This document discusses different pathways that chemolithotrophic prokaryotes use to fix inorganic carbon during metabolism. The most common pathway is the Calvin cycle, which fixes carbon dioxide into 3-phosphoglycerate. Some bacteria use the reductive tricarboxylic acid cycle or the acetyl-CoA pathway. The reductive TCA cycle operates in reverse using different enzymes than the forward cycle. The acetyl-CoA pathway reduces carbon dioxide to formate and then to acetyl-CoA. These pathways allow chemolithotrophs to use inorganic compounds as energy sources and carbon dioxide as their carbon source.

1. Chemolithotrophic Metabolism

2. Introduction
• Some prokaryotes grow by using reduced inorganic
compounds as their energy source and CO2 as the carbon
source. These are called chemolithotrophs.
• The electron donors used by chemolithotrophs include
nitrogen and sulfur compounds, Fe(II), H2, and CO.
• The Calvin cycle is the most common CO2 fixation
mechanism, and the reductive TCA cycle, acetyl-CoA
pathway and 3-ydroxypropionate cycle are found in
some Chemolithotrophic prokaryotes.
• Some can use organic compounds as their carbon source
while metabolizing an inorganic electron donor.
• This kind of bacterial metabolism is referred to as
mixotrophy.

3. CO2 fixation pathways in chemolithotrophs
Calvin Cycle
Most Aerobic chemolithotrophs
and photolithotrophs (95%)
Reductive TCA
cycle
Some Aerobic chemolithotrophs
and photolithotrophs (04%)
acetyl-CoA
pathway
The anaerobic chemolithotrophs
3-hydroxypropionate
cycle
Rare Aerobic chemolithotrophs
and photolithotrophs (01%)

4. • The Calvin cycle is the most common CO2
fixation pathway in aerobic chemolithotrophs
and in photolithotrophs, and some fix CO2
through the reductive TCA cycle.
• The anaerobic chemolithotrophs, including
methanogens, homoacetogens and
sulfidogens, employ the acetyl-CoA pathway
to fix CO2.
• A fourth CO2 fixation pathway, the 3-
hydroxypropionate cycle, is known in some
chemo- and photolithotrophs.
• The Calvin cycle is the only known CO2-fixing
metabolism in eukaryotes.

5. CALVIN CYCLE
• The Calvin cycle, elucidated by American
biochemist Melvin Calvin, is the most widely
distributed pathway, operating in plants,
algae, photosynthetic bacteria, and most
aerobic lithoautotrophic bacteria.

6. • CO2 is condensed to ribulose-1,5-
bisphosphate to produce two molecules of 3-
phosphoglycerate.

7. • The molecules of 3-phosphoglycerate, then
reduced to glyceraldehyde-3-phosphate.

8. • A molecule of glyceraldehyde-3-phosphate is
isomerized to dihydroxyacetone phosphate.
• Dihydroxyacetone phosphate then
condensed with second glyceraldehyde-3-
phosphate molecule through the reverse
reactions of the EMP pathway to fructose-
1,6- diphosphate.
• Fructose-1,6- diphosphate is
dephosphorylated to fructose-6-phosphate
by the action of fructose-1,6-diphosphatase

9. Isomerase
Aldolase
Phosphatase

10. • Through carbon rearrangement two molecules
of fructose-6-phosphate (2 x C6) and six
molecules of glyceraldehyde-3-phosphate (6 x
C3) are converted to six molecules of ribulose-
5-phosphate (6 x C5).
• Ribulose-5-phosphate then phosphorylated
to ribulose-1,5-bisphosphate to begin the next
round of reactions.

11. Overall Reaction
• 6CO2+18ATP+12NADPH+12H++12H2O
• C6H12O6{glucose} +12NADP++18ADP +18Pi

12. 1
2
3
4
CALVIN CYCLE

13. Key enzymes of Calvin cycle
• Ribulose-1,5-bisphosphate carboxylase and
phosphoribulokinase are key enzymes of the
Calvin cycle, and are present only in the
organisms fixing CO2 through this highly
energy-demanding pathway.
• Their activities are controlled at the
transcriptional level and also after they are
expressed.
• The enzymes are encoded by cbb genes
organized in cbb operons

14. • CbbR is a transcriptional regulator and the key
activator protein of cbb operons.
• The cbbR gene is located adjacent to its
cognate operon.
• Phosphoenolpyruvate is a negative effector of
CbbR, whereas NADPH is a coactivator of the
protein.

15. • Ribulose-1,5-bisphosphate carboxylase is the
most abundant single protein on Earth and is
synthesized by all organisms fixing CO2
through the Calvin cycle including plants.
• Two forms: Type I- Most common, consist of 8
large and 8 small subunits – form
hexadecameric structure- widely distributed in
CO2 fixing organisms.
• Type-II – consist only large subunits (2, 4 or 8)-
found in anaerobic organism.

16. Reductive TCA cycle
• The bacterium does not have enzymes of the
Calvin cycle, and fixes CO2 through the
reductive TCA cycle.
• This CO2-fixing metabolism shares TCA cycle
enzymes that catalyze the reverse reactions.
• Irreverse reactions are replaced by another set
of enzymes.

18. • Some bacteria and archaea reduce CO2 to
acetyl-CoA in the reverse direction of the TCA
cycle.
• The TCA cycle enzymes unable to catalyze the
reverse reaction, i.e. citrate synthase, 2-
ketoglutarate dehydrogenase and succinate
dehydrogenase, are replaced by ATP:citrate
lyase (8), 2-ketoglutarate synthase (5) and
fumarate reductase (3).

19. • Acetyl-CoA is reduced to pyruvate by
pyruvate:ferredoxin oxidoreductase.
• Pyruvate converted to PEP by the action of
PEP synthetase.

20. Co2 fixation through the acetyl CoA
pathway
• This pathway is employed for co2 fixation by
anaerobic chemolithotrophs, including
sulfidogens, methanogens and
homoacetogens.

22. • 1. CO2 is reduced to formate by – formate
dehydrogenase
• 2. Formate is bound to tetrahydrofolate (H4F)
and form formyl with help of enzyme formyl-
H4F synthatase followed by reduced to
methenyl---methylele---methyl formate.
• This methyl group is transferred to coenzyme
B12 (corrinoid).
• Simillarly- second CO2 is reduced to CO
(carbon monoxide) by – carbon monoxide
dehydrogenase (CODH).

23. • This CO is enzyme bound and with help of CO-
dehyderogenase, it form acetyl-CoA with
involving methyl group from coenzyme B12.
• CODH is duel function enzyme catalyzing CO
oxidation and reduction and acetyl CoA
synthesis/cleavage.