Evaluation of 3-hydroxypropionate biosynthesis in vitro by partial introduction of the 3-hydroxypropionate/4-hydroxybutyrate cycle from <Emphasis Type="Italic">Metallosphaera sedula</Emphasis>

The chemical 3-hydroxypropionate (3HP) is an important starting reagent for the commercial synthesis of specialty chemicals. In this study, a part of the 3-hydroxypropionate/4-hydroxybutyrate cycle from Metallosphaera sedula was utilized for 3HP production. To study the basic biochemistry of this pathway, an in vitro-reconstituted system was established using acetyl-CoA as the substrate for the kinetic analysis of this system. The results indicated that 3HP formation was sensitive to acetyl-CoA carboxylase and malonyl-CoA reductase, but not malonate semialdehyde reductase. Also, the competition between 3HP formation and fatty acid production was analyzed both in vitro and in vivo. This study has highlighted how metabolic flux is controlled by different catalytic components. We believe that this reconstituted system would be valuable for understanding 3HP biosynthesis pathway and for future engineering studies to enhance 3HP production.     

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.     

Presence of Acetyl Coenzyme A (CoA) Carboxylase and Propionyl-CoA Carboxylase in Autotrophic Crenarchaeota and Indication for Operation of a 3-Hydroxypropionate Cycle in Autotrophic Carbon Fixation

The pathway of autotrophic CO₂ fixation was studied in the phototrophic bacterium Chloroflexus aurantiacus and in the aerobic thermoacidophilic archaeon Metallosphaera sedula. In both organisms, none of the key enzymes of the reductive pentose phosphate cycle, the reductive citric acid cycle, and the reductive acetyl coenzyme A (acetyl-CoA) pathway were detectable. However, cells contained the biotin-dependent acetyl-CoA carboxylase and propionyl-CoA carboxylase as well as phosphoenolpyruvate carboxylase. The specific enzyme activities of the carboxylases were high enough to explain the autotrophic growth rate via the 3-hydroxypropionate cycle. Extracts catalyzed the CO₂-, MgATP-, and NADPH-dependent conversion of acetyl-CoA to 3-hydroxypropionate via malonyl-CoA and the conversion of this intermediate to succinate via propionyl-CoA. The labelled intermediates were detected in vitro with either ¹⁴CO₂ or [¹⁴C]acetyl-CoA as precursor. These reactions are part of the 3-hydroxypropionate cycle, the autotrophic pathway proposed for C. aurantiacus. The investigation was extended to the autotrophic archaea Sulfolobus metallicus and Acidianus infernus, which showed acetyl-CoA and propionyl-CoA carboxylase activities in extracts of autotrophically grown cells. Acetyl-CoA carboxylase activity is unexpected in archaea since they do not contain fatty acids in their membranes. These aerobic archaea, as well as C. aurantiacus, were screened for biotin-containing proteins by the avidin-peroxidase test. They contained large amounts of a small biotin-carrying protein, which is most likely part of the acetyl-CoA and propionyl-CoA carboxylases. Other archaea reported to use one of the other known autotrophic pathways lacked such small biotin-containing proteins. These findings suggest that the aerobic autotrophic archaea M. sedula, S. metallicus, and A. infernus use a yet-to-be-defined 3-hydroxypropionate cycle for their autotrophic growth. Acetyl-CoA carboxylase and propionyl-CoA carboxylase are proposed to be the main CO₂ fixation enzymes, and phosphoenolpyruvate carboxylase may have an anaplerotic function. The results also provide further support for the occurrence of the 3-hydroxypropionate cycle in C. aurantiacus.     

Substrate and binding specificities of bacterial polyhydroxybutyrate depolymerases

The substrate specificities of three extracellular polyhydroxybutyrate (PHB) depolymerases from Alcaligenes faecalis (PhaZ_Afa), Pseudomonas stutzeri (PhaZ_Pst), and Comamonas acidovorans (PhaZ_Cac), which are grouped into types A and B based on the position of a lipase box sequence in the catalytic domain, were examined for films of 12 different aliphatic polyesters. Each of these PHB depolymerases used was capable of hydrolyzing poly(3-hydroxybutyrate) (P(3HB)), poly(3-hydroxypropionate) (P(3HP)), poly(4-hydroxybutyrate) (P(4HB)), poly(ethylene succinate) (PESU), and poly(ethylene adipate) (PEA) but could not hydrolyze another seven polyesters. In addition, the binding characteristics of substrate binding domains from PhaZ_Afa, PhaZ_Cac, and PHB depolymerase from Comamonas testosteroni (PhaZ_Cte) were studied by using fusions with glutathione S-transferase (GST). All of fusion proteins adsorbed strongly on the surfaces of polyester granules of P(3HB), P(3HP), and poly(2-hydroxypropionate) (P(2HP)) which was not hydrolyzed by the PHB depolymerases used in this study, while they did not bind on Avicel and chitin granules. The adsorption kinetics of the fusion proteins to the surface of P(3HB) and P(2HP) granules were found to obey the Langmuir isotherm. The cross-area per molecule of fusion protein bound to P(3HB) granules was estimated to be 12±4 nm²/molecule. It has been suggested that the active sites in catalytic domains of PHB depolymerases have a similar conformational structure, and that several amino acids in substrate-binding domains of PHB depolymerases interact specifically with the surface of polyesters.     

Chloroflexus aurantiacus secretes 3-hydroxypropionate,a possible intermediate in the assimilation of CO2 and acetate

Chloroflexus aurantiacus OK-70 fl secreted 3-hydroxypropionate (3HP) during phototrophic growth. The greatest amounts were secreted by cells grown on propionate (0.35 mM 3HP) while the lowest levels were found in autotrophically grown cultures (1.5 M). Large amounts of 2-fluoro,3-hydroxypropionate were formed by autotrophically grown cells exposed to fluoroacetate (FAc). Increased levels of 3HP were observed in these cultures when incubated with acctate. The secretion of 3HP was further stimulated by 0.2 mM KCN, an inhibitor of CO₂ fixation, but only in the presence of acetate. The pathway of 3HP formation was studied by using ¹³C-labelled substrates and NMR. The 3HP formed in the presence of C1-labelled acetate and FAc was labelled at C3 and somewhat less at C2 while with C2-labelled acetate as the tracer 3HP was labelled predominantly at C2. The carboxyl group was derived from CO₂. The 3HP formed by cells grown on propionate and ¹³CO₂ was labelled at all carbon atoms, the label content of C2 and C3 was about 25 and 65% of that of C1 respectively. It is suggested that 3HP is an intermediate in a pathway for acetate assimilation and in a new reductive carboxylic acid cycle for autotrophic CO₂ fixation.Abbreviations 3HP 3-hydroxypropionate - 2F3HP 2,fluoro,3-hydroxypropionate - FAc fluoroacetate - GC gas chromatography - MS mass spectrometry - NMR nuclear magnetic resonance     

Candida albicans Utilizes a Modified β-Oxidation Pathway for the Degradation of Toxic Propionyl-CoA

Propionyl-CoA arises as a metabolic intermediate from the degradation of propionate, odd-chain fatty acids, and some amino acids. Thus, pathways for catabolism of this intermediate have evolved in all kingdoms of life, preventing the accumulation of toxic propionyl-CoA concentrations. Previous studies have shown that fungi generally use the methyl citrate cycle for propionyl-CoA degradation. Here, we show that this is not the case for the pathogenic fungus Candida albicans despite its ability to use propionate and valerate as carbon sources. Comparative proteome analyses suggested the presence of a modified β-oxidation pathway with the key intermediate 3-hydroxypropionate. Gene deletion analyses confirmed that the enoyl-CoA hydratase/dehydrogenase Fox2p, the putative 3-hydroxypropionyl-CoA hydrolase Ehd3p, the 3-hydroxypropionate dehydrogenase Hpd1p, and the putative malonate semialdehyde dehydrogenase Ald6p essentially contribute to propionyl-CoA degradation and its conversion to acetyl-CoA. The function of Hpd1p was further supported by the detection of accumulating 3-hydroxypropionate in the hpd1 mutant on propionyl-CoA-generating nutrients. Substrate specificity of Hpd1p was determined from recombinant purified enzyme, which revealed a preference for 3-hydroxypropionate, although serine and 3-hydroxyisobutyrate could also serve as substrates. Finally, virulence studies in a murine sepsis model revealed attenuated virulence of the hpd1 mutant, which indicates generation of propionyl-CoA from host-provided nutrients during infection.     

3-Hydroxypropionyl-coenzyme A synthetase from Metallosphaera sedula, an enzyme involved in autotrophic CO2 fixation

A modified 3-hydroxypropionate cycle has been proposed as the autotrophic CO₂ fixation pathway for the thermoacidophilic crenarchaeon Metallosphaera sedula. The cycle requires the reductive conversion of 3-hydroxypropionate to propionyl-coenzyme A (propionyl-CoA). The specific activity of the 3-hydroxypropionate-, CoA-, and MgATP-dependent oxidation of NADPH in autotrophically grown cells was 0.023 μmol min⁻¹mg protein⁻¹. The reaction sequence is catalyzed by at least two enzymes. The first enzyme, 3-hydroxypropionyl-CoA synthetase, catalyzes the following reaction: 3-hydroxypropionate + ATP + CoA → 3-hydroxypropionyl-CoA + AMP + PP_i. The enzyme was purified 95-fold to a specific activity of 18 μmol min⁻¹ mg protein⁻¹ from autotrophically grown M. sedula cells. An internal peptide sequence was determined and a gene encoding a homologous protein identified in the genome of Sulfolobus tokodaii; similar genes were found in S. solfataricus and S. acidocaldarius. The gene was heterologously expressed in Escherichia coli, and the His-tagged protein was purified. Both the native enzyme from M. sedula and the recombinant enzyme from S. tokodaii not only activated 3-hydroxypropionate to its CoA ester but also activated propionate, acrylate, acetate, and butyrate; however, with the exception of propionate, the affinities for these substrates were reduced. 3-Hydroxypropionyl-CoA synthetase is up-regulated eightfold in autotrophically versus heterotrophically grown M. sedula, supporting its proposed role during CO₂ fixation in this archaeon and possibly other members of the Sulfolobaceae family.     

Occurrence,biochemistry and possible biotechnological application of the 3-hydroxypropionate cycle

The 3-hydroxypropionate cycle, a pathway for autotrophic carbon dioxide fixation, is reviewed with special emphasis on the biochemistry of CO₂ fixing enzymes in Acidianus brierleyi, a thermophilic and acidophilic archeon. In the 3-hydroxypropionate cycle, two enzymes, acetyl-CoA carboxylase and propionyl-CoA carboxylase, catalyze CO₂ fixation. It has been shown in A. brierleyi, and subsequently in Metallosphaera sedula, that acetyl-CoA carboxylase is promiscuous, acting equally well on acetyl-CoA and propionyl-CoA. The subunit structure of the acyl-CoA carboxylase was shown to be ₄₄₄. Gene cloning revealed that the genes encoding the three subunits are adjacent to each other. accC encodes the -subunit (59 kDa subunit, biotin carboxylase subunit), accB encodes the -subunit (20 kDa subunit, biotin carboxyl carrier protein), and pccB encodes the -subunit (62 kDa subunit, carboxyltransferase subunit). Sequence analyses showed that accC and accB are co-transcribed and that pccB is transcribed separately. Potential biotechnological applications for the 3-hydroxypropionate cycle are also presented.     

Expression of the sub-pathways of the Chloroflexus aurantiacus 3-hydroxypropionate carbon fixation bicycle in E. coli: Toward horizontal transfer of autotrophic growth

The 3-hydroxypropionate (3-HPA) bicycle is unique among CO₂-fixing systems in that none of its enzymes appear to be affected by oxygen. Moreover, the bicycle includes a number of enzymes that produce novel intermediates of biotechnological interest, and the CO₂-fixing steps in this pathway are relatively rapid. We expressed portions of the 3-HPA bicycle in a heterologous organism, E. coli K12. We subdivided the 3-HPA bicycle into four sub-pathways: (1) synthesis of propionyl-CoA from acetyl-CoA, (2) synthesis of succinate from propionyl-CoA, (3) glyoxylate production and regeneration of acetyl-CoA, and (4) assimilation of glyoxylate and propionyl-CoA to form pyruvate and regenerate acetyl-CoA. We expressed the novel enzymes of the 3-HPA bicycle in operon form and used phenotypic tests for activity. Sub-pathway 1 activated a propionate-specific biosensor. Sub-pathway 2, found in non-CO₂-fixing bacteria, was reassembled in E. coli using genes from diverse sources. Sub-pathway 3, operating in reverse, generated succinyl-CoA sufficient to rescue a sucAD⁻ double mutant of its diaminopimelic acid (DAP) auxotrophy. Sub-pathway 4 was able to reduce the toxicity of propionate and allow propionate to contribute to cell biomass in a prpC⁻(2 methylcitrate synthase) mutant strain. These results indicate that all of the sub-pathways of the 3-HPA bicycle can function to some extent in vivo in a heterologous organism, as indicated by growth tests. Overexpression of certain enzymes was deleterious to cell growth, and, in particular, expression of MMC-CoA lyase caused a mucoid phenotype. These results have implications for metabolic engineering and for bacterial evolution through horizontal gene transfer.     

Malonic Semialdehyde Reductase,Succinic Semialdehyde Reductase,and Succinyl-Coenzyme A Reductase from Metallosphaera sedula: Enzymes of the Autotrophic 3-Hydroxypropionate/4-Hydroxybutyrate Cycle in Sulfolobales

A 3-hydroxypropionate/4-hydroxybutyrate cycle operates during autotrophic CO₂ fixation in various members of the Crenarchaea. In this cycle, as determined using Metallosphaera sedula, malonyl-coenzyme A (malonyl-CoA) and succinyl-CoA are reductively converted via their semialdehydes to the corresponding alcohols 3-hydroxypropionate and 4-hydroxybutyrate. Here three missing oxidoreductases of this cycle were purified from M. sedula and studied. Malonic semialdehyde reductase, a member of the 3-hydroxyacyl-CoA dehydrogenase family, reduces malonic semialdehyde with NADPH to 3-hydroxypropionate. The latter compound is converted via propionyl-CoA to succinyl-CoA. Succinyl-CoA reduction to succinic semialdehyde is catalyzed by malonyl-CoA/succinyl-CoA reductase, a promiscuous NADPH-dependent enzyme that is a paralogue of aspartate semialdehyde dehydrogenase. Succinic semialdehyde is then reduced with NADPH to 4-hydroxybutyrate by succinic semialdehyde reductase, an enzyme belonging to the Zn-dependent alcohol dehydrogenase family. Genes highly similar to the Metallosphaera genes were found in other members of the Sulfolobales. Only distantly related genes were found in the genomes of autotrophic marine Crenarchaeota that may use a similar cycle in autotrophic carbon fixation.The thermoacidophilic autotrophic crenarchaeum Metallosphaera sedula uses a 3-hydroxypropionate/4-hydroxybutyrate cycle for CO₂ fixation (9, 28, 29, 35) (Fig. ​(Fig.1).1). A similar cycle may operate in other autotrophic members of the Sulfolobales (31) and in mesophilic marine group I Crenarchaea (Cenarchaeum sp., Nitrosopumilus sp.). This cycle uses elements of the 3-hydroxypropionate cycle that was originally discovered in the phototrophic bacterium Chloroflexus aurantiacus (15, 22-25, 41, 42). It involves the carboxylation of acetyl coenzyme A (acetyl-CoA) to malonyl-CoA by a biotin-dependent acetyl-CoA carboxylase (12, 29). The carboxylation product is reduced to malonic semialdehyde by malonyl-CoA reductase (1). Malonic semialdehyde is further reduced to 3-hydroxypropionate, the characteristic intermediate of the pathway (9, 31, 35). 3-Hydroxypropionate is further reductively converted to propionyl-CoA (3), which is carboxylated to (S)-methylmalonyl-CoA by propionyl-CoA carboxylase. Only one copy of the genes encoding the acetyl-CoA/propionyl-CoA carboxylase subunits is present in most Archaea, indicating that this enzyme is a promiscuous enzyme that acts on both acetyl-CoA and propionyl-CoA (12, 29). (S)-Methylmalonyl-CoA is isomerized to (R)-methylmalonyl-CoA, which is followed by carbon rearrangement to succinyl-CoA catalyzed by coenzyme B₁₂-dependent methylmalonyl-CoA mutase.Open in a separate windowFIG. 1.Proposed 3-hydroxypropionate/4-hydroxybutyrate cycle in M. sedula and other autotrophic Sulfolobales. Enzymes: 1, acetyl-CoA carboxylase; 2, malonyl-CoA reductase (NADPH); 3, malonate semialdehyde reductase (NADPH); 4, 3-hydroxypropionate-CoA ligase (AMP forming); 5, 3-hydroxypropionyl-CoA dehydratase; 6, acryloyl-CoA reductase (NADPH); 7, propionyl-CoA carboxylase, identical to acetyl-CoA carboxylase; 8, (S)-methylmalonyl-CoA epimerase; 9, methylmalonyl-CoA mutase; 10, succinyl-CoA reductase (NADPH), identical to malonyl-CoA reductase; 11, succinic semialdehyde reductase (NADPH); 12, 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 highlighted steps are catalyzed by the enzymes studied here.Succinyl-CoA is converted via succinic semialdehyde and 4-hydroxybutyrate to two molecules of acetyl-CoA (9), thus regenerating the starting CO₂ acceptor molecule and releasing another acetyl-CoA molecule 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 molecule and two bicarbonate molecules into succinyl-CoA (Fig. ​(Fig.1,1, steps 1 to 9), and the second part converts succinyl-CoA to two acetyl-CoA molecules (Fig. ​(Fig.1,1, steps 10 to 16).The second part of the autotrophic cycle also occurs in the dicarboxylate/4-hydroxybutyrate cycle, which operates in autotrophic CO₂ fixation in Desulfurococcales and Thermoproteales (Crenarchaea) (27, 37), raising the question of whether the enzymes in these two lineages have common roots (37). The first part of the cycle also occurs in the 3-hydroxypropionate cycle for autotrophic CO₂ fixation in Chloroflexus aurantiacus and a few related green nonsulfur phototrophic bacteria (19, 22, 23, 32, 49).The two-step reduction of malonyl-CoA to 3-hydroxpropionate in Chloroflexus is catalyzed by a single bifunctional 300-kDa enzyme (30). The M. sedula malonyl-CoA reductase is completely unrelated and forms only malonic semialdehyde (1), and the enzyme catalyzing the second malonic semialdehyde reduction step that forms 3-hydroxypropionate is unknown. In the second part of the 3-hydroxypropionate/4-hydroxybutyrate cycle a similar reduction of succinyl-CoA via succinic semialdehyde to 4-hydroxybutyrate takes place. The enzymes responsible for these reactions also have not been characterized.In this work we purified the enzymes malonic semialdehyde reductase, succinyl-CoA reductase, and succinic semialdehyde reductase from M. sedula. The genes coding for these enzymes were identified in the genome, and recombinant proteins were studied in some detail. Interestingly, succinyl-CoA reductase turned out to be identical to malonyl-CoA reductase. We also show here that enzymes that are highly similar to succinyl-CoA reductase in Thermoproteus neutrophilus do not function as succinyl-CoA reductases in M. sedula.     

Structural Basis for a Bispecific NADP+ and CoA Binding Site in an Archaeal Malonyl-Coenzyme A Reductase

Autotrophic members of the Sulfolobales (crenarchaeota) use the 3-hydroxypropionate/4-hydroxybutyrate cycle to assimilate CO₂ into cell material. The product of the initial acetyl-CoA carboxylation with CO₂, malonyl-CoA, is further reduced to malonic semialdehyde by an NADPH-dependent malonyl-CoA reductase (MCR); the enzyme also catalyzes the reduction of succinyl-CoA to succinic semialdehyde onwards in the cycle. Here, we present the crystal structure of Sulfolobus tokodaii malonyl-CoA reductase in the substrate-free state and in complex with NADP⁺ and CoA. Structural analysis revealed an unexpected reaction cycle in which NADP⁺ and CoA successively occupy identical binding sites. Both coenzymes are pressed into an S-shaped, nearly superimposable structure imposed by a fixed and preformed binding site. The template-governed cofactor shaping implicates the same binding site for the 3′- and 2′-ribose phosphate group of CoA and NADP⁺, respectively, but a different one for the common ADP part: the β-phosphate of CoA aligns with the α-phosphate of NADP⁺. Evolution from an NADP⁺ to a bispecific NADP⁺ and CoA binding site involves many amino acid exchanges within a complex process by which constraints of the CoA structure also influence NADP⁺ binding. Based on the paralogous aspartate-β-semialdehyde dehydrogenase structurally characterized with a covalent Cys-aspartyl adduct, a malonyl/succinyl group can be reliably modeled into MCR and discussed regarding its binding mode, the malonyl/succinyl specificity, and the catalyzed reaction. The modified polypeptide surrounding around the absent ammonium group in malonate/succinate compared with aspartate provides the structural basis for engineering a methylmalonyl-CoA reductase applied for biotechnical polyester building block synthesis.     

Deletion of acetyl-CoA synthetases I and II increases production of 3-hydroxypropionate by the metabolically-engineered hyperthermophile Pyrococcus furiosus

The heterotrophic, hyperthermophilic archaeon Pyrococcus furiosus is a new addition to the growing list of genetically-tractable microorganisms suitable for metabolic engineering to produce liquid fuels and industrial chemicals. P. furiosus was recently engineered to generate 3-hydroxypropionate (3-HP) from CO₂ and acetyl-CoA by the heterologous-expression of three enzymes from the CO₂ fixation cycle of the thermoacidophilic archaeon Metallosphaera sedula using a thermally-triggered induction system. The acetyl-CoA for this pathway is generated from glucose catabolism that in wild-type P. furiosus is converted to acetate with concurrent ATP production by the heterotetrameric (α₂β₂) acetyl-CoA synthetase (ACS). Hence ACS in the engineered 3-HP production strain (MW56) competes with the heterologous pathway for acetyl-CoA. Herein we show that strains of MW56 lacking the α-subunit of either of the two ACSs previously characterized from P. furiosus (ACSI and ACSII) exhibit a three-fold increase in specific 3-HP production. The ΔACSIα strain displayed only a minor defect in growth on either maltose or peptides, while no growth defect on these substrates was observed with the ΔACSIIα strain. Deletion of individual and multiple ACS subunits was also shown to decrease CoA release activity for several different CoA ester substrates in addition to acetyl-CoA, information that will be extremely useful for future metabolic engineering endeavors in P. furiosus.     

The parageneses thermodynamic analysis of chemoautotrophic CO2 fixation archaic cycle components, their stability and self-organization in hydrothermal systems

The parageneses physico-chemical analysis based on a method of thermodynamic potentials has been used to study the system of C-H-O organic compounds, which are, in particular, components of biomimetically built primordial cycles of carbon dioxide chemoautotrophic fixation. Thermodynamic data for aqueous organic compounds allowed one to construct the chemical potential diagrams and establish the areas of thermodynamic stability (facies) of components of CO₂ fixation pathways in hydrothermal systems, in particular, a reductive citric cycle (RCC), 3-hydroxypropionate cycle (3-HPC) and acetyl-CoA pathway. An alternative deep source of carbon (hydrocarbons) proved by the data on endogenous emission of hydrocarbons in hydrothermal fields of oceanic ridges was suggested. The system was determined, which combines hydrocarbons, CO₂ and components of RCC, 3-HPC and acetyl-CoA pathway with characteristic parageneses of methane and ethylene with acetate in two-component CH₄-CO₂ and C₂H₄-O₂ subsystems, respectively. The thermodynamic analysis of a redox mode at various pressures and temperatures allowed one to uniquely determine hydrocarbon-organic system able to independently generate acetate and succinate at oxidation of deep hydrothermal hydrocarbon fluids emerging on sea surface. The limits for thermodynamic stability of CO₂ archaic fixation (CAF) components responsible for generation and self-organization in hydrothermal environment was identified. The tentative integrated system of CAF was developed as a combined acetyl-CoA pathway, 3-HPC and RCC containing a succinate-fumarate core, capable of switching electron flow in forward or reverse direction depending on redox potential of geochemical environment that is governed by the (CH)₂(COOH)₂+H₂(CH₂)₂(COOH)₂ reaction. This core is a “redox switch”, which is sensitive to certain conditions of hydrothermal environment and defines electron flow direction. The redox geochemical mode caused by temperature, pressure, composition of a hydrothermal fluid and a mineralogical setting defines stability of CAF cycle components in paragenesis with hydrocarbons and possibility of cycle self-organization.     

Enzymatic hydrolysis of bacterial poly(3-hydroxybutyrate-co-3-hydroxypropionate)s by poly(3-hydroxyalkanoate) depolymerase from Acidovorax Sp. TP4

Enzymatic degradability has been investigated for a series of bacterial poly(3-hydroxybutyrate-co-3-hydroxypropionate)s (P(3HB-co-3HP)s) with 3-hydroxypropionate (3HP) unit contents from 11 to 86 mol % as well as poly(3-hydroxybutyrate) (P(3HB)) and chemosynthesized poly(3-hydroxypropionate) (P(3HP)). The behavior of degradation by two types of extracellular poly(3-hydroxyalkanoate) (PHA) depolymerases purified from Ralstonia pikettii T1 and Acidovorax Sp. TP4, defined respectively as PHA depolymerase types I and II according to the position of the lipase box in the catalytic domain, were compared in relation to the thermal properties and crystalline structures of the PHA samples elucidated by differential scanning calorimetry and wide-angle X-ray diffraction. The degradation products were characterized by high-performance liquid chromatography and one- (1D) and two-dimension (2D) (1)H NMR spectroscopy. It was found that the PHA depolymerase of Acidovorax Sp. TP4 showed degradation behavior different from that shown by depolymerase of R. pikettii T1. PHA depolymerase from Acidovorax Sp. TP4 degraded the P(3HB-co-3HP) films with lower crystallinity in higher rates than those with higher crystallinity, no matter what kinds of crystalline structures they formed. In contrast, PHA depolymerase from R. pikettii T1 degraded P(3HB-co-3HP) films forming P(3HB) crystalline structure in higher rates than those forming P(3HP)s. The increase in amorphous nature of the P(3HB-co-3HP) films with P(3HB)-homopolymer-like crystalline structure increases and then decreases the rate of degradation by depolymerase from R. pikettii T1. The 3-hydroxybutyrate (3HB) monomer was produced as a major product by the hydrolysis of P(3HB) film by PHA depolymerase from Acidovorax Sp. TP4. The P(3HB-co-3HP) films could be degraded into 3HB and 3-hydroxypropionate (3HP) monomer at last, indicating that the catalytic domain of the enzyme recognized at least two monomeric units as substrates. While the PHA depolymerase from R. pikettii T1 hydrolyzed P(3HB) film into 3HB dimer as a major product, and the catalytic domain recognized at least three monomeric units. The degradation behavior of P(3HB-co-3HP) films by the PHA depolymerase of Acidovorax Sp. TP4 could be distinguished from that by the depolymerase of R. pikettii T1.     

Rhodobacter sphaeroides uses a reductive route via propionyl coenzyme A to assimilate 3-hydroxypropionate

3-Hydroxypropionate is a product or intermediate of the carbon metabolism of organisms from all three domains of life. However, little is known about how carbon derived from 3-hydroxypropionate is assimilated by organisms that can utilize this C(3) compound as a carbon source. This work uses the model bacterium Rhodobacter sphaeroides to begin to elucidate how 3-hydroxypropionate can be incorporated into cell constituents. To this end, a quantitative assay for 3-hydroxypropionate was developed by using recombinant propionyl coenzyme A (propionyl-CoA) synthase from Chloroflexus aurantiacus. Using this assay, we demonstrate that R. sphaeroides can utilize 3-hydroxypropionate as the sole carbon source and energy source. We establish that acetyl-CoA is not the exclusive entry point for 3-hydroxypropionate into the central carbon metabolism and that the reductive conversion of 3-hydroxypropionate to propionyl-CoA is a necessary route for the assimilation of this molecule by R. sphaeroides. Our conclusion is based on the following findings: (i) crotonyl-CoA carboxylase/reductase, a key enzyme of the ethylmalonyl-CoA pathway for acetyl-CoA assimilation, was not essential for growth with 3-hydroxypropionate, as demonstrated by mutant analyses and enzyme activity measurements; (ii) the reductive conversion of 3-hydroxypropionate or acrylate to propionyl-CoA was detected in cell extracts of R. sphaeroides grown with 3-hydroxypropionate, and both activities were upregulated compared to the activities of succinate-grown cells; and (iii) the inactivation of acuI, encoding a candidate acrylyl-CoA reductase, resulted in a 3-hydroxypropionate-negative growth phenotype.     

Production of acrylic acid and propionic acid by constructing a portion of the 3-hydroxypropionate/4-hydroxybutyrate cycle from <Emphasis Type="Italic">Metallosphaera sedula</Emphasis> in <Emphasis Type="Italic">Escherichia coli</Emphasis>

Acrylic acid and propionic acid are important chemicals requiring affordable, renewable production solutions. Here, we metabolically engineered Escherichia coli with genes encoding components of the 3-hydroxypropionate/4-hydroxybutyrate cycle from Metallosphaera sedula for conversion of glucose to acrylic and propionic acids. To construct an acrylic acid-producing pathway in E. coli, heterologous expression of malonyl-CoA reductase (MCR), malonate semialdehyde reductase (MSR), 3-hydroxypropionyl-CoA synthetase (3HPCS), and 3-hydroxypropionyl-CoA dehydratase (3HPCD) from M. sedula was accompanied by overexpression of succinyl-CoA synthetase (SCS) from E. coli. The engineered strain produced 13.28 ± 0.12 mg/L of acrylic acid. To construct a propionic acid-producing pathway, the same five genes were expressed, with the addition of M. sedula acryloyl-CoA reductase (ACR). The engineered strain produced 1430 ± 30 mg/L of propionic acid. This approach can be expanded to synthesize many important organic chemicals, creating new opportunities for the production of chemicals by carbon dioxide fixation.     

Labeling and enzyme studies of the central carbon metabolism in Metallosphaera sedula

Metallosphaera sedula (Sulfolobales, Crenarchaeota) uses the 3-hydroxypropionate/4-hydroxybutyrate cycle for autotrophic carbon fixation. In this pathway, acetyl-coenzyme A (CoA) and succinyl-CoA are the only intermediates that can be considered common to the central carbon metabolism. We addressed the question of which intermediate of the cycle most biosynthetic routes branch off. We labeled autotrophically growing cells by using 4-hydroxy[1-¹⁴C]butyrate and [1,4-¹³C₁]succinate, respectively, as precursors for biosynthesis. The labeling patterns of protein-derived amino acids verified the operation of the proposed carbon fixation cycle, in which 4-hydroxybutyrate is converted to two molecules of acetyl-CoA. The results also showed that major biosynthetic flux does not occur via acetyl-CoA, except for the formation of building blocks that are directly derived from acetyl-CoA. Notably, acetyl-CoA is not assimilated via reductive carboxylation to pyruvate. Rather, our data suggest that the majority of anabolic precursors are derived from succinyl-CoA, which is removed from the cycle via oxidation to malate and oxaloacetate. These C₄ intermediates yield pyruvate and phosphoenolpyruvate (PEP). Enzyme activities that are required for forming intermediates from succinyl-CoA were detected, including enzymes catalyzing gluconeogenesis from PEP. This study completes the picture of the central carbon metabolism in autotrophic Sulfolobales by connecting the autotrophic carbon fixation cycle to the formation of central carbon precursor metabolites.Sulfolobales (Crenarchaeota) comprise extreme thermoacidophiles from volcanic areas that grow best at a pH of around 2 and a temperature of 60 to 90°C (32, 33). Most Sulfolobales can grow chemoautotrophically on sulfur, pyrite, or H₂ under microaerobic conditions, which also applies to Metallosphaera sedula (31), the organism studied here. Its genome has been sequenced (2). Some species of the Sulfolobales secondarily returned to a facultative anaerobic or even strictly anaerobic life style (33), and some laboratory strains appear to have lost their ability to grow autotrophically (8). Autotrophic representatives of the Sulfolobales use a 3-hydroxypropionate/4-hydroxybutyrate cycle (in short, hydroxypropionate/hydroxybutyrate cycle) for autotrophic carbon fixation (Fig. ​(Fig.1)1) (6-8, 38). The enzymes of this cycle are oxygen tolerant, which predestines the cycle for the lifestyle of the aerobic Crenarchaeota (8). The presence of genes coding for key enzymes of the hydroxypropionate/hydroxybutyrate cycle in the mesophilic aerobic “marine group I” Crenarchaeota suggests that these abundant marine archaea use a similar autotrophic carbon fixation mechanism (6, 24, 68) (for a review of autotrophic carbon fixation in Archaea, see reference 7).Open in a separate windowFIG. 1.Proposed 3-hydroxypropionate/4-hydroxybutyrate cycle functioning in autotrophic carbon fixation in Sulfolobales and its relation to the central carbon metabolism, as studied in this work for Metallosphaera sedula. The situation may be similar in other Sulfolobales and possibly in autotrophic marine Crenarchaeota. Enzymes: 1, acetyl-CoA/propionyl-CoA carboxylase; 2, malonyl-CoA reductase (NADPH); 3, malonic semialdehyde reductase (NADPH); 4, 3-hydroxypropionate-CoA ligase (AMP forming); 5, 3-hydroxypropionyl-CoA dehydratase; 6, acryloyl-CoA reductase (NADPH); 7, acetyl-CoA/propionyl-CoA carboxylase; 8, methylmalonyl-CoA epimerase; 9, methylmalonyl-CoA mutase; 10, succinyl-CoA reductase (NADPH); 11, succinic semialdehyde reductase (NADPH); 12, 4-hydroxybutyrate-CoA ligase (AMP forming); 13, 4-hydroxybutyryl-CoA dehydratase; 14 and 15, crotonyl-CoA hydratase/(S)-3-hydroxybutyryl-CoA dehydrogenase (NAD⁺); 16, acetoacetyl-CoA β-ketothiolase; 17, succinyl-CoA synthetase (ADP forming); 18, succinic semialdehyde dehydrogenase; 19, succinate dehydrogenase (natural electron acceptor unknown); 20, fumarate hydratase; 21, malate dehydrogenase; 22, malic enzyme; 23, PEP carboxykinase (GTP); 24, pyruvate:water dikinase (ATP); 25, enolase; 26, phosphoglycerate mutase; 27, phosphoglycerate kinase; 28, glyceraldehyde 3-phosphate dehydrogenase; 29, triosephosphate isomerase; 30, fructose 1,6-bisphosphate aldolase/phosphatase; 31, (si)-citrate synthase; 32, aconitase; 33, isocitrate dehydrogenase.In the cycle, one molecule of acetyl-coenzyme A (CoA) is formed from two molecules of bicarbonate. The key carboxylating enzyme is a bifunctional biotin-dependent acetyl-CoA/propionyl-CoA carboxylase (10, 11, 36, 38, 48, 49). In Bacteria and Eukarya, acetyl-CoA carboxylase catalyzes the first step in fatty acid biosynthesis. However, archaea do not contain fatty acids, and therefore acetyl-CoA carboxylase obviously plays a different metabolic role. The hydroxypropionate/hydroxybutyrate cycle can be divided into two parts. The first transforms acetyl-CoA and two bicarbonate molecules via 3-hydroxypropionate to succinyl-CoA, and the second converts succinyl-CoA via 4-hydroxybutyrate to two acetyl-CoA molecules. In brief, the product of the acetyl-CoA carboxylase reaction, malonyl-CoA, is reduced via malonic semialdehyde to 3-hydroxypropionate, which is further reductively converted to propionyl-CoA. Propionyl-CoA is carboxylated to (S)-methylmalonyl-CoA by the same carboxylase as that that carboxylates acetyl-CoA (11, 36). (S)-Methylmalonyl-CoA is isomerized to (R)-methylmalonyl-CoA, followed by carbon rearrangement to succinyl-CoA catalyzed by coenzyme B₁₂-dependent methylmalonyl-CoA mutase.Succinyl-CoA then is converted into two molecules of acetyl-CoA via succinic semialdehyde, 4-hydroxybutyrate, 4-hydroxybutyryl-CoA, crotonyl-CoA, 3-hydroxyacetyl-CoA, and acetoacetyl-CoA. This reaction sequence apparently is common to the autotrophic Crenarchaeota, as it also is used by autotrophic Crenarchaeota of the orders Thermoproteales and Desulfurococcales, which use a dicarboxylate/4-hydroxybutyrate cycle for autotrophic carbon fixation (8, 34, 55, 56) (also see the accompanying work [57]).From the list of intermediates of the hydroxypropionate/hydroxybutyrate cycle, acetyl-CoA and succinyl-CoA are the only intermediates considered common to the central carbon metabolism. In this work, we addressed the question of which intermediate of the cycle most biosynthetic routes branch off, and we came to the conclusion that succinyl-CoA serves as the main precursor for cellular carbon. This requires one turn of the cycle to regenerate the CO₂ acceptor and to generate one extra molecule of acetyl-CoA from two molecules of bicarbonate. Acetyl-CoA plus another two bicarbonate molecules are converted by an additional half turn of the cycle to succinyl-CoA. This strategy differs from that of the anaerobic pathways, in which acetyl-CoA is reductively carboxylated to pyruvate, and from there the other precursors for building blocks ultimately are derived (discussed in reference 7).     

Biosynthetic pathway for poly(3-Hydroxypropionate) in recombinant Escherichia coli

Poly(3-hydroxypropionate) (P3HP) is a biodegradable and biocompatible thermoplastic. In this study, we engineered a P3HP biosynthetic pathway in recombinant Escherichia coli. The genes for malonyl-CoA reductase (mcr, from Chloroflexus aurantiacus), propionyl-CoA synthetase (prpE, from E. coli), and polyhydroxyalkanoate synthase (phaC1, from Ralstonia eutropha) were cloned and expressed in E. coli. The E. coli genes accABCD encoding acetyl-CoA carboxylase were used to channel the carbon into the P3HP pathway. Using glucose as a sole carbon source, the cell yield and P3HP content were 1.32 g/L and 0.98% (wt/wt [cell dry weight]), respectively. Although the yield is relatively low, our study shows the feasibility of engineering a P3HP biosynthetic pathway using a structurally unrelated carbon source in bacteria.     

Metabolic engineering of type II methanotroph,Methylosinus trichosporium OB3b,for production of 3-hydroxypropionic acid from methane via a malonyl-CoA reductase-dependent pathway

We engineered a type II methanotroph, Methylosinus trichosporium OB3b, for 3-hydroxypropionic acid (3HP) production by reconstructing malonyl-CoA pathway through heterologous expression of Chloroflexus aurantiacus malonyl-CoA reductase (MCR), a bifunctional enzyme. Two strategies were designed and implemented to increase the malonyl-CoA pool and thus, increase in 3HP production. First, we engineered the supply of malonyl-CoA precursors by overexpressing endogenous acetyl-CoA carboxylase (ACC), substantially enhancing the production of 3HP. Overexpression of biotin protein ligase (BPL) and malic enzyme (NADP⁺-ME) led to a ∼22.7% and ∼34.5% increase, respectively, in 3HP titer in ACC-overexpressing cells. Also, the acetyl-CoA carboxylation bypass route was reconstructed to improve 3HP productivity. Co-expression of methylmalonyl-CoA carboxyltransferase (MMC) of Propionibacterium freudenreichii and phosphoenolpyruvate carboxylase (PEPC), which provides the MMC precursor, further improved the 3HP titer. The highest 3HP production of 49 mg/L in the OB3b-MCRMP strain overexpressing MCR, MMC and PEPC resulted in a 2.4-fold improvement of titer compared with that in the only MCR-overexpressing strain. Finally, we could obtain 60.59 mg/L of 3HP in 42 h using the OB3b-MCRMP strain through bioreactor operation, with a 6.36-fold increase of volumetric productivity compared than that in the flask cultures. This work demonstrates metabolic engineering of type II methanotrophs, opening the door for using type II methanotrophs as cell factories for biochemical production along with mitigation of greenhouse gases.