CO 2 fixation. In the methanotrophic prokaryotes, we know two different pathways
for carbon fixation: the ribulose monophosphate pathway and the serine pathway. A
third pathway is operative in methanotrophic yeasts the dihydroxyacetone pathway,
also known as the xylulose monophosphate pathway, with as key reaction the
formation of glyceraldehyde-3-phosphate + dihydroxyacetone from xylulose-5phosphate and formaldehyde. The ribulose monophosphate pathway is based on
the reaction of formaldehyde with ribulose-5-phosphate to yield 3-hexulose-6-phosphate that is then converted to fructose-6-phosphate. The formaldehyde is derived
from the oxidation of methane via methanol. The 3-hexulose-6-phosphate synthase
and 6-phospho-3-hexuloisomerase are the key enzymes. This pathway is found in
methanotrophs with Type I membranes, which include members of the
Gammaproteobacteria such as the genera Methylococcus, Methylomonas, and
Methylobacter (Colby et al. 1979; Hanson and Hanson 1996). The ribulose
monophosphate pathway is widespread in the prokaryotic world, not only in organisms that grow on methane, but it is also involved in formaldehyde fixation and
detoxification, and in some archaea its enzymes catalyze the reverse reaction for the
biosynthesis of pentose phosphate (Kato et al. 2006). Methanotrophs with Type II
membranes such as Methylosinus and Methylocystis (Betaproteobacteria) employ
the serine pathway that combines the assimilation of formaldehyde (formation of
serine from glycine) and CO 2 (carboxylation of phosphoenolpyruvate to oxaloacetate) (Colby et al. 1979; Hanson and Hanson 1996).
10.11 How Much Free Energy Should a Reaction
Minimally Yield to Support Microbial Growth?
The amount of Gibbs free energy needed to form one mole of ATP from ADP and
inorganic phosphate is about À32 kJ under standard conditions and approximately
À44 kJ when taking the true intracellular concentrations of the compounds into
account (Thauer et al. 1977).
It is always surprising that many anaerobic prokaryotes thrive on dissimilatory
reactions that yield even less free energy than needed for the synthesis of one ATP
molecule. An example is the sulfur-reducing, acetate-oxidizing bacterium
Desulfuromonas acetoxidans, which obtains its energy from the reaction:
2 Acetate
À
þ 2 H
þ
þ S 8 þ 4 H 2 O ! 4 CO 2 þ 8 H 2 S;
ΔG
0
o ¼ À16:7 kJ per acetate oxidized
Desulfuromonas forms robust growing syntrophic mixed cultures with
phototrophic green sulfur bacteria that re-oxide the sulfide formed (Pfennig and
Biebl 1976).
Syntrophic growth, i.e., the growth of two types of organisms together in which
one partner consumes metabolic end products excreted by the other, is a well-known
10 The Grand Microbial Variety Show
175
for carbon fixation: the ribulose monophosphate pathway and the serine pathway. A
third pathway is operative in methanotrophic yeasts the dihydroxyacetone pathway,
also known as the xylulose monophosphate pathway, with as key reaction the
formation of glyceraldehyde-3-phosphate + dihydroxyacetone from xylulose-5phosphate and formaldehyde. The ribulose monophosphate pathway is based on
the reaction of formaldehyde with ribulose-5-phosphate to yield 3-hexulose-6-phosphate that is then converted to fructose-6-phosphate. The formaldehyde is derived
from the oxidation of methane via methanol. The 3-hexulose-6-phosphate synthase
and 6-phospho-3-hexuloisomerase are the key enzymes. This pathway is found in
methanotrophs with Type I membranes, which include members of the
Gammaproteobacteria such as the genera Methylococcus, Methylomonas, and
Methylobacter (Colby et al. 1979; Hanson and Hanson 1996). The ribulose
monophosphate pathway is widespread in the prokaryotic world, not only in organisms that grow on methane, but it is also involved in formaldehyde fixation and
detoxification, and in some archaea its enzymes catalyze the reverse reaction for the
biosynthesis of pentose phosphate (Kato et al. 2006). Methanotrophs with Type II
membranes such as Methylosinus and Methylocystis (Betaproteobacteria) employ
the serine pathway that combines the assimilation of formaldehyde (formation of
serine from glycine) and CO 2 (carboxylation of phosphoenolpyruvate to oxaloacetate) (Colby et al. 1979; Hanson and Hanson 1996).
10.11 How Much Free Energy Should a Reaction
Minimally Yield to Support Microbial Growth?
The amount of Gibbs free energy needed to form one mole of ATP from ADP and
inorganic phosphate is about À32 kJ under standard conditions and approximately
À44 kJ when taking the true intracellular concentrations of the compounds into
account (Thauer et al. 1977).
It is always surprising that many anaerobic prokaryotes thrive on dissimilatory
reactions that yield even less free energy than needed for the synthesis of one ATP
molecule. An example is the sulfur-reducing, acetate-oxidizing bacterium
Desulfuromonas acetoxidans, which obtains its energy from the reaction:
2 Acetate
À
þ 2 H
þ
þ S 8 þ 4 H 2 O ! 4 CO 2 þ 8 H 2 S;
ΔG
0
o ¼ À16:7 kJ per acetate oxidized
Desulfuromonas forms robust growing syntrophic mixed cultures with
phototrophic green sulfur bacteria that re-oxide the sulfide formed (Pfennig and
Biebl 1976).
Syntrophic growth, i.e., the growth of two types of organisms together in which
one partner consumes metabolic end products excreted by the other, is a well-known
10 The Grand Microbial Variety Show
175