The decarboxylation reaction usually proceeds from the dissociated form of a carboxyl group. As a result, the primary reaction intermediate is more or less a carbanion-like species. In one case, the carbanion is stabilized by the adjacent car-bonyl group to form an enolate intermediate as seen in the case of decarboxylation of malonic acid and tropic acid derivatives. In the other case, the anion is stabilized by the aid of the thiazolium ring of TPP. This is the case of transketolases. The formation of carbanion equivalents is essentially important in the synthetic chemistry no matter what methods one takes, i.e., enzymatic or ordinary chemical. They undergo C—C bond-forming reactions with carbonyl compounds as well as a number of reactions with electrophiles, such as protonation, Michael-type addition, substitution with pyrophosphate and halides and so on. In this context, decarboxylases and related enzymes are the key enzymes to develop the usefulness of enzymes in synthetic chemistry. They are expected to extend the border of biotransformation beyond the transformation between functional groups.
Decarboxylases are one of the members of the enolase superfamily. The most important and interesting point of this class of enzymes is that they are mechanistically diverse and catalyze different overall reactions. However, each enzyme shares a partial reaction in which an active site base abstracts a proton to form a nucleophile. The intermediates are directed to different products in the different active sites of different members. However, some enzymes of this class exhibit catalytic promiscuity in their natural form.90 This fact is considered to be strongly related to the evolution of enzymes. Reflecting the similarity of the essential step of the total reaction, there are some successful examples of artificial-directed evolution of these enzymes to catalyze distinctly different chemical transformation.91 The changing of decarboxylase to racemase described in Section 2.5 is also one of these examples.
Even an entirely different enzyme can be changed to the one that has enolase activity. One representative example is the changing of a lipase to an aldolase utilizing the basicity of the catalytic triad via a simple mutation. The resulting promiscuous lipase has been demonstrated to catalyze the aldol reaction92 and Michael addition93 as shown in Fig. 23.
As described above, simple mutation, regardless of rational or random, sometimes changes the function of enzymes in a drastic manner. Especially, in the case of enzymes belonging to enolase superfamily, including decarboxylases, consideration of the reaction mechanism is important because the apparently different transformations proceed via a similar key intermediate. Thus, the well-designed mutation and structure of the substrates will lead to a successful expansion of the application of enzymes in organic synthesis.
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