3 Advanced Developments of Artificial Metalloenzymes
3.1 Cascade Reactions
Enzymes function in vivo in the presence of other biomacromolecules including
other enzymes and small molecules other than their substrates. In vitro, combinations of enzymes are routinely employed, to catalyze cascades of reactions,
including at the industrial scale [100, 101]. The combination of transition metal
catalysts with enzymes has however often led to mutual inhibition [102–105]. To
overcome this issue, scientists rely mainly on compartmentalization strategies
[106, 107] but ArMs offer another strategy as they may operate in harmony with
other enzymes to achieve a synthetic goal. After all, the protein shell constitutes a
protective environment for the metal of the active site of an ArM.
3.1.1 Cascade Reactions Employing Artificial Transfer
Hydrogenase
By incorporating a d
6 -piano stool iridium complex bound to biotin within SAV,
Hollmann et al. [108] generated an artificial transfer hydrogenase (ATHase) that
catalyzed the racemic reduction of imines by formate. They subsequently combined
this ATHase with the natural monoaminooxidase-N-9 (MAO-N-9) that uses
dioxygen to selectively oxidize the (S)-stereoisomer of a variety of amine substrates
and provide the corresponding imines along with hydrogen peroxide. While the
ATHase reduced imines into racemic (RS)-amine, only the (S)-isomer was reoxidized by MAO-N-9 into imine leading to the accumulation of the (R)-amine isomer.
The reaction did not proceed unless either catalase or peroxidase was also added to
the combination as hydrogen peroxide produced by MAO-N-9 was found to be
detrimental to the ATHase activity. The combination of the three enzymes was
efficient in the dynamic kinetic resolution of various chiral amines. It is noteworthy
that inactivation was observed when the free iridium complex was used instead of
the ATHase emphasizing the protective role of the protein shell and thus of artificial
enzymes. HRP was also used as a catalase alternative that consumed hydrogen
peroxide to bleach scopoletin and thus allowed to follow the ATHase activity
(Scheme 4, left). In a similar approach, Ward et al. prepared another ATHase that
was able to reduce imines using NADPH instead of formate as it relied on
4,7-dihydroxy-1,10-phenanthroline d
6 -piano stool iridium complex. Glucose
dehydrogenase (GDH) uses glucose to reduce NADP
+ to NADPH and the combination of the ATHase/MAO/catalase and GDH proved to be efficient in the
dynamic kinetic resolution of various amines using simply glucose and dioxygen in
the presence of catalytic amount of NADPH (Scheme 4, left) [109]. Finally, MAO
could also be replaced by L -amino acid oxidase (LAAO) and D -amino acid oxidase
(DAAO) that resulted in the accumulation of L -pipecolic acid starting from L -lysine
(Scheme 4, right).
Additionally, the ATHase was used in combination with ene reductase-catalyzed
asymmetric reduction of a,b-unsaturated compounds. NADH mimics (mNADHs)
390
J.-P. Mahy et al.
3.1 Cascade Reactions
Enzymes function in vivo in the presence of other biomacromolecules including
other enzymes and small molecules other than their substrates. In vitro, combinations of enzymes are routinely employed, to catalyze cascades of reactions,
including at the industrial scale [100, 101]. The combination of transition metal
catalysts with enzymes has however often led to mutual inhibition [102–105]. To
overcome this issue, scientists rely mainly on compartmentalization strategies
[106, 107] but ArMs offer another strategy as they may operate in harmony with
other enzymes to achieve a synthetic goal. After all, the protein shell constitutes a
protective environment for the metal of the active site of an ArM.
3.1.1 Cascade Reactions Employing Artificial Transfer
Hydrogenase
By incorporating a d
6 -piano stool iridium complex bound to biotin within SAV,
Hollmann et al. [108] generated an artificial transfer hydrogenase (ATHase) that
catalyzed the racemic reduction of imines by formate. They subsequently combined
this ATHase with the natural monoaminooxidase-N-9 (MAO-N-9) that uses
dioxygen to selectively oxidize the (S)-stereoisomer of a variety of amine substrates
and provide the corresponding imines along with hydrogen peroxide. While the
ATHase reduced imines into racemic (RS)-amine, only the (S)-isomer was reoxidized by MAO-N-9 into imine leading to the accumulation of the (R)-amine isomer.
The reaction did not proceed unless either catalase or peroxidase was also added to
the combination as hydrogen peroxide produced by MAO-N-9 was found to be
detrimental to the ATHase activity. The combination of the three enzymes was
efficient in the dynamic kinetic resolution of various chiral amines. It is noteworthy
that inactivation was observed when the free iridium complex was used instead of
the ATHase emphasizing the protective role of the protein shell and thus of artificial
enzymes. HRP was also used as a catalase alternative that consumed hydrogen
peroxide to bleach scopoletin and thus allowed to follow the ATHase activity
(Scheme 4, left). In a similar approach, Ward et al. prepared another ATHase that
was able to reduce imines using NADPH instead of formate as it relied on
4,7-dihydroxy-1,10-phenanthroline d
6 -piano stool iridium complex. Glucose
dehydrogenase (GDH) uses glucose to reduce NADP
+ to NADPH and the combination of the ATHase/MAO/catalase and GDH proved to be efficient in the
dynamic kinetic resolution of various amines using simply glucose and dioxygen in
the presence of catalytic amount of NADPH (Scheme 4, left) [109]. Finally, MAO
could also be replaced by L -amino acid oxidase (LAAO) and D -amino acid oxidase
(DAAO) that resulted in the accumulation of L -pipecolic acid starting from L -lysine
(Scheme 4, right).
Additionally, the ATHase was used in combination with ene reductase-catalyzed
asymmetric reduction of a,b-unsaturated compounds. NADH mimics (mNADHs)
390
J.-P. Mahy et al.