Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 532

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 distin-
guish between reactions in the cytoplasm and reactions in the mitochondria.
Formation of NADH

H þ , FADH 2 , and ATP at different stages of the aerobic catabolism
of glucose are summarized in Table 10.6 . The overall reaction (assuming 3 ATP/NADH) of
aerobic glucose catabolism in eukaryotes

þ

glucose þ 36 H PO

4 þ 36

ADP þ 6 O

2 / 6

CO

2 þ 6

H O þ 36 ATP (10.42)

3 2

The energy deposited in 36 mol of ATP is 1100 kJ/mol-glucose. The free energy change in
the direct oxidation of glucose is 2870 kJ/mol-glucose. Therefore, the energy efficiency of
glycolysis is 38% under standard conditions. With the correction for nonstandard conditions,
this efficiency is estimated to be greater than 60%, which is significantly higher than the effi-
ciency of man-made machines. The remaining energy stored in glucose is dissipated as heat.
However, in prokaryotes, the conversion of the reducing power to ATP is less efficient. The
number of ATP generated from NADH H þ is usually

2, and only one ATP may be gener-
ated from FADH 2 . Thus, in prokaryotes, a single glucose molecule will yield less than 24 ATP,
and the P/O ratio is generally between 1 and 2.

þ

10.7.6. Control Sites in Aerobic Glucose Metabolism
Several enzymes involved in glycolysis are regulated by feedback inhibition. The
major control site in glycolysis is the phosphorylation of fructose-6-phosphate by
phosphofructokinase.

fructose -6- phosphate þ ATP fructose- 1; 6- diphosphate þ ADP (10.43)

/

The enzyme phosphofructokinase is an allosteric enzyme activated by ADP and H 3 PO 4 but
inactivated by ATP.

ATP
ADP

ð Þ % ð Þ (10.44)

Phosphofructokinase active phosphofructokinase inactive

At high ATP/ADP ratios, this enzyme is inactivated, resulting in a reduced rate of glycolysis
and reduced ATP synthesis. The concentration of dissolved oxygen or oxygen partial

TABLE 10.6 Summary of NADH, FADH 2 , and ATP Formation During Aerobic Catabolism
of Glucose (Based on the Consumption of one mole of Glucose)

NADH FADH 2 ATP

Glycolysis 2 2

Oxidative decarboxylation
of pyruvate

2

TCA cycle 6 2 2

Total 10 2 4