From Cellular Function to Industrial Products
27
In the EMP pathway G
6
P is converted to pyruvate, and the overall stoichiometry from glucose is
given by Eq. (2.6). One reaction (the conversion of F
6
P to fructose-1,6-diphosphate) requires a
concomitant hydrolysis of ATP to proceed, but two other reactions which run twice for every
molecule of G
6
P produce enough Gibbs free energy to give a net production of ATP in the pathway
(see also Section 4.1). Since ATP (or PEP) is used for formation of G
6
P from glucose, the net yield
of ATP is 2 moles per mole of glucose converted to pyruvate. The four electrons liberated by the
partial oxidation of 1
mole glucose to 2 moles pyruvate are captured by 2 moles of NAD* leading to
formation of 2 moles NADH.
2 PYR + 2 ATP + 2 H20 + 2 NADH + 2 H’ - Glucose - 2 ADP - 2 - P - 2 NAD* = 0
(2.6)
The major function of the PP pathway is to supply the anabolic reactions with reducing equivalents
in the form of NADPH and to produce the precursor metabolites ribose-5-phosphate (R5P) and
erythrose-4-phosphate. Due to the branch points present in the PP pathway (see Fig. 2.4), it is
possible to adjust the fate of G
6
P in this pathway exactly to the cellular need for R5P and NADPH.
If necessary a glucose molecule can be ground up to form 12 NADPH and
6
C 0
2
by six “passages”
in the loop G
6
P
PP pathway ~> F
6
P
G
6
P.
2.1.3.2 TCA Cycle and Oxidative Phosphorylation
The pyruvate formed in the glycolysis can be oxidized completely to carbon dioxide and water in
the tricarboxylic acid (TCA) cycle, which is entered via acetyl-CoA (see Fig. 2.5). Here one mole
of GTP, an “energy package” equivalent to ATP, and five reduced cofactor molecules are formed
for each pyruvate molecule. Four of these are NADH, the fifth is FADH,. A prerequisite for the
complete conversion of pyruvate in the TCA cycle is that NAD’ and FAD can be regenerated from
NADH and FADH2. This is done in the respiratory chain (see Fig. 4.1 for details), an oxidative
process involving free oxygen and therefore operable only in aerobic organisms. In the respiratory
chain, electrons are passed from NADH to a co-enzyme called ubiquinone (UQ) by NADH
dehydrogenase. They are carried on from UQ through a sequence of cytochromes (proteins
containing a heme group), and are finally donated to oxygen, resulting in the formation of water.
The cytochromes and the co-enzyme UQ are positioned at or near the cytoplasmic membrane (or
the inner mitochondrial membrane in eucaryotes), and when electrons pass through the respiratory
chain protons are pumped across the membrane (in prokaryotes it is the cytosolic membrane and in
eukaryotes it is the inner mitochondrial membrane). When the protons re-enter the cell (or the
mitochondria) through the action of the enzyme F
0
F,-ATPase, as shown in Fig. 2.3A, ADP may be
phosphorylated to form ATP, and the respiratory chain is therefore often referred to as
oxidative
phosphorylation.
The number of sites where protons are pumped across the membrane in the
respiratory chain depends on the organism. In many organisms there are three sites, and here ideally
three moles of ATP can be formed by the oxidation of NADH. FADH
2
enters the respiratory chain
at UQ. The electrons therefore do not pass the NADH dehydrogenase and the oxidation of FADH
2
therefore results only in the pumping of protons across the membrane at two sites. The number of
moles of ATP formed for each oxygen atom used in the oxidative phosphorylation is normally
referred to as the P/O ratio, and the value of this stoichiometric coefficient indicates the overall
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