Green Chemistry and the Ten Commandments

Chap. 12. Feedstocks 321

O

-+

K Potassium phenolate

with carbon dioxide under high pressure at 220 ̊ C, which converts slightly less than half
of the potassium phenolate to the desired product and produces substantial impurities.
The process dates back to the early 1860s almost 150 years ago, long before there were
any considerations of pollutants and wastes. It requires severe conditions and produces
metal and phenol wastes. Reactive alumina powder (Al 2 O 3 ) used to catalyze the process
has been implicated in a 1995 explosion at a facility to produce p-hydroxybenzoic acid
that killed 4 workers.
A biosynthetic alternative to the synthesis described above has been attempted with
Pseudomonas putida bacteria genetically engineered to carry out several steps in the
synthesis of p-hydroxybenzoic acid starting with toluene. A key to the process is the
attachment at the para position on toluene of a hydroxyl group by the action of toluene-
4-monooxygenase (T4MO) enzyme system transferred to Pseudomonas putida from
Pseudomonas mendocina:

(12.10.1)

p -Cresol

Para position on the aromatic ring

H 3 C OH

T4MO

O 2

H 3 C

The next step is carried out by p-cresol methylhydroxylase (PCMH) enzyme from a strain
of Pseudomonas putida that yields p-hydroxybenzyl alcohol followed by conversion to
p-hydroxybenzaldehyde:

C OH (12.10.2)

O

H

PCMH

H 2 O

C OH

H

H

HO

PCMH

H 2 O

H 3 C OH

The last step is carried out by an aromatic aldehyde dehydrogenase enzyme designated
PHBZ also obtained from a strain of Pseudomonas putida and consists of the conversion
of the aldehyde to the p-hydroxybenzoic acid product:

(12.10.1)

H 2 O

C OH

O

C OH HO

O

H

Through elegant genetic manipulation, the chemical processes described above were
achieved leading to the desired product. In addition to providing the enzymes to carry