A Bicyclic Autotrophic CO2 Fixation Pathway in Chloroflexus aurantiacus

Abstract

Phototrophic CO 2 assimilation by the primitive, green eubacterium Chloroflexus aurantiacus has been shown earlier to proceed in a cyclic mode via 3-hydroxypropionate, propionyl-CoA, succinyl-CoA, and malyl-CoA. The metabolic cycle could be closed by cleavage of malylCoA affording glyoxylate (the primary CO 2 fixation product) with regeneration of acetyl-CoA serving as the starter unit of the cycle. Cell extracts of C. aurantiacus were also shown to catalyze the conversion of citramalate into pyruvate and acetyl-CoA in a succinyl-CoAdependent reaction. The data suggest that glyoxylate obtained by the cleavage of malyl-CoA can be utilized by condensation with propionyl-CoA affording erythro-␤-methylmalyl-CoA, which is converted to acetyl-CoA and pyruvate. This reaction sequence regenerates acetylCoA, which serves as the precursor of propionyl-CoA in the 3-hydroxypropionate cycle. Autotrophic CO 2 fixation proceeds by combination of the 3-hydroxypropionate cycle with the methylmalyl-CoA cycle. The net product of that bicyclic autotrophic CO 2 fixation pathway is pyruvate serving as an universal building block for anabolic reactions.Autotrophic CO 2 fixation in the phototrophic bacterium Chloroflexus aurantiacus has been proposed to proceed via a novel pathway, the 3-hydroxypropionate cycle ( Fig. 1) (1-7). Briefly, acetyl-CoA (1) serves as starting unit, and biotin-dependent carboxylation of acetyl-CoA and propionyl-CoA (4) are the main CO 2 fixation reactions. One turn of the proposed cycle results in conversion of acetyl-CoA into malyl-CoA (8) with consumption of 2 HCO 3 Ϫ and 3 NADPH. Malyl-CoA is cleaved by malyl-CoA lyase with regeneration of the starting molecule acetyl-CoA. Glyoxylate (9) is believed to be the initial CO 2 fixation product (7).The pathway of glyoxylate assimilation into cell material is incompletely understood (5-12). Glycine has been ruled out as an intermediate (7). So far, in vitro transformation of glyoxylate has not been observed, except for pyridine nucleotide-dependent reduction to glycolate (7). An acetyl-CoA-dependent conversion of glyoxylate to malyl-CoA and malate was ascribed to the reverse reaction of malyl-CoA lyase forming malyl-CoA, combined with a side reaction of citrate synthase or acyl-CoA thioesterase, which hydrolyzes malyl-CoA to malate and CoA (7,(13)(14)(15). Previous studies have shown that C. aurantiacus can use pyruvate for anaplerotic reactions (3,7,11,16). Pyruvate is converted to phosphoenolpyruvate (PEP) 1 by pyruvate phosphate dikinase, followed by PEP carboxylation to oxaloacetate by PEP carboxylase. However, pyruvate synthase activity was hardly detectable (12), and the origin of pyruvate in C. aurantiacus is still unknown. To serve as a central intermediate for anaplerotic reactions, it should be formed ultimately from one of the intermediates of the 3-hydroxypropionate cycle and/or from glyoxylate.The aim of this work was to elucidate reactions for glyoxylate assimilation. We show that a reaction sequence starting with glyoxylate and propionyl-CoA afford...

Characterization of the 2-Hydroxy-acid Dehydrogenase McyI, Encoded within the Microcystin Biosynthesis Gene Cluster of Microcystis aeruginosa PCC7806

“…The oily crystals were washed briefly with cold chloroform, in which hydroxymethylsuccinic acids are insoluble, leaving a colorless crystalline solid (0.54 g). The 600- (12). The resonances are also consistent with earlier NMR data recorded at lower fields and thus lacking some of the dispersion reported here for both the free acids and some derivatives (13)(14)(15).…”

l -Malyl-Coenzyme A Lyase/β-Methylmalyl-Coenzyme A Lyase from Chloroflexus aurantiacus , a Bifunctional Enzyme Involved in Autotrophic CO 2 Fixation

“…The volume of the supernatant was reduced to 5 ml by flash evaporation at 30°C (3 kPa) and subjected to a reversedphase column (250 by 20 mm, 10 m; Grom-Sil 120 ODS-4 HE; Crom, Herrenberg-Kayh, Germany), which was developed by a step gradient (64 ml each) of 2.9, 4.8, 5.7, 6.7, 8.6, and 10.5% acetonitrile (vol/vol) in 50 mM potassium phosphate buffer, pH 6.7, with a flow rate of 8 ml min Ϫ1 . The effluent was monitored by a radiomonitor and photometrically (260 nm (18).…”

Design and analysis of synthetic carbon fixation pathways

“…These are the reductive tricarboxylic acid (rTCA) cycle, postulated in the 1960s (14); the oxygen-sensitive reductive acetyl-CoA (rAcCoA) pathway (15); the extensively researched 3-hydroxypropionate cycle (16); the 3-hydroxypropionate/4-hydroxybutyrate cycle (17); and the recently discovered dicarboxylate/4-hydroxybutyrate cycle (18).…”