it may yield less energy (for dissimilatory reactions) or may be energetically more
costly (for assimilatory processes).
Sugars such as glucose can be degraded to pyruvate by the Embden-MeyerhofParnas glycolytic pathway via fructose-1,6-bisphosphate or by the Entner-Doudoroff
pathway with 6-phosphogluconate and 2-keto-3-deoxy-6-phosphogluconate as
intermediates. In the latter pathway, only one molecule of ATP is formed for each
two pyruvate generated instead of two ATP in the classical glycolytic pathway. The
Entner-Doudoroff pathway is also operative in barley and possibly in other higher
plants (Chen et al. 2016). There are additional ways to degrade sugars, such as the
oxidative pentose phosphate cycle in which three molecules of 6-phosphogluconate
are decarboxylated to three ribulose-5-phosphate, which are then converted to one
glyceraldehyde-3-phosphate and two fructose-6-phosphate.
For fermentative organisms, the difference between one and two ATP formed is
highly significant. Still, besides the alcohol fermentation of yeasts (the EmbdenMeyerhof-Parnas glycolytic pathway, yielding two ATP per glucose fermented), we
have the alcohol fermentation by Zymomonas that yields the same end products with
generation of 1 ATP only, as the organism uses the Entner-Doudoroff pathway
(Swings and De Ley 1977). Another case in which the same fermentation products
can be formed by using different metabolic pathways is the fermentation of lactate
with the formation of propionate, acetate, and CO 2 . In most propionate-forming
bacteria, succinate formed by carboxylation of pyruvate is an intermediate; however,
in Anaerotignum propionicum (basonym: Clostridium propionicum) and
Megasphaera elsdenii, no such carboxylation reaction occurs, and the three-carbon
compound acrylate is a key intermediate (Gottschalk 1985).
When the reactions of the reductive pentose phosphate cycle (the Calvin-BensonBassham cycle) were discovered in 1950, the cycle was proposed as the universal
pathway for autotrophic carbon dioxide assimilation, the most important biosynthetic process in biology. However, a second pathway for autotrophic CO 2 fixation,
the reductive citric acid cycle, had been discovered by 1966. The question must be
asked why nature has devised many different ways to active the same goal (Berg
2011). Today we know no less than six different pathways used by different
autotrophic prokaryotes for the assimilation of inorganic carbon into their biomass.
These include:
The Calvin-Benson-Bassham cycle with ribulose-1,5-bisphosphate carboxylase/
oxygenase (RuBisCO) and phosphoribulokinase as the key enzymes. To some
extent, it is surprising that most autotrophic CO 2 fixation is mediated by RuBisCo:
the enzyme has a low affinity for CO 2 , its catalytic turnover rate is slow, and it has a
wasteful oxygenase side reaction that generates phosphoglycolate, leading to photorespiration in oxygenic phototrophs. There appears to be a sharp upper temperature
limit (~70
C–75
C) for the functioning of the cycle. This may be due to the heat
instability of glyceraldehyde-3-phosphate and other intermediates of the cycle.
The reductive citric acid cycle, also known as the Arnon-Buchanan cycle. In this
pathway, most reactions of the tricarboxylic acid cycle (the Krebs cycle) are
reversed. Four carboxylation reactions are operative: carboxylation of acetyl-CoA
to pyruvate, phosphoenolpyruvate to oxaloacetate, succinyl-CoA to 2-oxoglutarate,
10 The Grand Microbial Variety Show
173
costly (for assimilatory processes).
Sugars such as glucose can be degraded to pyruvate by the Embden-MeyerhofParnas glycolytic pathway via fructose-1,6-bisphosphate or by the Entner-Doudoroff
pathway with 6-phosphogluconate and 2-keto-3-deoxy-6-phosphogluconate as
intermediates. In the latter pathway, only one molecule of ATP is formed for each
two pyruvate generated instead of two ATP in the classical glycolytic pathway. The
Entner-Doudoroff pathway is also operative in barley and possibly in other higher
plants (Chen et al. 2016). There are additional ways to degrade sugars, such as the
oxidative pentose phosphate cycle in which three molecules of 6-phosphogluconate
are decarboxylated to three ribulose-5-phosphate, which are then converted to one
glyceraldehyde-3-phosphate and two fructose-6-phosphate.
For fermentative organisms, the difference between one and two ATP formed is
highly significant. Still, besides the alcohol fermentation of yeasts (the EmbdenMeyerhof-Parnas glycolytic pathway, yielding two ATP per glucose fermented), we
have the alcohol fermentation by Zymomonas that yields the same end products with
generation of 1 ATP only, as the organism uses the Entner-Doudoroff pathway
(Swings and De Ley 1977). Another case in which the same fermentation products
can be formed by using different metabolic pathways is the fermentation of lactate
with the formation of propionate, acetate, and CO 2 . In most propionate-forming
bacteria, succinate formed by carboxylation of pyruvate is an intermediate; however,
in Anaerotignum propionicum (basonym: Clostridium propionicum) and
Megasphaera elsdenii, no such carboxylation reaction occurs, and the three-carbon
compound acrylate is a key intermediate (Gottschalk 1985).
When the reactions of the reductive pentose phosphate cycle (the Calvin-BensonBassham cycle) were discovered in 1950, the cycle was proposed as the universal
pathway for autotrophic carbon dioxide assimilation, the most important biosynthetic process in biology. However, a second pathway for autotrophic CO 2 fixation,
the reductive citric acid cycle, had been discovered by 1966. The question must be
asked why nature has devised many different ways to active the same goal (Berg
2011). Today we know no less than six different pathways used by different
autotrophic prokaryotes for the assimilation of inorganic carbon into their biomass.
These include:
The Calvin-Benson-Bassham cycle with ribulose-1,5-bisphosphate carboxylase/
oxygenase (RuBisCO) and phosphoribulokinase as the key enzymes. To some
extent, it is surprising that most autotrophic CO 2 fixation is mediated by RuBisCo:
the enzyme has a low affinity for CO 2 , its catalytic turnover rate is slow, and it has a
wasteful oxygenase side reaction that generates phosphoglycolate, leading to photorespiration in oxygenic phototrophs. There appears to be a sharp upper temperature
limit (~70
C–75
C) for the functioning of the cycle. This may be due to the heat
instability of glyceraldehyde-3-phosphate and other intermediates of the cycle.
The reductive citric acid cycle, also known as the Arnon-Buchanan cycle. In this
pathway, most reactions of the tricarboxylic acid cycle (the Krebs cycle) are
reversed. Four carboxylation reactions are operative: carboxylation of acetyl-CoA
to pyruvate, phosphoenolpyruvate to oxaloacetate, succinyl-CoA to 2-oxoglutarate,
10 The Grand Microbial Variety Show
173