Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 537

Many organisms grow without using the electron transport chain. The generation of
energy without the electron transport chain is called fermentation. This definition is the exact
and original meaning of the term fermentation, although currently it is often used in
a broader context. Since no electron transport is used, the organic substrate must undergo
a balanced series of oxidative and reductive reactions. This constraint requires that the rates
of conversion of NAD þ and NADP þ to NADH and NADPH must equal the rates of conver-
sion of NADH and NADPH to NAD þ and NADP þ . For example, with the EMP pathway, the
2 mol of NAD reduced in this pathway in the production of pyruvate are reoxidized by
oxidation of pyruvate to other products. Two prime examples are lactic acid and ethanol
production (see Fig. 10.31 ). Both lactic acid and ethanol are important commercial products
from bioprocesses. Other partially oxidized by-products from fermentation are or have been
commercially important [acetone-butanol fermentation, propionic acid, acetic acid (for
vinegar), 2,3-butanediol, isopropanol, and glycerol].
Figure 10.31 summarizes common routes to some of these fermentation end products.
Pyruvate is a key metabolite in these pathways. In most cases, pyruvate is formed through
glycolysis. However, alternative pathways to form pyruvate exist. The most common of these
is the ED pathway (see Fig. 10.32 ). This pathway is important in the fermentation of glucose by
the bacterium Zymomonas. The use of Zymomonas to convert glucose into ethanol is of poten-
tial commercial interest, because the use of the ED pathway produces only 1 mol of ATP per
mole of glucose. This low energy yield forces more glucose into ethanol and less into cell
mass than for yeast, which uses glycolysis to produce pyruvate, which yields 2 mol ATP
per mole of glucose. No one organismmakes all the products indicated in Fig. 10.33 . Different
organisms will contain different combinations of pathways. Thus, it is important to screen

(a)

Glucose Fructose-biphosphate

2 ATP

2 ADP

2 phosphoglyceric acid 2 triose phosphate

2 ADP

2 NADH

2 NAD +

2 ATP

2 lactic acid

2 pyruvic acid

(b)

Fructose-biphosphate

Glucose

2 ATP 2 ADP

2 ADP

2 triose phosphate

2 phosphoglyceric acid

2 NADH

2 NAD +

2 ATP

2 pyruvic acid 2 acetaldehyde 2 ethanol

FIGURE 10.31 Comparison between (a) lactic acid and (b) alcoholic fermentations.