Biochemical Reaction Networks
133
The catabolic pathways supply the required ATP and NADPH for biomass synthesis. Excess
NADH formed in the biosynthetic reactions and NADH formed in the catabolic pathways are
both reoxidized by transfer of electrons to oxygen via the electron transport chain. Reactions
(5.8) -(5.10) summarize the overall stoichiometry for the catabolic pathways. Reaction (5.8)
specifies NADPH formation in the pentose phosphate pathway, when glucose is completely
oxidized to C 0 2. A complete oxidation of glucose to C 0 2 is in fact possible in the pentose
phosphate pathway (Fig. 2.4) since the pentoses can be fed back to the fructose-6-P/glucose-6-P
pool and hereby be returned to the pentose phosphate pathway. Six cycles in the oxidative
pentose
phosphate pathway will result in complete oxidation o f a glucose molecule. Reaction
(5.9) is the overall stoichiometry for the combined EMP pathway and the TCA cycle. Finally,
reaction (5.10) is the overall stoichiometry for oxidative phosphorylation. Compartmentation is
not considered in the stoichiometry
i.e.,
no differentiation is made between cytosolic and
mitochondrial NADH, and FADH2 formed in the TCA cycle is pooled together with NADH. The
P/O ratio in reaction (5.10) is therefore the overall (or operational) P/O ratio for oxidative
phosphorylation.
C 0 2 + 2 NADPH - CH20 = 0
(5.8)
C 0 2 + 2 NADH + 0.667 ATP - CH20 = 0
(5.9)
P/O ATP - 0.5 0 2 - NADH = 0
(5.10)
As argued in Section 3.4 we have omitted ADP, NAD* and NADP in the stoichiometry. Finally,
consumption of ATP for maintenance is included as a separate reaction where ATP is hydrolyzed
to less energy containing adenylates:
- ATP = 0
(5.11)
Note that, with the stoichiometry defined on a C-mole basis, the stoichiometric coefficients
extracted from the biochemistry,
e.g.,
formation of 2 mol ATP per mole of glucose in the EMP
pathway, are divided by 6, because 1 mole of glucose contains 6 moles of carbon.
Table 5.2 Calculated values of the requirements of NADPH for biomass synthesis and the amount of
NADH formed in connection with biomass synthesis
Organism
Y
iNADPH
mmoles NADPH
(z
D W 11
^xNADH
mmoles NADH
(z
DWV1
Reference
E. coli
13.91
16.97
Ingraham
et al.
(1983)
S. cerevisiae
8.24
15.43
O ura (1983)a
P. chrvseoeenum
8.49
16.00
N ielsen (1997)
‘ See also Albers
et al.
(1998) for a detailed calculation of the NADH formation in connection with biomass
synthesis. The calculations are based on growth on one specific carbon source (glucose) and one specific nitrogen
source (ammonia). Furthermore, a certain composition of the biomass in terms of protein, lipid content etc. has been
used. The original references should be consulted for detailed information.
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