132
Chapter 5
5.2.3 Energetics of Aerobic Processes
During aerobic growth it is possible to oxidize part of the energy source completely to carbon
dioxide in the TCA cycle. Since much more Gibbs free energy (and consequently ATP) is gained
by complete oxidation of the energy source, it is possible to obtain much higher biomass yields
from the energy source in an aerobic process than in an anaerobic process. In aerobic processes
most of the ATP is formed in the oxidative phosphorylation, and therefore this process has a central
position when energetic balances are to be set up. Unfortunately, the value of the P/O ratio is not
exactly known as discussed in Section 4.3, and it may also vary with the operating conditions.
From an energetic analysis of a growth process with carbon dioxide as the only metabolic product
we may, however, calculate the operational stoichiometry of the oxidative phosphorylation as
illustrated in the following. Consider a simple, but still quite general model for aerobic growth
without metabolite formation. As described in Section 2.1, the growth process starts with formation
of precursor metabolites. Next, building blocks are synthesized from the precursor metabolites, and
finally the building blocks are polymerized into macromolecules. In the synthesis of the precursor
metabolites, both carbon dioxide and NADH are produced as by-products. In the formation of
building blocks from the precursor metabolites, NADPH and ATP are required, and these
compounds are produced in the catabolism. Finally, the polymerization requires ATP. The overall
synthesis of biomass can therefore be described by:
biomass +
Yxc
C 0 2 + 7xNADH
NADH - (1+7XC
) CH20
- TxATP ATP - TxNADPH
NADPH = 0
(5.7)
As argued in Section 3.3 we need not consider the nitrogen source (and other nutrients) in the
analysis. In Eq. (5.7) the stoichiometric coefficients are given relative to the formation of
biomass and are therefore the yield coefficients. Thus, 7XC
specifies the moles of carbon dioxide
formed per C-mole biomass produced. If a C-mole basis is applied the carbon balance directly
gives the stoichiometric coefficient for glucose as 1+7XC
. Notice that in the overall reaction
NADH is formed and NADPH is consumed in connection with biomass formation, which is the
typical situation. The stoichiometric coefficients for NADH and NADPH can be calculated from
detailed information about the biosynthesis of biomass, i.e., if the exact requirement for the
individual building blocks is known and the biosynthetic routes to the individual building blocks
have been unraveled. This has been illustrated for different microbial cells, and Table 5.2 collects
some of these results. The values in Table 5.2 are given on a gram dry weight basis for the
biomass, but using a molecular mass of 25 g (C-mole)'1
the yield coefficients 7xNADH
and 7xNADPH
are found for
P. chrysogenum to
be 0.458 mmoles (C-mole)'1
and 0.243 mmoles (C-mole)
respectively1.
1
Note that the NADPH requirement for growth is considerably higher for the bacterium
E. coli
than for the two
fungi. Prokaryotes have a higher lipid and protein content than eukaryotes and a substantial portion of the NADPH
is spent in the sythesis of these cell constituents. In a more recent textbook Lengeler
et al.
(1999) gives a figure of
about 19 mmoles NADPH per g DW, which is slightly higher than the value specified in Table 5.2. The calculations
are complicated by the presence of isoenzymes using different co-factors for some of the steps. Therefore the
calculated values should in reality be presented as ranges [see e.g. Albers
et al.
(1996)].
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