Biochemical Reaction Networks
129
such as methanogenic bacteria can not grow in the presence of oxygen, but many organisms,
including
S. cerevisiae
and
E. coli
are
facultative anaerobes,
i.e., they grow both anaerobically and
aerobically. Both
S. cerevisiae
and
E. coli
can gain ATP by respiration, but many facultative
anaerobes for one reason or another lack the oxidative phosphorylation pathway. Thus, in lactic
acid bacteria the lack of Cytochrome C (a constituent of the electron transport chain) impairs the
functioning of the electron transport chain in oxidative phosphorylation. Although
Lactococcus
lactis
is unable to gain free energy by respiration growth at aerobic conditions can give the bacteria
advantages. For example the presence of oxygen can help to keep a low NADH level through the
action of NADH-oxidases that use oxygen as electron acceptor.
During anaerobic growth the products of the catabolism are typically ethanol, lactic acid, and
acetate, as illustrated in Fig. 2.6. Many other products may, however, be formed by other
microorganisms grown at anaerobic conditions, e.g., acetoin, acetone and butanol by
Clostridium
acetobutylicum
(see Problem 5.4). Microorganisms that are used industrially to produce metabolites
as products of the catabolism may be divided into two groups, the
homofermentative
and the
heterofermentative.
A homofermentative microorganism will under most operating conditions
produce a single product, whereas a heterofermentative microorganism produces many different
products. In most cases it is necessary to form several metabolic products in order to balance the
co-factors NADH and NADPH, except if growth is on a rich medium where there is very little net
requirement of NADPH and formation of NADH in connection with biomass synthesis. At these
conditions the ATP balance can be used to derive the linear rate equation (5.5) as shown in
Example 5.2. When several different products are formed in order to balance the co-factors NADH
and NADPH the metabolic network becomes somewhat more complex as illustrated in Example
5.4, but linear rate equations can be derived also in this case.
Example 5.2 ATP requirements for growth of
Lactococcus cremoris
Lactococcus cremoris
is used as a starter culture in the dairy industry for the production of butter, yogurt,
and cheese. It is also used in some types of fermented sausage and sour bread.
L. cremoris
is characterized
as a homofermentative Gram-positive bacteria mainly producing lactic acid, but it may produce many other
products at conditions of very low sugar concentrations.
L. cremoris
is a multiple-amino acid auxotroph;
i.e., it requires a supply of several amino acids for growth, and it is therefore normally grown on a rich
medium. This is a drawback when a detailed analysis of the growth process is to be carried out, since it is
difficult to identify the growth-limiting substrate. However, it also means that there is no net
production/consumption of NADH and NADPH in connection with biomass synthesis and the overall
biomass synthesis reaction can therefore be specified as:
biomass -
a
glucose -
b
nitrogen source -
YxATPATP
= 0
(1)
where the nitrogen source is a complex mixture of amino acids
etc. a
and
b
are yield coefficients in the
overall biomass synthesis process. Since the stoichiometric coefficient for biomass is 1
the forward rate of
this reaction is given by the specific growth rate P.
The ATP required for biomass synthesis is supplied by the catabolic reactions, which at homofermenative
conditions are limited to conversion of glucose to lactic acid. The overall stoichiometry for this process is
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