Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 535

Although many additional pathways exist, we will describe just two more, one in the context
of biosynthesis and the other under anaerobic metabolism.
The first is the PP pathway or HMP (see Fig. 10.20 ). Although this pathway produces signif-
icant reducing power, which could be used, in principle, to supply energy to the cell, its
primary role is to provide carbon skeletons for biosynthetic reactions and the reducing power
necessary to support anabolism. Normally, NADPH is used in biosynthesis, whereas NADH
is used in energy production. This pathway provides an array of small organic compounds
with three, four, five, and seven carbon atoms. These compounds are particularly important
for the synthesis of ribose, purines, coenzymes, and the aromatic amino acids. The glyceral-
dehyde-3-phosphate formed can be oxidized to yield energy through conversion to pyruvate
and further oxidation of pyruvate in the TCA cycle.
A vital component of biosynthesis, which consumes a large amount of cellular building
blocks, is the production of amino acids. Many amino acids are also important commercial
products, and the alteration of pathways to induce overproduction is critical to commercial
success. The 20 amino acids can be grouped into various families. Figure 10.30 summarizes
these families and the compounds from which they are derived. The amino acid, histidine,
is not included in Fig. 10.30 . Its biosynthesis is fairly complicated and cannot be easily
grouped with the others. However, ribose-5-phosphate from HMP is a key precursor in
its synthesis.
In addition to the synthesis of amino acids and nucleic acids, the cell must be able to
synthesize lipids and polysaccharides. The key precursor is acetyl-CoA (see Fig. 10.22 for

F F m m l l y y P P r r c c u u r r o o r r P P r r o o u u t t

Glutamine

Proline

α -Ketogluterate
(TCA cycle)

1. Glutamate Glutamate

Ornithine Arginine

Asparagine
Homoserine

Methionine

Oxaloacetate
(TCA cycle)

2. Aspartate Aspartate

Threonine

Lysine

Pyruvate

Isoleucine

Pyruvate
(EMP)

Alonine

3. Pyruvate

Leucine
Valine

α -ketoisovaleric acid

H 2 S

Cysteine

3-Phosphoglycerate
(EMP)

Serine

4. Serine

Glycine

Serine + anthranilic acid

Phosphoenolpyruvate
(EMP)
+ Erythrose-4-phosphate
(PP or HMP)

5. Aromatic

Tryptophan
Phenylalanine
Tyrosine

Chorisate

FIGURE 10.30 Summary of the amino acid families and their synthesis from intermediates in the EMP, TCA,
and HMP pathways. The amino acids are underlined.