28
Chapter 2
thermodynamic efficiency of the process. If NADH was the only co-enzyme formed in the
catabolic reactions, the theoretical P/O ratio would be exactly 3, but since some FADH
2
is also
formed the P/O ratio is always less than 3. Furthermore, the proton and electrochemical gradient is
also used for solute transport and the overall stoichiometry in the process is therefore substantially
smaller than the upper value of 3 (see Section 4.3). As the different reactions in the oxidative
phosphorylation are not directly coupled the P/O-ratio varies with the growth conditions, and the
overall stoichiometry is therefore written as:
NAD* + (1 + P/O) H20 + P/O ATP - NADH - 0.5 0
2
- P/O ADP - H+
- P/O - P = 0
(2.8)
In many microorganisms one or more of the sites of proton pumping are lacking, and this of course
results in a substantially lower P/O-ratio.
Since the electron transport chain is located in the inner mitochondrial membrane in eukaryotes,
and since NADH cannot be transported from the cytosol into the mitochondrial matrix NADH
formed in the cytosol needs to be oxidized by another route. Strain specific NADH dehydrogenases
face the cytosol, and these proteins donate the electrons to the electron transport chain at a later
stage than the mitochondrial NADH dehydrogenase. The theoretical P/O ratio for oxidation of
cytoplasmic NADH is therefore lower than that for mitochondrial NADH. To calculate the overall
P/O ratio it is therefore necessary to distinguish between reactions in the cytoplasm and reactions in
the mitochondria.
ATP
AMP
3
( 10)
I-------
Oxaloacatata
Pyruvate
NAD**
NADH *
(
1
)
CO*
Acetyl-Co A
____I____
(2)
—V
Citrate
ATP ADP
NADH NAD*
Figure 2.5 TCA cycle, pyruvate carboxylation and the glyoxylate cycle. To simplify the diagram CoA is
not shown. The reaction from succinyl-CoA to succinate does in reality involve GTP, an analogue of ATP,
but again to simplify the diagram the two species are lumped together. The enzymes are: (1) pyruvate
dehydrogenase; (2) citrate synthase; (3) aconitase; (4) isocitrate dehydrogenase; (5) 2-oxoglutarate
dehydrogenase;
(
6
) succinate thiokinase;
(7) succinate dehydrogenase;
(
8
)
fumarase; (9) malate
dehydrogenase; (
10
) pyruvate carboxylase; (
1 1
) isocitrate lyase; (
12
) malate synthase.
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