148
Chapter 5
Example 5.6 Heterofermentative metabolism by lactic acid bacteria
Lactic acid bacteria are often grown on complex media that are rich in amino acids
(e.g
., yeast extract or
casein peptone), providing the carbon skeletons for biosynthesis. There is, therefore, no (or very little)
net consumption of redox equivalents in the anabolic pathways. This results in a conservation of redox
equivalents in the conversion of the energy source (glucose or lactose) to the primary metabolites
(lactate, ethanol, acetate, formate, and carbon dioxide). The catabolic pathway utilized by lactic acid
bacteria, which is shown in Fig. 5.4, is therefore decoupled from growth and, as such, can be analyzed
separately. Because the drain of precursor metabolites for growth is negligible for growth on a complex
medium, the EMP pathway can be considered as a linear pathway with no branch points, so that all
intermediates between glucose and pyruvate are eliminated. Under conditions of good growth, some
species of lactic acid bacteria use
only the pathway from
glucose to
lactate
(often called
homofermentative metabolism as leading to a single product). In this case the redox balance closes
exactly, as the NADH formed in the conversion of glucose to pyruvate is regenerated in the conversion
of pyruvate to lactate. Under conditions of extreme starvation, however, the cells strive to gain more
ATP in the catabolic reactions, and channeling some pyruvate toward acetate does this. When this
happens, a redox imbalance results, requiring that a fraction of the pyruvate be channeled toward ethanol
where NAD+
is regenerated. Thus, in order to obtain more ATP in the catabolism of glucose, several
metabolic products are formed, and the metabolism in this case is called heterofermentative (or mixed
acid fermentation).
The metabolic map shown in Fig. 5.4 is simplified, as in reality there are other reactions, e.g. an NADH
oxidase, and there are also regulation that ensures that not all pathways operate at the same time, e.g. the
pyruvate formate lyase, is not operating in the presence of oxygen whereas the pyruvate dehydrogenase
is not operating at very low oxygen concentrations. Here we will, however, use the simplified network as
it illustrates very well how the formation of different metabolic products are coupled via constrains
imposed via the co-factor NADH.
'A
Glucose
NAOH
ATP
Pyruvate -
NAUM
2
^ >
Lactate
Vsj
\
H
Formate
CO,
I
v 4
v ?
NADH-
Acetyl*
NADH
Acetaldehyde
NADH
Acetvl-P
Acetaldeh)
Jf
Acetate
6
Ethanol
Figure 5.4 A simple model for the catabolic pathways in lactic acid bacteria. The metabolism from
pyruvate is also shown in Fig. 2.6B and discussed in Section 2.1.3.3.
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