30
Chapter 2
with a free -SH group that can be acetylated to CH
3
C O -S - either directly from acetate or by
capturing two of the carbon atoms of pyruvate with the last carbon atom liberated as carbon dioxide
or formic acid, HCOOH. Lactic acid bacteria (Fig. 2.6B) have both pathways for conversion of
pyruvate to acetyl-CoA, whereas
E. coli
only has the pyruvate formate lyase catalyzed reaction. In
yeast the fermentative pathway does not proceed via acetyl-CoA, but instead by decarboxylation of
pyruvate to acetaldehyde. From acetate cytosolic acetyl-CoA may be synthesized, and this serves as
precursor for fatty acid biosynthesis, whereas the mitocondrial acetyl-CoA that is formed directly
from pyruvate serves as an entry point to the TCA cycle. In yeast the primary metabolic product is
ethanol, but even with respiratory growth, where complete re-oxidation of NADH is possible by
oxidative phosphorylation the pyruvate dehydrogenase complex (reaction (2) in Fig. 2.6C), which
catalyzes the direct conversion of pyruvate to acetyl-CoA, may be by-passed as indicated. Above a
certain glucose uptake rate the respiratory capacity becomes limiting and this leads to overflow in
the by-pass and consequently ethanol is formed. This over-flow metabolism is traditionally referred
to as the
Crabtree effect.
Acetyl-CoA can be regarded as an activated form of acetic acid as it can be converted to acetic acid
via reactions (4) and (5) in Fig. 2.6A and B. As seen in reaction (5) an ATP is released, hereby
doubling the ATP yield by catabolism of glucose from 2 to 4 ATP per glucose molecule. This is the
reason why bacteria use the mixed acid pathways at very low glucose fluxes. To obtain a complete
regeneration of NAD* the flow of carbon to the metabolic end products formic acid, ethanol and
acetic acid must, however, be balanced as will be discussed in Chapter 5.
Finally it should be noted that the pathways shown in Fig. 2.6 are of necessity quite simplified.
Thus, in
E. coli
succinate may be an end product. Furthermore, in some bacteria alternative
pathways from pyruvate to other end products such as butanol (together with butyric acid and
acetone) or to 2,3 butanediol (together with acetoin) may be active.
2.1.4 Anabolism
Formation of macromolecules which constitute the major part of the cell mass requires production
of the necessary building blocks followed by polymerization of the building blocks. In Table 2.5
the composition of an
E. coli
cell is shown together with the energy requirement for synthesis of the
individual macromolecules, i.e., requirements for both biosynthesis and polymerization. It is
observed that approximately 70%, of the total requirements for energy and reduction equivalents
are used for synthesis of proteins. The precise values should therefore be used with caution for
other microbial species since the protein content may vary considerably, not only among microbial
species, but also with the operating conditions. It is furthermore observed that the requirements for
Gibbs free energy and reduction equivalents are strongly dependent on whether the building blocks
are present in the medium. It is therefore difficult to make detailed physiological studies when
2
The Crabtree effect is a term often misused to describe over-flow metabolism. In
S. cerevisiae
the mechanisms
behind over-flow metabolism are quite complex as discussed in Example 7.3, and it involves both redirection of
carbon fluxes and repression of respiration. The term is named after Herbert G. Crabtree who studied sugar
metabolism in tumor cells.
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