Identifying the missing steps of the autotrophic 3-hydroxypropionate CO2 fixation cycle in Chloroflexus aurantiacus.
The phototrophic bacterium Chloroflexus aurantiacus uses a yet unsolved 3-hydroxypropionate cycle for autotrophic CO(2) fixation. It starts from acetyl-CoA, with acetyl-CoA and propionyl-CoA carboxylases acting as carboxylating enzymes. In a first cycle, (S)-malyl-CoA is formed from acetyl-CoA and 2 molecules of bicarbonate. (S)-Malyl-CoA cleavage releases the CO(2) fixation product glyoxylate and regenerates the starting molecule acetyl-CoA. Here we complete the missing steps devoted to glyoxylate assimilation. In a second cycle, glyoxylate is combined with propionyl-CoA, an intermediate of the first cycle, to form beta-methylmalyl-CoA. This condensation is followed by dehydration to mesaconyl-C1-CoA. An unprecedented CoA transferase catalyzes the intramolecular transfer of the CoA moiety to the C4 carboxyl group of mesaconate. Mesaconyl-C4-CoA then is hydrated by an enoyl-CoA hydratase to (S)-citramalyl-CoA. (S)-Citramalyl-CoA is cleaved into acetyl-CoA and pyruvate by a tri-functional lyase, which previously cleaved (S)-malyl-CoA and formed beta-methylmalyl-CoA. Thus, the enigmatic disproportionation of glyoxylate and propionyl-CoA into acetyl-CoA and pyruvate is solved in an elegant and economic way requiring only 3 additional enzymes. The whole bicyclic pathway results in pyruvate formation from 3 molecules of bicarbonate and involves 19 steps but only 13 enzymes. Elements of the 3-hydroxypropionate cycle may be used for the assimilation of small organic molecules. The 3-hydroxypropionate cycle is compared with the Calvin-Benson-Bassham cycle and other autotrophic pathways.
Zarzycki
Jan
J
Mikrobiologie, Fakultät für Biologie, Universität Freiburg, Schänzlestrasse 1, D-79104 Freiburg, Germany.
Brecht
Volker
V
Müller
Michael
M
Fuchs
Georg
G
D000105:Acetyl Coenzyme A / Q000378:metabolism; D000103:Acetyl-CoA Carboxylase / Q000378:metabolism; D000214:Acyl Coenzyme A / Q000378:metabolism; D052818:Autotrophic Processes; D002245:Carbon Dioxide / Q000378:metabolism*; D041861:Chloroflexus / Q000378:metabolism*; D006038:Glyoxylates / Q000378:metabolism; D019344:Lactic Acid / Q000031:analogs & derivatives* / Q000378:metabolism; D053858:Metabolic Networks and Pathways; D043586:Methylmalonyl-CoA Decarboxylase / Q000378:metabolism
D000214:Acyl Coenzyme A; D006038:Glyoxylates; D002245:Carbon Dioxide; C009061:propionyl-coenzyme A; D019344:Lactic Acid; D000105:Acetyl Coenzyme A; C031601:hydracrylic acid; D000103:Acetyl-CoA Carboxylase; D043586:Methylmalonyl-CoA Decarboxylase; C031150:glyoxylic acid