Biochemical Reactions - A First Look
63
Table 3.4 Stoichiometry of pathway reactions. The oxidized co-factor and H;0 are not included. Also
shown is the ATP yield Y5ATP (in ATP per C-mole glucose except for respiration where the yield is per
NADH used). In some reactions this yield coefficient can have different, organism specific values.
Reaction______________ Stoichiometry_______________________________________________ ATP gain
glucose
pyruvate
-
CH 20 + CH 4nO +
4
NADH
= 0
1/3
glucose
lactate
CH20 + CHzO
= 0
1/3
glucose
—>
acetaldehyde
-
CH2
O +
j
C 0 2
+ -j
CH2
0 U2
+ j
NADH
= 0
1/3
glucose
ethanol
-
CH20
+ 1
C 0 2
+ 1
CH2Ol!2
= 0
1/3
glucose
glycerol
-
CH2 O
+
CH%
,
3
O
- I
NADH
= 0
-1/3
glucose -+ acetate
-
CH2
0 + ± C 0 2 + j CH2
O + j NADH
= 0
1/3*
glucose ~^ C 02
CH20 + C 0 2
+ 2NADH
= 0
1/3 or 2/3
Respiration
-
NADH —\ 0 2
= 0
P/O
glucose -+ ribulose-5-P
- C H 20 - ± H P 0 ; 2
+±C H 20 ( \H P O ? ) + \C 0 2
+ {
NADPH
= 0
-1/6
glucose
glutamate
CH 2O —
j N H 2 +
f
CH 9/50 6/sN l!5 + \ C 0 2
+ -j
NAD (P)H
= 0
1/3
“ The value shown is for eukaryotes. For prokaryotes the ATP gain for acetate formation is 2/3 moles ATP per C-
mole glucose (see also Fig. 2.6).
If oxygen is a reactant (i.e. in aerobic bioreactions) an arbitrary number o f NADH generating
pathways can be redox balanced by respiration for which the net stoichiometry is included in
Table 3.4. Consequently a black box model for a net production of metabolites with lower degree
of reduction than the carbon substrate can be written without involvement of redox carrying co-
factor if 0 2 is included as a reactant. The yield coefficients of the black box stoichiometry are
then found by redox balancing the net reaction, using 0 2 (or another oxidizing agent) to balance
the difference in redox level between substrates and products. This will be illustrated in Example
3.2, as well as several examples will be used to illustrate the concept of redox balancing.
Example 3.1. Anaerobic yeast fermentation
We now examine the stoichiometry of reaction (3.23) a little closer. The pathway leading from glucose
to ethanol is redox neutral, but some carbon is lost as C02. Production of biomass (K = 4.18) is, however,
not redox neutral since carbon is lost in connection with formation of precursor metabolites needed for
biomass synthesis, e.g. ribulose-5-P that is a precursor metabolite for synthesis of nucleic acids and
acetyl-CoA that is a precursor metabolite for lipid biosynthesis, and NADH (and FADH2) is formed in
the TCA cycle, which must be somewhat active even at anaerobic growth to produce precursors for
amino acid biosynthesis. Conversion of some glucose to the more reduced glycerol counterbalances this
biomass related production of redox equivalents. Using the stoichiometry of reaction (3.23) one obtains
the following redox balance:
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