Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 536

the TCA cycle). Fatty acid synthesis consists of the stepwise buildup of acetyl-CoA. Also, CO 2
is an essential component in fatty acid biosynthesis. Acetyl-CoA and CO 2 produce malonyl-
CoA, which is a three-carbon-containing intermediate in fatty acid synthesis. This require-
ment for CO 2 can lengthen the start-up phase (or lag phase; see Chapter 11) for commercial
fermentations if the system is not operated carefully. The requirement for CO 2 can be elim-
inated if the medium is formulated to supply key lipids, such as oleic acid.
The synthesis of most of the polysaccharides from glucose or other hexoses is readily
accomplished in most organisms. However, if the carbon-energy source has less than six
carbons, special reactions need to be used. Essentially, the EMP pathway needs to be oper-
ated in reverse to produce glucose. The production of glucose is called gluconeogenesis.
Since several of the key steps in the EMP pathway are irreversible, the cell must circum-
vent these irreversible reactions with energy-consuming reactions. Since pyruvate can be
synthesized from a wide variety of pathways, it is the starting point. However, in glycolysis,
the final step to convert PEP into pyruvate is irreversible. In gluconeogenesis, PEP is
produced from pyruvate from

H þ

pyruvate þ CO

2 þ

ATP þ H O oxaloacetate þ ADP þ H PO

4 þ 2

(10.51)

/ 2 3

and

oxaloacetate þ ATP PEP þ ADP þ CO (10.52)

/

2

or a net reaction of

H þ (10.53)

The reactions in gluconeogenesis are reversible (under appropriate conditions) up to the
formation of fructose-1,6-diphosphate. To complete gluconeogenesis, two enzymes (fruc-
tose-1, 6-diphosphatase and glucose-6-phosphatase) not in the EMP pathway are required.
Thus, an organism with these two enzymes and the ability to complete reaction 10.53 should
be able to grow a wide variety of nonhexose carbon-energy sources.
So far we have concentrated on aerobic metabolism. Many of the reactions we have
described would be operable under anaerobic conditions. The primary feature of anaerobic
metabolism is energy production in the absence of oxygen and in most cases the absence of
other external electron acceptors. The cell must also balance its generation and consumption
of reducing power. In the next section, we show how the pathways we have discussed can be
adapted to the constraints of anaerobic metabolism.

pyruvate þ 2 ATP þ H O PEP þ 2 ADP þ 2

/

2

10.9. OVERVIEW OF ANAEROBIC METABOLISM

The production of energy in the absence of oxygen can be accomplished by anaerobic respi-
ration (see also Section 10.7.1 ). The same pathways as employed in aerobic metabolism can be
used; the primary difference is the use of an alternative electron acceptor. One excellent
example is the use of nitrate NO 3 , which can act as an electron acceptor. Its product, nitrous
oxide (N 2 O), is also an acceptor leading to the formation of dinitrogen (N 2 ). This process,
denitrification, is an important process environmentally. Many advanced biological waste-
treatment systems are operated to promote denitrification.