Diversity of Metabolism in Procaryotes (page 3)
(This chapter has 8 pages)
© Kenneth Todar, PhD
Heterotrophic Types of
Heterotrophy (i.e. chemoheterotrophy) is the use of an organic
as a source of carbon and energy. It is the complete metabolism
The cell oxidizes organic molecules in order to produce energy
and then uses the energy to synthesize cellular material from these the
organic molecules (anabolism). We animals are familiar with
metabolism. Many Bacteria (but just a few Archaea) are
particularly those that live in associations with animals.
bacteria are the masters of decomposition and biodegradation in the
Heterotrophic metabolism is driven mainly by two metabolic processes:
Fermentation is an ancient mode of metabolism, and it must have
evolved with the appearance of organic material on the planet.
is metabolism in which energy is derived from the partial oxidation
of an organic compound using organic intermediates as electron
and electron acceptors. No outside electron acceptors are involved;
no membrane or electron transport system is required; all ATP is
by substrate level phosphorylation.
By definition, fermentation may be as simple as two steps
in the following model. Indeed, some amino acid fermentations by the
are this simple. But the pathways of fermentation are a bit
complex, usually involving several preliminary steps to prime the
source for oxidation and substrate level phosphorylations.
Figure 7. Model fermentation.
L. The substrate is oxidized to an organic intermediate; the usual
agent is NAD. Some of the energy released by the oxidation is conserved
during the synthesis of ATP by the process of substrate level
Finally, the oxidized intermediate is reduced to end products. Note
NADH2 is the reducing agent, thereby balancing its redox
to drive the energy-producing reactions. R. In lactic fermentation by Lactobacillus,
the substrate (glucose) is oxidized to pyruvate, and pyruvate becomes
to lactic acid. Redox balance is maintained by coupling oxidations to
within the pathway. For example, in lactic acid fermentation via the
pathway, the oxidation of glyceraldehyde phosphate to phosphoglyceric
is coupled to the reduction of pyruvic acid to lactic acid.
In biochemistry, for the sake of convenience, fermentation pathways
start with glucose. This is because it is the simplest molecule,
the fewest catalytic steps, to enter into a pathway of glycolysis and
metabolism. In procaryotes there exist three major pathways of
(the dissimilation of sugars): the classic Embden-Meyerhof pathway,
which is also used by most eucaryotes, including yeast (Saccharomyces):
the phosphoketolase or heterolactic pathway related to the
shunt; and the Entner-Doudoroff pathway. Whether or not a
is a fermenter, it will likely dissimilate sugars through one or more
these pathways (See Table 1 below).
This is the pathway of glycolysis most familiar to biochemists and
biologists, as well as to brewers, breadmakers and cheeseheads. The
is operated by Saccharomyces to produce ethanol and CO2.
The pathway is used by the (homo)lactic acid bacteria to produce lactic
acid, and it is used by many other bacteria to produce a variety of
acids, alcohols and gases. Some end products of Embden-Meyerhof
are essential components of foods and beverages, and some are useful
and industrial solvents. Diagnostic microbiologists use bacterial
profiles (e.g. testing an organism's ability to ferment certain sugars,
or examining an organism's array of end products) in order to identify
them, down to the genus level.
Figure 8. The Embden Meyerhof
pathway for glucose dissimilation. The overall reaction is the
of glucose to 2 pyruvic acid. The two branches of the pathway after the
cleavage are identical, drawn in this manner for comparison with other
bacterial pathways of glycolysis.
The first three steps of the pathway prime (phosphorylate) and
the hexose for cleavage into 2 trioses (glyceraldehyde-phosphate). Fructose
1,6-diphosphate aldolase is the key (cleavage) enzyme in the E-M
Each triose molecule is oxidized and phosphorylated followed by two
level phosphorylations that yield 4 ATP during the pathway to pyruvate.
Lactic acid bacteria reduce the pyruvate to lactic acid (lactate);
the pyruvate to alcohol (ethanol) and CO2 as shown in Figure
The oxidation of glucose to lactate yields a total of 56 kcal per
of glucose. Since the cells harvest 2 ATP (16 kcal) as useful energy,
efficiency of the lactate fermentation is about 29 percent (16/56).
fermentations have a similar efficiency.
Figure 9. (a) The Embden
pathway of lactic acid fermentation in lactic acid bacteria
and (b) the Embden Meyerhof pathway of alcohol fermentation in yeast
The pathways yield two moles of end products and two moles of ATP per
of glucose fermented. The steps in the breakdown of glucose to pyruvate
are identical. The difference between the pathways is the manner of
pyruvic acid, thereby giving rise to different end products.
Besides lactic acid, Embden-Meyerhof fermentations in bacteria can
to a wide array of end products depending on the pathways taken in the
reductive steps after the formation of pyruvic acid. Figure 10 below
some of the pathways proceeding from pyruvic acid in certain bacteria.
Usually, these bacterial fermentations are distinguished by their end
into the following groups.
1. Homolactic Fermentation. Lactic acid is the sole
product. Pathway of the homolactic acid bacteria (Lactobacillus,
and most streptococci). The bacteria are used to ferment milk and milk
products in the manufacture of yogurt, buttermilk, sour cream, cottage
cheese, cheddar cheese, and most fermented dairy products.
2. Mixed Acid Fermentations. Mainly the pathway of the Enterobacteriaceae.
products are a mixture of lactic acid, acetic acid,
succinate and ethanol, with the possibility of
gas formation (CO2 and H2) if the
possesses the enzyme formate dehydrogenase, which cleaves formate to
2a. Butanediol Fermentation. Forms mixed acids and gases as
but, in addition, 2,3 butanediol from the condensation of 2
The use of the pathway decreases acid formation (butanediol is neutral)
and causes the formation of a distinctive intermediate, acetoin.
Water microbiologists have specific tests to detect low acid and
in order to distinguish non fecal enteric bacteria (butanediol formers,
such as Klebsiella and Enterobacter) from fecal
(mixed acid fermenters, such as E. coli, Salmonella and Shigella).
3. Butyric acid fermentations, as well as the
fermentation (below), are run by the clostridia, the masters of
In addition to butyric acid, the clostridia form acetic acid, CO2
and H2 from the fermentation of sugars. Small amounts of
and isopropanol may also be formed.
3a. Butanol-acetone fermentation. Butanol and acetone were
as the main end products of fermentation by Clostridium
during the World War I. This discovery solved a critical problem of
manufacture (acetone is required in the manufacture gunpowder) and is
to have affected the outcome of the War. Acetone was distilled from the
fermentation liquor of Clostridium acetobutylicum, which worked
out pretty good if you were on our side, because organic chemists
figured out how to synthesize it chemically. You can't run a war
gunpowder, at least you couldn't in those days.
4. Propionic acid fermentation. This is an unusual
carried out by the propionic acid bacteria which include
and Bifidobacterium. Although sugars can be fermented straight
to propionate, propionic acid bacteria will ferment lactate (the end
of lactic acid fermentation) to acetic acid, CO2 and
acid. The formation of propionate is a complex and indirect process
5 or 6 reactions. Overall, 3 moles of lactate are converted to 2 moles
of propionate + 1 mole of acetate + 1 mole of CO2, and 1
of ATP is squeezed out in the process. The propionic acid bacteria are
used in the manufacture of Swiss cheese, which is distinguished by the
distinct flavor of propionate and acetate, and holes caused by
Figure 10. Fermentations in
that proceed through the Embden-Meyerhof pathway. Representive bacteria
that utilize these pathways are in shown in BLUE.
The Embden-Meyerhof pathway for glucose dissimilation (Figure 8), as
well as the TCA cycle discussed below (Figure 14), are two
that are at the center of metabolism in nearly all bacteria and
eucaryotes. Not only
do these pathways dissimilate organic compounds and provide energy,
also provide the precursors for biosynthesis of macromolecules that
up living systems (see Figure 25 below). These are rightfully called amphibolic
pathways since the have both an anabolic and a catabolic function.
The phosphoketolase pathway (Figure 11) is distinguished by the key
enzyme, phosphoketolase, which cleaves pentose phosphate into
and acetyl phosphate. As a fermentation pathway, it is employed mainly
by the heterolactic acid bacteria, which include some species
and Leuconostoc. In this pathway, glucose-phosphate is oxidized
6-phosphogluconic acid, which becomes oxidized and decarboxylated to
pentose phosphate. Unlike the Embden-Meyerhof pathway, NAD-mediated
take place before the cleavage of the substrate being utilized. Pentose
phosphate is subsequently cleaved to glyceraldehyde-3-phosphate (GAP)
acetyl phosphate. GAP is converted to lactic acid by the same enzymes
the E-M pathway. This branch of the pathway contains an oxidation
to a reduction while 2 ATP are produced by substrate level
Acetyl phosphate is reduced in two steps to ethanol, which balances the
two oxidations before the cleavage but does not yield ATP. The overall
reaction is Glucose ---------->1 lactic acid + 1 ethanol +1 CO2
with a net gain of 1 ATP. The efficiency is about half that of the E-M
Heterolactic species of bacteria are occasionally used in the
industry. For example, kefir, a type of fermented milk to yogurt, is
produced by is produced
a heterolactic Lactobacillus species. Likewise, sauerkraut
use Leuconostoc, a heterolactic bacterium, to complete the
Figure 11. The heterolactic
(phosphoketolase) pathway of fermentation. Compare with the
pathway in Figure 9. This pathway differs in the early steps before the
cleavage of the molecule. The overall reaction in the fermentation of
is Glucose -------> Lactic acid + ethanol + CO2 + 1 ATP
Only a few bacteria, most notably Zymomonas, employ the
pathway as a strictly fermentative way of life. However, many bacteria,
grouped around the pseudomonads, use the pathway as a way to degrade
for respiratory metabolism (see Table 1 below). The E-D pathway yields
2 pyruvic acid from glucose (same as the E-M pathway) but like the
pathway, oxidation occurs before the cleavage, and the net energy
yield is one mole of ATP per mole of glucose utilized.
In the E-D pathway, glucose phosphate is oxidized to
acid (KDPG) which is cleaved by KDPG aldolase to pyruvate and
The latter is oxidized to pyruvate by E-M enzymes wherein 2 ATP are
by substrate level phosphorylations. Pyruvic acid from either branch of
the pathway is reduced to ethanol and CO2, in the same
as yeast, by the "yeast-like bacterium", Zymomonas (Figure 12
Thus, the overall reaction is Glucose ---------->2 ethanol +2 CO2,
and a net gain of 1 ATP.
Zymomonas is a bacterium that lives on the
including the succulent Maguey cactus which is indigenous to Mexico.
as grapes are crushed and fermented by resident yeast to wine, so may
Maguey flesh be crushed and allowed to ferment with Zymomonas,
which gives rise to "cactus beer" or "pulque", as it is known in
pulque yields tequila in the state of Jalisco, or mescal in the state
Oaxaca. Many cultures around the world prepare their native fermented
with Zymomonas in deference to the yeast, Saccharomyces,
although they may not have a choice in the matter. Zymomonas
potential advantageous over yeast for the industrial production of
but the industry is geared to do what it can do, and no change in
Figure 12. The
Pathway of Fermentation. The overall reaction is Glucose -------> 2
+ 2 CO2 + 1 ATP (net).
Table 1. Oxidative pathways
glycolysis employed by various bacteria.
||Phosphoketolase (heterolactic) pathway
||Entner Doudoroff pathway
Table 2. End product yields
|fructose 1,6-diP aldolase
|fructose 1,6-diP aldolase
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