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4.3 Linear Cascade Reactions Involving ω-Transaminases 69

O NH
ω-TA, PLP 2
R 1 R 2 R 1 * R 2
O (a) Decarboxylase
NH 2
R CH O + CO 2
R CO 2 H R CO H
2
(b) Acetolacetate OH
synthase
H 2 O NH 3 * + CO 2
(d) Amino acid dehydrogenase
O
(additional cofactor recycling)
(c) Dehydrogenase OH
(additional cofactor R CO H
recycling) 2
Scheme 4.4 Various enzymatic strategies for the coproduct removal to shift the equilib-
rium in ω-TA-catalyzed reactions toward amine formation.

strategies were accomplished by coupling the ω-TA with either a pyruvate decar-
boxylase (PDC, Scheme 4.4a) [19] or an acetolactate synthase (Scheme 4.4b) [20].
The PDC method has the advantage that the formed pyruvate (R = Me) is decom-
posed to acetaldehyde and CO , making the reaction irreversible. Unfortunately,
2
acetaldehyde is, in general, also a good substrate for the ω-TA, which may lead to
unwanted side reactions.
Alternatively, the produced pyruvate can be removed by reduction to the corre-
sponding α-hydroxy acid employing NAD(P)H-dependent dehydrogenases such as
lactate dehydrogenases (LDHs) [21]. As a consequence, an additional enzymatic
step catalyzing the recycling of the required nicotinamide cofactor is required
(Scheme 4.4c). To avoid the formation of lactate, pyruvate can be recycled to
alanine using an AADH and a suitable cofactor regeneration system, enabling
the ketone to be converted to the chiral amine at the expense of only low-cost
reducing agents such as formate or glucose and ammonia [22] (Scheme 4.4d). Even
though all these techniques allow shifting the equilibrium toward amine formation
and were already successfully used, further compatible chemical and biocatalytical
alternatives are expected to appear in the coming years.

4.3
Linear Cascade Reactions Involving -Transaminases

A vast number of enzymatic cascade reactions have been published during the
past years in which single enzymatic transformations were coupled with chemical
transformations or wherein several biocatalysts operate in one pot. Usually, they
were conducted in a sequential or orthogonal manner, but also parallel cascades
were established providing at least two chiral products simultaneously [23]. Several
of these approaches have reached an impressive level of complexity, whereby the
biggest challenge is in identifying suitable operating conditions compatible with