could catalyze a reaction analogous to that of the enzyme, but
preparation of the intermediate lactoylthiamin from acet-
aldehyde was unsuccessful. Thus, it is difficult to reverse the
reaction without an enzyme, and pyruvate decarboxylase is the
best catalyst for carboxylation of acetaldehyde.
Recently, some enzymatic reactions utilizing supercritical
carbon dioxide have been reported.9,10 In our method, a higher
concentration of carbon dioxide is required. The supercritical
conditions might be effective at improving the yield of pyruvic
acid synthesis, because they should provide a much higher
carbon dioxide concentration. The reaction in supercritical
carbon dioxide conditions will be our next subject.
Although the effect is much weaker than methane, carbon
dioxide is considered to be a greenhouse gas, and therefore its
immobilization is desired. The methods currently reported are
mainly catalytic or electrochemical reactions, which require
much energy.11–13 One alternative method has been reported,
which utilized carbonic anhydrase for the immobilization of
carbon dioxide.14 This biomimetic approach needs almost no
energy for the reaction. However, the carbonic anhydrase just
improves the solubility of carbon dioxide in aqueous media, and
further treatment of dissolved gas is required. Although the
reaction requires a large excess of CO2, our approach can be
useful for such a purpose. By our method, the carbon dioxide in
the aqueous phase can be condensed with acetaldehyde to
produce the pyruvic acid. Because pyruvic acid can easily be
converted into lactic acid, which is a constituent of a
biodegradable plastic, the carbon dioxide can be immobilized
into the biological cycle. An enzymatic reaction by lactic
dehydrogenase can be used for hydrogenation of pyruvic acid
into lactic acid. Thus, it is possible to design a two-step
enzymatic process as a completely environmentally safe
method for CO2 immobilization.
In summary, we have demonstrated the usefulness of the
reverse reaction of pyruvate decarboxylase. This reaction might
become a recommendable, environmentally safe carboxylation
procedure for acetaldehyde. Further studies, such as the reaction
utilizing supercritical carbon dioxide, and two-step enzymatic
production of lactic acid, are in progress in our laboratories.
We thank Drs Masao Shibata, Tsuyoshi Sakaki, Hiroaki
Kodama and Shoji Ando, for their continuous supports.
Notes and references
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