W. Eisenreich et al., RETROBIOSYNTHETIC ANALYSIS OF CARBON FIXATION IN THE PHOTOTROPHIC EUBACTERIUM CHLOROFLEXUS-AURANTIACUS, European journal of biochemistry, 215(3), 1993, pp. 619-632
The phototrophic bacterium Chloroflexus aurantiacus does not use any o
f the known autotrophic CO2 fixation pathways. There is evidence for a
new cyclic autotrophic pathway in which acetyl-CoA is converted to 3-
hydroxypropionate and further to succinate and malate. This hypothesis
was tested by feeding growing cultures during several generations wit
h 3-hydroxy[1-C-13]propionate, [1-C-13]acetate, or [2-C-13]acetate, in
addition to unlabeled CO2. The relative C-13 content of individual ca
rbon atoms in biosynthetic amino acids and nucleosides was determined
by H-1- and C-13-NMR spectroscopy. C-13 coupling patterns were analyze
d by two-dimensional C-13-TOCSY experiments which were optimized for t
he analysis of multiply C-13-labeled biosynthetic samples. From the C-
13 enrichments of amino acids and nucleosides, the labeling patterns o
f central metabolic intermediates were evaluated by a retrobiosyntheti
c approach. Both 3-hydroxypropionate and acetate were incorporated int
o all central metabolic pools. The C-13 labeling and coupling patterns
suggest a novel carbon fixation pathway via 3-hydroxypropionate. Spec
ifically, we propose that acetyl-CoA is carboxylated to malonyl-CoA wh
ich is reduced under formation of 3-hydroxypropionyl-CoA. Dehydration
and reduction yield propionyl-CoA which is converted to succinate by a
second carboxylation reaction. The net product of autotrophic carbon
fixation appears to be glyoxylate. However, it is not yet known how gl
yoxylate is channeled into anabolic metabolism. Assimilation of acetat
e can proceed via the CO2 fixation pathway, but also via the glyoxylat
e pathway.