Thermodynamics of Bioreactions
111
organism. The theoretical P/O ratio for this yeast is therefore 2. Another yeast
Candida utilis,
can
use all three stages to produce ATP from oxidation of NADH, and the theoretical P/O ratio for this
yeast is therefore 3.
Fermentation physiologists have studied - as ardently as the theoretical bioscientists - the ATP
yield on redox equivalents oxidized in respiration. Their goal has been to get a true (rather than a
theoretical) value of ATP gained in catabolism in order to find biomass yield on different
substrates, i.e. to find the ATP required for balanced growth, typically in steady state continuous
cultures. Using aerobic cultivations of the yeast
S. cerevisiae
growing on different substrates
(acetate, lactate, ethanol, glucose etc.) an average value of 1.2-1.3 for the effective P/O ratio was
derived by van Gulik and Heijnen (1995). In their study they used a rather complete metabolic
network model for
S. cerevisiae
to obtain the distribution of metabolites (and hence the ATP gained
in catabolism). Vanrolleghem and Heijnen (1998) used mixtures of glucose and ethanol and
measured the variation in
and
Ym
with changing ratio of ethanol and glucose in the feed and
compared this with simulations, again based on a large metabolic network. A rather low value for
P/O of 1.05-1.15 was obtained by least squares fitting of the model parameters to the data. In
Section 5.2.3 we will look further into the energetics of aerobic processes and illustrate how the
P/O ratio can be derived from fermentation data.
One may ask why the experimentally obtained P/O ratios are so much smaller than the theoretical
values. A sound theoretical explanation of the ATP production process coupled with oxidation of
redox equivalents might presumably give some hints for construction of strains which produce ATP
more effectively than those used today (whether this is beneficial for the organism or for the user of
the organism is of course an open question). The chemoosmotic hypothesis is one o f at least two
competing explanations of oxidative phosphorylation. In Note 4.1 we review some of the main
steps in the mechanism, which in a remarkably efficient way is able to generate free energy rather
than heat out of the oxidation of H2 - the real chemical process that occurs when the redox
cofactors are converted to their oxidized form.
Note 4.1 What is the operational P/O ratio?
The mechanism of the oxidative phosphorylation is outlined in Fig.
4.1.
It has been studied by some of
the most innovative physiologists over the last
40-50
years and is reviewed by Senior
(1988).
This
reference and semiquantitative treatments by Rottenberg
(1979)
and by Stucki
(1980)
- both referred to
by Roels
(1983)
- of the thermodynamic efficiency of the overall process were used in the preparation of
the present note. Basically, the huge amount of free energy made available by the redox process Eq. (1)
is used to drive the phosphorylation process Eq. (2).
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