intermediate product. The latter, however, can undergo a second aldol reaction
with another acetaldehyde donor, forming a β,δ-dihydroxy aldehyde. At this
stage, this aldol cascade (which would lead to the formation of a polymeric
product if uninterrupted) is terminated by the (spontaneous) formation of a stable
hemiacetal (lactol). The latter does not possess a free aldehydic group and
therefore cannot serve as acceptor any more.
The dihydroxylactols thus obtained can be oxidized by NaOCl to the
corresponding lactones, which represent the chiral side chains of several
cholesterol-lowering 3-hydroxy-3-methylglutaryl-(HMG)-CoA reductase inhibitors, collectively denoted as ‘statins’ [1585].
42 Several derivatives thereof are
produced on industrial scale using DER-aldolase mutants at product concentrations
exceeding 100 g/L [1586, 1587].
This concept provides rapid access to polyfunctional complex products from
cheap starting materials in a one-pot reaction. It has recently been extended by
combining various types of aldolases together to perform three- and four-substrate
cascade reactions [1588, 1589].
Group V: Glycine-Dependent Aldolases
One remarkable feature of group V aldolases is their requirement for an amino
acid as donor – glycine (Scheme 2.192) [1590, 1591]. Since the donor bears an
amino group, their mechanism of action is related to Type I aldolases, with the
difference that umpolung of the (glycine) donor to an enamine species is not
effected by an ε-amino-moiety of lysine within the protein, but via Schiff-base
formation with the aldehyde group of a pyridoxal-5
0 -phosphate cofactor (PLP,
Sect. 2.6.2, Scheme 2.221). However, the nucleophilic attack of Cα onto an
aldehyde acceptor forming an α-amino-β-hydroxy acid is essentially the same.
Since two new stereocenters are formed, four possible stereoisomers can by
formally obtained. However, in contrast to DHAP-dependent aldolases (Scheme
2.180), the complete set of stereo-complementary threonine aldolases has not yet
been found [1592, 1593]. β-Hydroxyamino acids are multifunctional compounds
with numerous applications in the synthesis of complex bioactive structures, such
as peptide mimetics, protease inhibitors and antibiotics. It is thus not surprising,
that threonine aldolases have been frequently used for their synthesis, also on
industrial scale [1594].
D- and L-Threonine Aldolases These enzymes are involved in the biosynthesis/
degradation of α-amino-β-hydroxyamino acids, such as threonine and they exquisitely control the stereochemistry of the α-amino configuration, which is either D
or L, depending on the type of enzyme. However, they show only low-moderate
specificities for the β-hydroxy-center, which leads to diastereomeric threo/erythro
product mixtures [1595].
42 For instance, atorvastatin (Lipitor™), rosuvastatin (Crestor™) or simvastatin.
216
2 Biocatalytic Applications
with another acetaldehyde donor, forming a β,δ-dihydroxy aldehyde. At this
stage, this aldol cascade (which would lead to the formation of a polymeric
product if uninterrupted) is terminated by the (spontaneous) formation of a stable
hemiacetal (lactol). The latter does not possess a free aldehydic group and
therefore cannot serve as acceptor any more.
The dihydroxylactols thus obtained can be oxidized by NaOCl to the
corresponding lactones, which represent the chiral side chains of several
cholesterol-lowering 3-hydroxy-3-methylglutaryl-(HMG)-CoA reductase inhibitors, collectively denoted as ‘statins’ [1585].
42 Several derivatives thereof are
produced on industrial scale using DER-aldolase mutants at product concentrations
exceeding 100 g/L [1586, 1587].
This concept provides rapid access to polyfunctional complex products from
cheap starting materials in a one-pot reaction. It has recently been extended by
combining various types of aldolases together to perform three- and four-substrate
cascade reactions [1588, 1589].
Group V: Glycine-Dependent Aldolases
One remarkable feature of group V aldolases is their requirement for an amino
acid as donor – glycine (Scheme 2.192) [1590, 1591]. Since the donor bears an
amino group, their mechanism of action is related to Type I aldolases, with the
difference that umpolung of the (glycine) donor to an enamine species is not
effected by an ε-amino-moiety of lysine within the protein, but via Schiff-base
formation with the aldehyde group of a pyridoxal-5
0 -phosphate cofactor (PLP,
Sect. 2.6.2, Scheme 2.221). However, the nucleophilic attack of Cα onto an
aldehyde acceptor forming an α-amino-β-hydroxy acid is essentially the same.
Since two new stereocenters are formed, four possible stereoisomers can by
formally obtained. However, in contrast to DHAP-dependent aldolases (Scheme
2.180), the complete set of stereo-complementary threonine aldolases has not yet
been found [1592, 1593]. β-Hydroxyamino acids are multifunctional compounds
with numerous applications in the synthesis of complex bioactive structures, such
as peptide mimetics, protease inhibitors and antibiotics. It is thus not surprising,
that threonine aldolases have been frequently used for their synthesis, also on
industrial scale [1594].
D- and L-Threonine Aldolases These enzymes are involved in the biosynthesis/
degradation of α-amino-β-hydroxyamino acids, such as threonine and they exquisitely control the stereochemistry of the α-amino configuration, which is either D
or L, depending on the type of enzyme. However, they show only low-moderate
specificities for the β-hydroxy-center, which leads to diastereomeric threo/erythro
product mixtures [1595].
42 For instance, atorvastatin (Lipitor™), rosuvastatin (Crestor™) or simvastatin.
216
2 Biocatalytic Applications