100
Chapter 4
Further down in the EMP pathway ATP is regenerated by conversion of phosphoenolpyruvate
(PEP) to pyruvate (PYR):
- PEP + PYR - ADP - Pj + ATP = 0 ; AG° =-61.9-(-30.5) =-31.4 kJm ole1
(4.9)
The many individual reactions, which together constitute the EMP pathway net-reaction
- (glucose + 2NAD+ + 2ADP + 2P,) + (2PYR + 2NADH + 2ATP) = 0
(4.10)
will, by careful harnessing of the large release of free energy in some of the reactions, proceed
voluntarily although some of the reaction steps have apositive AG0 (see Example 4.3).
The ATP that participates in the reactions is not to be construed as a stationary pool. ATP is formed
and consumed all the time, and the turnover frequency is high. Left alone ATP has a half life time
of seconds or at most a few minutes, depending on the cell type and on the environment It is tacitly
assumed that in steady state fermentations the ATP generation in catabolic pathways is always
balanced by its consumption in anabolic pathways.
It should finally be remarked that the AG° = -30.5 U mole'1
hydrolyzed ATP is a more or less
empirical value. In living cells concentrations of ions, coenzymes and metabolites might vary by
Example 4.3. Free energy changes of reactions in the EMP pathway
To determine the free energy changes of cellular reactions, it is necessary to know the concentration of
all metabolites and co-factors participating in these reactions. Such data are available only for a few
pathways, and thermodynamic considerations are therefore often based on evaluation of standard free
energy changes. This may, however, lead to erroneous conclusions since the rise of standard free energy
changes assumes certain fixed concentrations for reactants and products (those of the standard state) that
may be different from the actual intracellular metabolite concentrations. To illustrate this point, we
calculate the free energy change for some of the reactions in the EMP pathway (see Fig. 2.4). Table 4.1
lists the measured intracellular concentrations of some of the intermediates, ATP, ADP, and
orthophosphate in the human erythrocyte, and Table 4.2 lists the calculated free energy changes.
Table 4.1 Concentrations of intermediates and co-factors of the EMP pathway in the human erythrocyte"
Metabolite/co-factor
Concentration
(PM)
Metabolite/co- factor
Concentration
fPMl
Glucose (GLC)
5000
2-Phosphoglycerate (2PG)
29.5
Glucose-6-P (G6P)
83
Phosphoenolpyruvate (PEP)
23
Fructose-6-P (F6P)
14
Pyruvate (PYR)
51
Fructose-1,6-bisP (FDP)
31
ATP
1850
Dihydroxyacetone P (DHAP)
138
ADP
138
Glyceraldehyde-3-P (GAP)
18.5
Pi
1000
3-Phosohoirlvcerate GPG1
118
The data are taken from Lehninger (1975).
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