60
Chapter 3
conditions (pH, T, substrate concentrations). The substrate concentrations
in the bioreactor are
time independent and equal to the effluent concentrations in the steady state, continuous
bioreactor - and in no other situation. Since the value of
depends on the input variable
D,
in a
fashion to be discussed in Chapter 7 it is very likely that a change in dilution rate leads to a
different stoichiometry since the flux of carbon is going to be distributed differently in the many
pathways of the metabolic network. The situation illustrated in Fig. 3.4 is, indeed, very rare. Here
the stoichiometry remains unchanged over the wide range of dilution rates (0.05
< D <
0.33 h'1),
which corresponds to the substrate utilization rates 0.047 to 0.292 C-moles glucose h'1. The
change in stoichiometry around
D^,
in the aerobic yeast fermentation schematically shown in
Fig. 3.2 is a much more typical situation. When
D
increases above
the yield coefficient
Yx
jumps from virtually zero to a value much closer to the yield coefficient of the anaerobic yeast
fermentation given in reaction (3.23) while
decreases rapidly. A complete change in
stoichiometry when a “secondary” growth substrate such as NH3
has been depleted has also been
illustrated in Fig. 3.3 in connection with changes in the biomass composition.
3.4 Degree of Reduction Balances
All carbon containing substrates can be oxidized to C 02 and relative to this end product of
biochemical reactions the substrates and the carbon containing metabolic products are in a
reduced state. A particular route through the metabolic network of a microorganism will convert
the substrate, i.e. the input to the pathway, to a product that may be either reduced or oxidized
relative to the substrate. Since all feasible pathways must be redox neutral in an organism
working at steady state in a constant environment the net-reaction from substrate to product must
be accompanied by consumption of or production of a separate chemical species in which redox
power is stored. Looking one step further the totality of pathway reactions occurring in steady
state in the cell must have a net production rate of zero for the redox carrying compound or
compounds (see Section 5.2). As discussed in Section 2.1.3 there are several compounds, which
can be used to transfer redox power from one pathway to another. These are:
NADH (NAD+)
NADPH (NADP4)
FADH2 (FAD)
In parenthesis are written the oxidized form of the compounds. The compounds NADH, NADPH
and FADH2 are related chemical species, but they occur as co-factors in different enzymatic
reactions. As already mentioned in Section 2.1.3 NADH is mainly produced and again consumed
in catabolic pathways whereas NADPH is largely synthesized in the pentose phosphate (PP)
pathway and consumed in the anabolic pathway reactions. Thus, in Fig. 2.4 it is seen that
NADPH is produced in the first two reactions of the PP pathway and that NADH is produced
when glyceraldehyde-3 phosphate (G3P) is oxidized to 1,3-diphosphoglycerate in the EMP
pathway. Sometimes a biochemical reaction is catalyzed by either one of several isoenzymes.
Each isoenzyme may use a different cofactor, NADH or NADPH to absorb the redox power
liberated by oxidation of the substrate. Thus, in
S. cervisiae
conversion of 2-oxoglutarate to
glutamate can proceed by catalysis with two different glutamate dehydrogenases. In
S. cerevisiae
these two glutamate dehydrogenases are encoded by
GDH1
and
GDH2
, respectively, and the
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