebrate enzyme showed an optimum at alkaline pH. The enzyme decarboxylated malonyl-CoA with a Km of 143 microM and V of 250 nmol min-1 mg-1. For the decarboxylation of methylmalonyl-CoA this enzyme showed the opposite stereospecificity to that shown by vertebrate enzyme; the (R) isomer was decarboxylated at 3% of the rate observed with malonyl-CoA while the (S) isomer was not a substrate. Neither avidin nor biotin affected the rate of malonyl-CoA decarboxylation, suggesting that biotin is not involved in catalysis. Acetyl-CoA and free CoA were found to be competitive inhibitors. Propionyl-CoA, butyryl-CoA, succinyl-CoA, and methylmalonyl-CoA showed little inhibition, and neither thiol-directed reagents nor chelating agents inhibited the enzyme. High ionic strength and sulfate ions caused reversible inhibition of the enzymatic activity. Under two different cultural conditions the time course of appearance of malonyl-CoA decarboxylase was determined by measuring the enzyme activity and the level of enzyme protein by an immunological method using rabbit antibodies prepared against the enzyme. In both cases the increase and decrease in the decarboxylase correlated with the rate of production of erythromycin, suggesting a possible role for this enzyme in the antibiotic production.     

Stereospecificity of malonyl-CoA decarboxylase, acetyl-CoA carboxylase, and fatty acid synthetase from the uropygial gland of goose

Malonyl-CoA decarboxylase from the uropygial gland of goose decarboxylated (R,S)-methylmalonyl-CoA at a slow rate and introduced 3H from [3H]2O into the resulting propionyl-CoA. Carboxylation of this labeled propionyl-CoA by propionyl-CoA carboxylase from pig heart and acetyl-CoA carboxylase from the uropygial gland completely removed 3H. Repeated treatment of (R,S)-[methyl-14C]methylmalonyl-CoA with the decarboxylase converted 50% of the substrate into propionyl-CoA, whereas (S)-methylmalonyl-CoA, generated by both carboxylases, was completely decarboxylated. Radioactive (R)- (S), and (R,S)-methylmalonyl-CoA were equally incorporated into fatty acids by fatty acid synthetase from the uropygial gland. The residual methylmalonyl-CoA remaining after fatty acid synthetase reaction on (R,S)-methylmalonyl-CoA was also racemic. These results show that: (a) the decarboxylase is stereospecific, (b) replacement of the carboxyl group by hydrogen occurs with retention of configuration, (c) acetyl-CoA carboxylase of the uropygial gland generates (S)-methylmalonyl-CoA from propionyl-CoA, and (d) fatty acid synthetase is not stereospecific for methylmalonyl-CoA.     

Inhibition in vitro of lipogenic enzymes from bovine (Bos taurus) mammary tissue by methylmalonyl-coenzyme A and coenzyme A

Bovine mammary fatty acid synthetase was inhibited by approximately 50% by 40 microM methylmalonyl-CoA; this inhibition was competitive with respect to malonyl-CoA (apparent Ki = 11 microM). Similarly, 6.25 microM coenzyme A inhibited the synthetase by 35% and this inhibition was again competitive (apparent Ki = 1.7 microM). Apparent Km for malonyl-CoA was 29 microM. The short-chain dicarboxylic acids malonic, methylmalonic and ethylmalonic at high concentrations (160-320 microM) and ATP (5 mM) enhanced the synthetase activity by about 50% respectively; the activating effects of methylmalonic acid and ATP on the synthetase were additive. Methylmalonyl-CoA at 50 microM concentration inhibited the partially purified acetyl-CoA carboxylase uncompetitively by 10% and the propionyl-CoA carboxylase activity of the enzyme preparation competitively (apparent Ki = 21 microM) by 40%. Malonyl-CoA also inhibited the acetyl-CoA carboxylase activity competitively (apparent Ki = 7 microM) by 35% and the propionyl-CoA carboxylating activity of the preparation competitively (apparent Ki = 4 microM) by 82%. The possibility that methylmalonyl-CoA may be a causal factor in the aetiology of the low milk-fat syndrome in high yielding dairy cows is discussed.     

Life by a new decarboxylation-dependent energy conservation mechanism with Na as coupling ion

We report here a new mode of ATP synthesis in living cells. The anaerobic bacterium Propionigenium modestum gains its total energy for growth from the conversion of succinate to propionate according to: succinate + H₂O → propionate + HCO₃^- (G^o' = -20.6 kJ/mol). The small free energy change of this reaction does not allow a substrate-linked phosphorylation mechanism, and no electron transport phosphorylation takes place. Succinate was degraded by cell-free extracts to propionate and CO₂ via succinyl-CoA, methyl-malonyl-CoA and propionyl-CoA. This pathway involves a membrane-bound methylmalonyl-CoA decarboxylase which couples the exergonic decarboxylation with a Na⁺ ion transport across the membrane. The organism also contained a membrane-bound ATPase which was specifically activated by Na⁺ ions and catalyzed and transport of Na⁺ ions into inverted bacterial vesicles upon ATP hydrolysis. The transport was abolished by monensin but not by the uncoupler carbonylcyanide-p-trifluoromethoxy phenylhydrazone. Isolated membrane vesicles catalyzed the synthesis of ATP from ADP and inorganic phosphate when malonyl-CoA was decarboxylated and malonyl-CoA synthesis from acetyl-CoA when ATP was hydrolyzed. These syntheses were sensitive to monensin which indicates that Na⁺ functions as the coupling ion. We conclude from these results that ATP synthesis in P. modestum is driven by a Na⁺ ion gradient which is generated upon decarboxylation of methylmalonyl-CoA.     

Interaction of malonyl-CoA and related compounds with mitochondria from different rat tissues. Relationship between ligand binding and inhibition of carnitine palmitoyltransferase I.

The sensitivity of carnitine palmitoyltransferase I (CPT I; EC 2.3.1.21) to inhibition by malonyl-CoA and related compounds was examined in isolated mitochondria from liver, heart and skeletal muscle of the rat. In all three tissues the same order of inhibitory potency emerged: malonyl-CoA much greater than succinyl-CoA greater than methylmalonyl-CoA much greater than propionyl-CoA greater than acetyl-CoA. For any given agent, suppression of CPT I activity was much greater in skeletal muscle than in liver, with the heart enzyme having intermediate sensitivity. With skeletal-muscle mitochondria a high-affinity binding site for [14C]malonyl-CoA was readily demonstrable (Kd approx. 25 nM). The ability of other CoA esters to compete with [14C]malonyl-CoA for binding to the membrane paralleled their capacity to inhibit CPT I. Palmitoyl-CoA also competitively inhibited [14C]malonyl-CoA binding, in keeping with its known ability to overcome malonyl-CoA suppression of CPT I. For reasons not yet clear, free CoA displayed anomalous behaviour in that its competition for [14C]malonyl-CoA binding was disproportionately greater than its inhibition of CPT I. Three major conclusions are drawn. First, malonyl-CoA is not the only physiological compound capable of suppressing CPT I, since chemically related compounds, known to exist in cells, also share this property, particularly in tissues where the enzyme shows the greatest sensitivity to malonyl-CoA. Second, malonyl-CoA and its analogues appear to interact with the same site on the mitochondrial membrane, as may palmitoyl-CoA. Third, the degree of site occupancy by inhibitors governs the activity of CPT I.     

Lipid biosynthesis in sebaceous glands: regulation of the synthesis of n- and branched fatty acids by malonyl-coenzyme A decarboxylase.

Crude cell-free extracts isolated from the uropygial glands of goose catalyzed the carboxylation of propionyl-CoA but not acetyl-CoA. However, a partially purified preparation catalyzed the carboxylation of both substrates and the characteristics of this carboxylase were similar to those reported for chicken liver carboxylase. The Km and Vmax for the carboxylation of either acetyl-CoA or propionyl-CoA were 1.5 times 10- minus-5 M and 0.8 mumol per min per mg, respectively. In the crude extracts an inhibitor of the acetyl-CoA carboxylase activity was detected. The inhibitor was partially purified and identified as a protein that catalyzed the rapid decarboxylation of malonyl-CoA. This enzyme was avidin-insenitive and highly specific for malonyl-CoA with very low rates of decarboxylation for methylmalonyl-CoA and malonic acid. Vmax and Km for malonyl-CoA decarboxylation, at the pH optimum of 9.5, were 12.5 mumol per min per mg and 8 times 10- minus-4 M, respectively. The relative activities of the acetyl-CoA carboxylase and malonyl-CoA decarboxylase were about 4 mumol per min per gland and 70 mumoles per min per gland, respectively. Therefore acetyl-CoA and methylmalonyl-CoA should be the major primer and elongating agent, respectively, present in the gland. The major fatty acid formed from these precursors by the fatty acid synthetase of the gland would be 2,4,6,8-tetramethyl-decanoic acid which is known to be the major fatty acid of the gland (Buckner, J. S. and Kolattukudy, P. E. (1975), Biochemistry, following paper). Therefore it is concluded that the malonyl-CoA decarboxylase controls fatty acid synthesis in this gland.     

3-Hydroxypropionyl-Coenzyme A Dehydratase and Acryloyl-Coenzyme A Reductase,Enzymes of the Autotrophic 3-Hydroxypropionate/4-Hydroxybutyrate Cycle in the Sulfolobales

A 3-hydroxypropionate/4-hydroxybutyrate cycle operates in autotrophic CO₂ fixation in various Crenarchaea, as studied in some detail in Metallosphaera sedula. This cycle and the autotrophic 3-hydroxypropionate cycle in Chloroflexus aurantiacus have in common the conversion of acetyl-coenzyme A (CoA) and two bicarbonates via 3-hydroxypropionate to succinyl-CoA. Both cycles require the reductive conversion of 3-hydroxypropionate to propionyl-CoA. In M. sedula the reaction sequence is catalyzed by three enzymes. The first enzyme, 3-hydroxypropionyl-CoA synthetase, catalyzes the CoA- and MgATP-dependent formation of 3-hydroxypropionyl-CoA. The next two enzymes were purified from M. sedula or Sulfolobus tokodaii and studied. 3-Hydroxypropionyl-CoA dehydratase, a member of the enoyl-CoA hydratase family, eliminates water from 3-hydroxypropionyl-CoA to form acryloyl-CoA. Acryloyl-CoA reductase, a member of the zinc-containing alcohol dehydrogenase family, reduces acryloyl-CoA with NADPH to propionyl-CoA. Genes highly similar to the Metallosphaera CoA synthetase, dehydratase, and reductase genes were found in autotrophic members of the Sulfolobales. The encoded enzymes are only distantly related to the respective three enzyme domains of propionyl-CoA synthase from C. aurantiacus, where this trifunctional enzyme catalyzes all three reactions. This indicates that the autotrophic carbon fixation cycles in Chloroflexus and in the Sulfolobales evolved independently and that different genes/enzymes have been recruited in the two lineages that catalyze the same kinds of reactions.In the thermoacidophilic autotrophic crenarchaeum Metallosphaera sedula, CO₂ fixation proceeds via a 3-hydroxypropionate/4-hydroxybutyrate cycle (8, 23, 24, 28) (Fig. ​(Fig.1).1). A similar cycle may operate in other autotrophic members of the Sulfolobales and in mesophilic Crenarchaea (Cenarchaeum sp. and Nitrosopumilus sp.) of marine group I. The cycle uses elements of the 3-hydroxypropionate cycle that was originally discovered in the phototrophic bacterium Chloroflexus aurantiacus (11, 16, 17, 19, 20, 32, 33). It involves the carboxylation of acetyl-coenzyme A (CoA) to malonyl-CoA by the biotin-dependent acetyl-CoA carboxylase. Malonyl-CoA is reduced via malonate semialdehyde to 3-hydroxypropionate (1), which is further reductively converted to propionyl-CoA (3). Propionyl-CoA is carboxylated to (S)-methylmalonyl-CoA by a propionyl-CoA carboxylase that is similar or identical to acetyl-CoA carboxylase. In fact, only one copy of the genes for the acetyl-CoA/propionyl-CoA carboxylase subunits is present in most Archaea, suggesting that this is a promiscuous enzyme that acts on both acetyl-CoA and propionyl-CoA (24). (S)-Methylmalonyl-CoA is epimerized to (R)-methylmalonyl-CoA, followed by carbon rearrangement to succinyl-CoA by coenzyme B₁₂-dependent methylmalonyl-CoA mutase.Open in a separate windowFIG. 1.Proposed 3-hydroxypropionate/4-hydroxybutyrate cycle in M. sedula and other members of the Sulfolobales. Enzymes are the following: 1, acetyl-CoA carboxylase; 2, malonyl-CoA reductase (NADPH); 3, malonate semialdehyde reductase (NADPH); 4, 3-hydroxypropionyl-CoA synthetase (3-hydroxypropionate-CoA ligase, AMP forming); 5, 3-hydroxypropionyl-CoA dehydratase; 6, acryloyl-CoA reductase (NADPH); 7, propionyl-CoA carboxylase; 8, methylmalonyl-CoA epimerase; 9, methylmalonyl-CoA mutase; 10, succinyl-CoA reductase (NADPH); 11, succinate semialdehyde reductase (NADPH); 12, 4-hydroxybutyryl-CoA synthetase (4-hydroxybutyrate-CoA ligase, AMP-forming); 13, 4-hydroxybutyryl-CoA dehydratase; 14, crotonyl-CoA hydratase; 15, (S)-3-hydroxybutyryl-CoA dehydrogenase (NAD⁺); 16, acetoacetyl-CoA β-ketothiolase. The two steps of interest are highlighted.In Chloroflexus succinyl-CoA is converted to (S)-malyl-CoA, which is cleaved by (S)-malyl-CoA lyase to acetyl-CoA (thus regenerating the CO₂ acceptor molecule) and glyoxylate (16). Glyoxylate is assimilated into cell material by a yet not completely resolved pathway (37). In Metallosphaera succinyl-CoA is converted via 4-hydroxybutyrate to two molecules of acetyl-CoA (8), thus regenerating the starting CO₂ acceptor molecule and releasing another acetyl-CoA for biosynthesis. Hence, the 3-hydroxypropionate/4-hydroxybutyrate cycle (Fig. ​(Fig.1)1) can be divided into two parts. The first part transforms one acetyl-CoA and two bicarbonates into succinyl-CoA, and the second part converts succinyl-CoA to two acetyl-CoA molecules.The reductive conversion of 3-hydroxypropionate to propionyl-CoA requires three enzymatic steps: activation of 3-hydroxypropionate to its CoA ester, dehydration of 3-hydroxypropionyl-CoA to acryloyl-CoA, and reduction of acryloyl-CoA to propionyl-CoA. In C. aurantiacus these three steps are catalyzed by a single large trifunctional enzyme, propionyl-CoA synthase (2). This 200-kDa fusion protein consists of a CoA ligase, a dehydratase, and a reductase domain. Attempts to isolate a similar enzyme from M. sedula failed. Rather, a 3-hydroxypropionyl-CoA synthetase was found (3), suggesting that the other two reactions may also be catalyzed by individual enzymes.Here, we purified the missing enzymes 3-hydroxypropionyl-CoA dehydratase and acryloyl-CoA reductase from M. sedula, identified the coding genes in the genome of M. sedula and other members of the Sulfolobales, produced recombinant enzymes as proof of function, and studied the enzymes in some detail. A comparison with the respective domains of propionyl-CoA synthase from C. aurantiacus indicates that the conversion of 3-hydroxypropionate to propionyl-CoA via the 3-hydroxypropionate route has evolved independently in these two phyla.     

Characterization of propionate CoA-transferase from Ralstonia eutropha H16

In this study, a propionate CoA-transferase (H16_A2718; EC 2.8.3.1) from Ralstonia eutropha H16 (Pct _Re ) was characterized in detail. Glu342 was identified as catalytically active amino acid residue via site-directed mutagenesis. Activity of Pct _Re was irreversibly lost after the treatment with NaBH₄ in the presence of acetyl-CoA as it is shown for all CoA-transferases from class I, thereby confirming the formation of the covalent enzyme-CoA intermediate by Pct _Re . In addition to already known CoA acceptors for Pct _Re such as 3-hydroxypropionate, 3-hydroxybutyrate, acrylate, succinate, lactate, butyrate, crotonate and 4-hydroxybutyrate, it was found that glycolate, chloropropionate, acetoacetate, valerate, trans-2,3-pentenoate, isovalerate, hexanoate, octanoate and trans-2,3-octenoate formed also corresponding CoA-thioesters after incubation with acetyl-CoA and Pct _Re . Isobutyrate was found to be preferentially used as CoA acceptor amongst other carboxylates tested in this study. In contrast, no products were detected with acetyl-CoA and formiate, bromopropionate, glycine, pyruvate, 2-hydroxybutyrate, malonate, fumarate, itaconate, β-alanine, γ-aminobutyrate, levulate, glutarate or adipate as potential CoA acceptor. Amongst CoA donors, butyryl-CoA, crotonyl-CoA, 3-hydroxybutyryl-CoA, isobutyryl-CoA, succinyl-CoA and valeryl-CoA apart from already known propionyl-CoA and acetyl-CoA could also donate CoA to acetate. The highest rate of the reaction was observed with 3-hydroxybutyryl-CoA (2.5 μmol mg^?1 min^?1). K _m values for propionyl-CoA, acetyl-CoA, acetate and 3-hydroxybutyrate were 0.3, 0.6, 4.5 and 4.3 mM, respectively. The rather broad substrate range might be a good starting point for enzyme engineering approaches and for the application of Pct _Re in biotechnological polyester production.     

Ethylmalonyl-CoA decarboxylase, a new enzyme involved in metabolite proofreading

A limited number of enzymes are known that play a role analogous to DNA proofreading by eliminating non-classical metabolites formed by side activities of enzymes of intermediary metabolism. Because few such "metabolite proofreading enzymes" are known, our purpose was to search for an enzyme able to degrade ethylmalonyl-CoA, a potentially toxic metabolite formed at a low rate from butyryl-CoA by acetyl-CoA carboxylase and propionyl-CoA carboxylase, two major enzymes of lipid metabolism. We show that mammalian tissues contain a previously unknown enzyme that decarboxylates ethylmalonyl-CoA and, at lower rates, methylmalonyl-CoA but that does not act on malonyl-CoA. Ethylmalonyl-CoA decarboxylase is particularly abundant in brown adipose tissue, liver, and kidney in mice, and is essentially cytosolic. Because Escherichia coli methylmalonyl-CoA decarboxylase belongs to the family of enoyl-CoA hydratase (ECH), we searched mammalian databases for proteins of uncharacterized function belonging to the ECH family. Combining this database search approach with sequencing data obtained on a partially purified enzyme preparation, we identified ethylmalonyl-CoA decarboxylase as ECHDC1. We confirmed this identification by showing that recombinant mouse ECHDC1 has a substantial ethylmalonyl-CoA decarboxylase activity and a lower methylmalonyl-CoA decarboxylase activity but no malonyl-CoA decarboxylase or enoyl-CoA hydratase activity. Furthermore, ECHDC1-specific siRNAs decreased the ethylmalonyl-CoA decarboxylase activity in human cells and increased the formation of ethylmalonate, most particularly in cells incubated with butyrate. These findings indicate that ethylmalonyl-CoA decarboxylase may correct a side activity of acetyl-CoA carboxylase and suggest that its mutation may be involved in the development of certain forms of ethylmalonic aciduria.     

Purification and characterization of a cytoplasmic enzyme component of the Na+-activated malonate decarboxylase system of Malonomonas rubra: acetyl-S-acyl carrier protein: malonate acyl carrier protein-SH transferase

Malonate decarboxylation by crude extracts of Malonomonas rubra was specifically activated by Na⁺ and less efficiently by Li⁺ ions. The extracts contained an enzyme catalyzing CoA transfer from malonyl-CoA to acetate, yielding acetyl-CoA and malonate. After about a 26-fold purification of the malonyl-CoA:acetate CoA transferase, an almost pure enzyme was obtained, indicating that about 4% of the cellular protein consisted of the CoA transferase. This abundance of the transferase is in accord with its proposed role as an enzyme component of the malonate decarboxylase system, the key enzyme of energy metabolism in this organism. The apparent molecular weight of the polypeptide was 67,000 as revealed from SDS-polyacrylamide gel electrophoresis. A similar molecular weight was estimated for the native transferase by gel chromatography, indicating that the enzyme exists as a monomer. Kinetic analyses of the CoA transferase yielded the following: pH-optimum at pH 5.5, an apparent K_m for malonyl-CoA of 1.9mM, for acetate of 54mM, for acetyl-CoA of 6.9mM, and for malonate of 0.5mM. Malonate or citrate inhibited the enzyme with an apparent K_i of 0.4mM and 3.0mM, respectively. The isolated CoA transferase increased the activity of malonate decarboxylase of a crude enzyme system, in which part of the endogenous CoA transferase was inactivated by borohydride, about three-fold. These results indicate that the CoA transferase functions physiologically as a component of the malonate decarboxylase system, in which it catalyzes the transfer of acyl carrier protein from acetyl acyl carrier protein and malonate to yield malonyl acyl carrier protein and acetate. Malonate is thus activated on the enzyme by exchange for the catalytically important enzymebound acetyl thioester residues noted previously. This type of substrate activation resembles the catalytic mechanism of citrate lyase and citramalate lyase.Abbreviations DTNB 5,5 Dithiobis (2-nitrobenzoate) - MES 2-(N-Morpholino)ethanesulfonic acid - TAPS N-[Tris(hydroxymethyl)-methyl]-3-aminopropanesulfonic acid - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis     

Pathways of acetate and propionate production in adult Fasciola hepatica

In adult F. hepatica pyruvate is decarboxylated via pyruvate dehydrogenase to acetyl-CoA; acetyl-CoA is then cleaved to acetate via three possible mechanisms (1) carnitine dependent hydrolysis, (2) CoA transferase, (3) reversal of a GTP dependent acyl-CoA synthetase. Of these three systems, CoA transferase has by far the greatest activity. Propionate production by F. hepatica is similar to the mammalian system, succinate being metabolized via succinic thiokinase, methylmalonyl-CoA isomerase, methyl-malonyl-CoA racemase and propionyl-CoA carboxylase to propionyl-CoA. Propionyl-CoA is then cleaved to propionate by the same three pathways as acetyl-CoA. No ATP or GTP production could be demonstrated when acetyl- or propionyl-CoA were incubated with homogenates of F. hepatica. This indicates that carnitine dependent hydrolysis or CoA transferase are the major pathways of acetyl- or propionyl-CoA breakdown. The CoA transferase reaction would result in the conservation of the bond energy although there is no net ATP synthesis.     

Malonyl-coenzyme A reductase in the modified 3-hydroxypropionate cycle for autotrophic carbon fixation in archaeal Metallosphaera and Sulfolobus spp

Autotrophic members of the Sulfolobales (Crenarchaeota) contain acetyl-coenzyme A (CoA)/propionyl-CoA carboxylase as the CO2 fixation enzyme and use a modified 3-hydroxypropionate cycle to assimilate CO2 into cell material. In this central metabolic pathway malonyl-CoA, the product of acetyl-CoA carboxylation, is further reduced to 3-hydroxypropionate. Extracts of Metallosphaera sedula contained NADPH-specific malonyl-CoA reductase activity that was 10-fold up-regulated under autotrophic growth conditions. Malonyl-CoA reductase was partially purified and studied. Based on N-terminal amino acid sequencing the corresponding gene was identified in the genome of the closely related crenarchaeum Sulfolobus tokodaii. The Sulfolobus gene was cloned and heterologously expressed in Escherichia coli, and the recombinant protein was purified and studied. The enzyme catalyzes the following reaction: malonyl-CoA + NADPH + H+ --> malonate-semialdehyde + CoA + NADP+. In its native state it is associated with small RNA. Its activity was stimulated by Mg2+ and thiols and inactivated by thiol-blocking agents, suggesting the existence of a cysteine adduct in the course of the catalytic cycle. The enzyme was specific for NADPH (Km = 25 microM) and malonyl-CoA (Km = 40 microM). Malonyl-CoA reductase has 38% amino acid sequence identity to aspartate-semialdehyde dehydrogenase, suggesting a common ancestor for both proteins. It does not exhibit any significant similarity with malonyl-CoA reductase from Chloroflexus aurantiacus. This shows that the autotrophic pathway in Chloroflexus and Sulfolobaceae has evolved convergently and that these taxonomic groups have recruited different genes to bring about similar metabolic processes.     

A highly sensitive high-throughput luminescence assay for malonyl-CoA decarboxylase

Malonyl-CoA decarboxylase (MCD) catalyzes the conversion of malonyl-CoA to acetyl-CoA and thereby regulates malonyl-CoA levels in cells. Malonyl-CoA is a potent inhibitor of mitochondrial carnitine palmitoyltransferase-1, a key enzyme involved in the mitochondrial uptake of fatty acids for oxidation. Abnormally high rates of fatty acid oxidation contribute to ischemic damage. Inhibition of MCD leads to increased malonyl-CoA and therefore decreases fatty acid oxidation, representing a novel approach for the treatment of ischemic heart injury. The commonly used MCD assay monitors the production of NADH fluorometrically, which is not ideal for library screening due to potential fluorescent interference by certain compounds. Here we report a luminescence assay for MCD activity. This assay is less susceptible to fluorescent interference by compounds. Furthermore, it is 150-fold more sensitive, with a detection limit of 20 nM acetyl-CoA, compared to 3 μM in the fluorescence assay. This assay is also amenable to automation for high-throughput screening and yields excellent assay statistics (Z′ > 0.8). In addition, it can be applied to the screening for inhibitors of any other enzymes that generate acetyl-CoA.