Diversity of Metabolism in Procaryotes (page 7)
(This chapter has 8 pages)
© Kenneth Todar, PhD
Autotrophic CO2
fixation
The use of RUBP carboxylase and the Calvin cycle is the most
common
mechanism for CO2 fixation among autotrophs. Indeed, RUBP
carboxylase
is said to be the most abundant enzyme on the planet (nitrogenase,
which
fixes N2 is second most abundant). This is the only
mechanism
of autotrophic CO2 fixation among eucaryotes, and it is
used,
as well, by all cyanobacteria and purple bacteria. Lithoautotrophic
bacteria
also use this pathway. But the green bacteria and the methanogens, as
well
as a few isolated groups of procaryotes, have alternative mechanisms of
autotrophic CO2 fixation and do not possess RUBP
carboxylase.
RUBP carboxylase (ribulose bisphosphate carboxylase) uses
ribulose
bisphosphate (RUBP) and CO2 as co-substrates. In a
complicated
reaction the CO2 is "fixed" by addition to the RUBP, which
is
immediately cleaved into two molecules of 3-phosphoglyceric acid (PGA).
The fixed CO2 winds up in the -COO group of one of the PGA
molecules.
Actually, this is the reaction which initiates the Calvin cycle (Figure
22 below).
The Calvin cycle is concerned with the conversion of PGA to
intermediates
in glycolysis that can be used for biosynthesis, and with the
regeneration
of RUBP, the substrate that drives the cycle. After the initial
fixation
of CO2, 2 PGA are reduced and combined to form
hexose-phosphate
by reactions which are essentially the reverse of the oxidative
Embden-Meyerhof
pathway. (Now is a good time to go back to Figure 8 and look at the E-M
pathway for the location of PGA and glucose-phosphate). The hexose
phosphate
is converted to pentose-phosphate, which is phosphorylated to
regenerate
RUBP. An important function of the Calvin cycle is to provide the
organic
precursors for the biosynthesis of cell material. Intermediates must be
constantly withdrawn from the Calvin cycle in order to make cell
material.
In this regard, the Calvin cycle is an anabolic pathway. The fixation
of
CO2 to the level of glucose (C6H12O6)
requires 18 ATP and 12 NADPH2.
Figure
22. The Calvin cycle
and its relationship to the synthesis of cell materials.
The methanogens, a very abundant group of procaryotes, use CO2
as a source of carbon for growth, and as a final electron acceptor in
an
energy-producing process that produces methane. If a methanogen is fed
labeled CO2 as a sole form of carbon, 95 percent of the
label
winds up in methane and 5 percent winds up in cell material. The
methanogens
fix CO2 by means of the enzyme CODH (carbon
monoxide
dehydrogenase) and the Acetyl CoA pathway (Figure 23
below).
The pathway of methanogenesis steadily reduces CO2 to the
methyl (CH3) level, mediated by the coenzyme methanopterin
(MP),
related to folic acid. MP-CH3 may be reduced to methane (not
shown) or the MP may be replaced by a vitamin B12-like
molecule
to enter the pathway of CO2 fixation. The "B12"-CH3
is substrate for CO fixation mediated by the CODH. CODH reduces CO2
to CO and adds the CO to "B12"-CH3 to form
acetyl-[CODH].
Coenzyme A (CoA) then replaces the CODH, resulting in the formation of
Acetyl CoA, which is in the heart of biosynthetic metabolism. The net
effect
is the reduction of 2 CO2 to Acetyl CoA.
Figure 23. The CODH or
acetyl
CoA pathway of CO2 fixation in the methanogens. See text for
explanation.
Finally, in the photosynthetic Green Bacteria, the pathway of
autotrophic
CO2 fixation involves the reversal of familiar
decarboxylation
reactions in and around the TCA cycle. The two primary reactions
utilized
by the Green Bacteria are two Ferredoxin (FD)-mediated reactions,
the reduction of Acetyl CoA to pyruvate, and the reduction of succinyl
CoA to alpha-ketoglutarate This is referred to as the reverse TCA
cycle
for CO2 uptake.
Figure 24. The two
ferredoxin
(FD)-mediated reactions used for CO2 uptake in the green
bacteria
are a reversal of the oxidation of keto acids mediated by NAD and CoA
(c.f. Figure
4).
chapter continued
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