Biocatalytic access to α,α-dialkyl-α-amino acids by a mechanism-based approach

Article abstract of DOI:10.1002/anie.200904395

(figure presented) New donors - new products: Threonine aldolases (L-TA, D-TA) have now been found to accept donors other than glycine. In a simple asymmetric biocatalytic aldol reaction alanine, serine, and cysteine reacted with a range of simple acceptor aldehydes to yielded α-substituted serine derivatives (see scheme; PLP=pyridoxal phosphate).

Full text of DOI:10.1002/anie.200904395

Angewandte
Chemie
DOI: 10.1002/anie.200904395
Enzyme Catalysis
Biocatalytic Access to a,a-Dialkyl-a-amino Acids by a Mechanism-
Based Approach**
Kateryna Fesko, Michael Uhl, Johannes Steinreiber, Karl Gruber, and Herfried Griengl*
a,a-Dialkyl-a-amino acids[1] are important as building blocks
of pharmaceuticals,[2a] as enzyme inhibitors,[2b] and as con-
formational modifiers of physiologically active peptides.[3]
These compounds are not naturally occurring, and their
synthesis is still a challenging task, comprising several steps
and with difficult control of stereoselectivity.[4]
of threonine aldolases and investigated in detail in our
group.[8] In a search for potential catalysts for the asymmetric
synthesis of a,a-dialkyl-a-amino acids, we found two natural
threonine aldolases that catalyze the cleavage of racemic a-
methylthreonine (1) to produce acetaldehyde and d-alanine:
an l-allo-threonine aldolase from Aeromonas jandaei (l-
TA)[9] and a d-threonine aldolase from Pseudomonas sp. (d-
TA; Scheme 1). The reactions proceeded with excellent
The application of an enzyme-catalyzed aldol reaction
between an aldehyde and an amino acid provides a stereo-
selective pathway to b-hydroxy-a-amino acids. Aldolases
applied for this purpose are tolerant towards the acceptor
aldehyde, but they are quite stringent for the donor com-
pound. Up to now only glycine has been accepted as a
donor.[5] Acceptance of other amino acids would increase the
possible product range to b-hydroxy-a,a-dialkyl-a-amino
acids. Recently, a-methyl-3-nitro-b-phenylserine was
obtained in low yield (12%) from the reaction of 3-nitro-
benzaldehyde and d-alanine using a mutated alanine race-
mase (replacement of Tyr265 by Ala).[6] An a-methylserine
hydroxymethyltransferase overexpressed in E. coli was
applied to transform formaldehyde and d-alanine into a-
methyl-l-serine in 64% yield and l-2-aminobutyric acid into
a-ethyl-l-serine (55% yield).[7] No further examples were
given.
Scheme 1. Retro-aldol cleavage of a-methylthreonine 1 using l- and
d-threonine aldolases.
enantioselectivity: only l-1 was stereoselectively cleaved by
l-TA and only the d isomers were accepted by d-TA (see the
Supporting Information). Owing to the reversibility of aldol
reactions, a-methylthreonine was produced in an aldol
addition of acetaldehyde with an excess of alanine as donor
catalyzed by either l-TA and d-TA.
Recently a biocatalytic approach to enantiopure b-
hydroxy-a-amino acids has been developed by application
To obtain more information on the donor-accepting
properties of the two enzymes and to access to other
b-hydroxy-a,a-dialkyl-a-amino acids,
a mechanism-based
[*] Prof. H. Griengl
approach was chosen. In threonine aldolases the cofactor
pyridoxal phosphate (PLP) is bound to a lysine residue within
the active site.[10] After entrance of the donor amino acid into
the active site it is bound to PLP, and by proton abstraction a
quinoid structure with an absorption maximum at 498 nm is
formed (Scheme 2).[10] Besides glycine, only d-alanine and
d-serine (but not the respective l enantiomers) form these
complexes with the previously identified l-TA and d-TA and
effect the corresponding spectral changes (Figure 1).
d-cysteine shows only a weak band at 498 nm and an
additional absorption maximum at 330 nm due to a thiozo-
lidine formed from PLP and cysteine.[11]
Kompetenzzentrum Angewandte Biokatalyse
Petersgasse 14, 8010 Graz (Austria)
and
Institut fꢀr Organische Chemie, Technische Universitꢁt Graz
Stremayrgasse 16, 8010 Graz (Austria)
Fax: (+43)316-873-8740
E-mail: herfried.griengl@a-b.at
Dr. K. Fesko
Institut fꢀr Organische Chemie
Technische Universitꢁt Graz (Austria)
M. Uhl, Prof. K. Gruber
Kompetenzzentrum Angewandte Biokatalyse
Institut fꢀr Molekulare Biowissenschaften
Universitꢁt Graz (Austria)
Dr. J. Steinreiber
Kompetenzzentrum Angewandte Biokatalyse, Graz (Austria)
[**] The Austrian Science Fund (FWF, within the project W901-B05 DK
Molecular Enzymology) and the Austrian Research Promotion
Agency (FFG, wthin the Kplus program) are acknowledged for
financial support. We thank Prof. Harald Grꢂger, University
Erlangen–Nꢀrnberg, for providing us with reference samples of a-
methylthreonine and C. Illaszewicz-Trattner for measuring NMR
spectra.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904395.
Scheme 2. Formation of a quinoid complex between amino acid
donors and the PLP cofactor in threonine aldolase.
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121

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Table 1: Stereoselective synthesis of b-hydroxy-a,a-dialkyl-a-amino acids
using l- or d-threonine aldolase.[a] For R see Scheme 3; in 1–6, R’=CH3;
in 7–10, R’=CH2OH; in 11 and 12, R’=SH2SH.
l-TA Aeromonas jandaei
d-TA Pseudomonas sp.
Entry Product [%] de [%][b]
Entry Product [%] de [%][b]
1
2
3
4
5
6
7
8
l-1
l-2
l-3
l-4
l-5
l-6
l-7
l-8
20
35
60
24
6
46 (anti) 13
6 (anti) 14
7 (anti) 15
35 (syn) 16
26 (anti) 17
8 (anti) 18
65 (anti) 19
40 (anti) 20
65 (anti) 21
45 (anti) 22
18 (anti) 23
12 (anti) 24
d-1
d-2
d-3
d-4
d-5
d-6
d-7
d-8
54
11
36
21
32
84
23
42 (syn)
65 (syn)
76 (syn)
95 (syn)
66 (syn)
33 (syn)
11 (anti)
Figure 1. UV/Vis spectra of l-TA in the presence of glycine, d-alanine,
d-serine, and d-cysteine in 50 mM phosphate buffer at pH 8.0.
58
6
10
15
30
27
30
<1 n.d.
<5
43
33
39
9
l-9
d-9
23 (anti)
Having identified potential donors, we synthesized
b-hydroxy-a,a-dialkyl-a-amino acids starting from a range
of aldehydes as acceptors and d-alanine, d-serine, or
d-cysteine as donors (Scheme 3; Table 1). A wide range of
l- and d-a-alkylserine derivatives were successfully produced
with excellent enantiospecificity at the a-carbon atom
(>99% ee at this center; Table 1). The acceptor specificity
of l-TA is similar to that previously reported for the reactions
when glycine was used as a donor.[8] The best substrates are
aromatic aldehydes bearing electron-withdrawing groups
which increase the electrophilic reactivity of the acceptor.
For instance, 3-nitrobenzaldehyde was efficiently converted
to l-3 with 60% yield, whereas unsubstituted l-2 was
obtained with only 35% yield (Table 1, entries 2 and 3). A
thermodynamic mixture of diastereomers (de ꢀ 40%) was
usually formed during the l-TA-catalyzed reactions. This can
be explained by an unfavorable position of the equilibrium,
which is reached very fast in this case.[12] On the other hand,
the reactions with d-TA achieve the equilibrium state later
10
11
12
l-10
l-11[c]
l-12[c]
d-10
d-11[c]
d-12[c]
24 (syn)
20 (anti)
6 (anti)
[a] Reaction conditions: 1 mL reaction volume, 0.5m d-alanine (products
1–6), d-serine (products 7–12), or d-cysteine (products 11, 12), 0.1m
corresponding aldehyde, PLP 50 mm, 308C for l-TA, 108C for d-TA, TA 4
U; conversions were determined after 24 h by HPLC and 1H NMR
analysis. [b] ee and de values were determined after precolumn
derivatization by HPLC analysis on a reversed-phase column; in all
reactions >99% ee was determined for reactions of both l-TA and d-TA
1
(see the Supporting Information). [c] Determined by H NMR analysis;
n.d.: not determined.
and, as a result, products with a high diastereomeric ratio (up
to 95% de) and moderate yields were obtained in the
kinetically controlled mode. d-TA shows a broad flexibility
for the acceptor carbonyl compound, among which aliphatic
aldehydes were the best substrates. Optically pure long-chain
d-6—a possible precursor for myriocin and sphingofungi-
nes E and F[13]—was obtained in 84% yield and 33%
diastereomeric excess. Reactions with d-serine and d-cysteine
usually gave lower yields than reactions of their a-methyl
analogues (Table 1, entries 7–12 and 19–24).
Our results represent the first example of a biocatalytic
asymmetric aldol synthesis of a-substituted serine derivatives
using threonine aldolases. Moreover, our finding offers the
possibility of accessing both enantiomers by choosing either
l-TA or d-TA.
Reactions catalyzed by natural enzymes often suffer from
low diastereoselectivities; this can be overcome by enzyme
engineering, for example by rational protein design and
directed evolution to enhance the utility of the biocatalyst.
Another promising alternative would be the removal of the
hydroxy group from the product b-hydroxy-a,a-dialkyl-a-
amino acids by hydrogenation[19] such that optically pure l- or
d-a,a-disubstituted a-amino acids would be obtained.
Finally, to find a possible explanation for the donor
specificity in the tested aldolases, we constructed the homol-
ogy model of l-TA from A. jandaei based on the known
Scheme 3. b-Hydroxy-a,a-dialkyl-a-amino acids obtained in aldol reac-
tions using l- and d-threonine aldolase.
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Angewandte
Chemie
crystal structure of l-TA from Thermatoga maritima[14] and a
phenylserine aldolase from Pseudomonas putida.[15] Neither
of these two enzymes can accept donors other than glycine.
However, the comparison of their structures with the
homology model of l-TA from A. jandaei does not show
clear differences within the substrate-binding site that could
explain the donor specificity in the investigated l-TA. It
seems that donor specificity might be a consequence of more
complex interactions.
The strict specificity of l-TA from A. jandaei towards the
d isomer of a donor can be explained by modeling both
enantiomers of alanine into the active site of the homology
model (Figure 2). The amino acid forms a Schiff base with the
binding and the protonation of the b-hydroxy group would
occur on the other side yielding a d-configured product. The
second base must be located rather far from Ca to avoid the
racemization of alanine.
In summary, we have found the first natural threonine
aldolases that accept other donors than glycine, for example,
alanine, serine and cysteine. The strict specificity for d-amino
acids as donors exhibited by both l-TA and d-TA provided
additional insight into mechanistic differences regarding the
location of active site bases. Our findings increase the
substrate range of aldolases and opens new routes for the
unique and simple biocatalytic synthesis of highly valuable
enantiopure l- or d-a-alkylserine derivatives. Moreover, the
high enantioselectivity of these enzymes can be used for the
kinetic resolution of chemically produced dl-syn or dl-anti
mixtures of b-hydroxy-a-quaternary amino acids to obtain
pure diastereomers. Genetic engineering of these threonine
aldolases to enhance diastereoselectivities and optimization
of reaction protocols will be the options to further improve
the outcome of these biocatalytic transformations.[20]
À
PLP cofactor; in this structure the Ca H bond must be
Received: August 6, 2009
Published online: November 26, 2009
Keywords: aldol reaction · aldolases · amino acids · biocatalysis
.
Figure 2. The active site of the l-TA Aeromonas jandaei model: a) com-
plex with d-alanine (the a proton points towards the potential catalytic
base); b) complex with l-alanine (the a proton points away from the
potential catalytic base).
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perpendicular to the plane of the PLP ring in order to be
cleaved.[16] In this case the a proton of d-alanine (which
corresponds to the pro-2S proton in glycine) is abstracted
most probably by a water molecule, which appears to be
activated by the negatively charged PLP phosphate, His85,
and His128 on the re face of the cofactor, opposite to the PLP-
binding lysine Lys199. The substrate aldehyde enters the
À
active site from the same side at which the C C bond
formation maintains the configuration of the a-carbon. His85
and possibly His128 are likely candidates for the further
protonation of the hydroxy group at Cb of the thus-formed
l product.[14] On the other hand, when l-alanine is bound to
PLP, the deprotonation of Ca must occur from the si face of
PLP. However, no residues were found in this area that could
serve as the catalytic bases. Consequently, only the d isomer
of a donor can be deprotonated and forms a quinoid complex
with the cofactor. A more detailed discussion of the
mechanism proposed is given in the Supporting Information.
At present no crystal structure exists for d-threonine
aldolase, but its similarity to bacterial alanine racemase (AR)
was postulated based on the amino acid sequence.[17] Anal-
ogous to AR from Bacillus stearothermophilus (but in
contrast to the situation in l-TA described above) the
mechanism of catalysis of d-TA might involve two catalytic
bases, which are located on opposite faces of PLP.[18] Thus, the
a proton would be abstracted from d-alanine by a base
located on one side of the PLP ring, whereas the substrate
[12] K. Fesko, C. Reisinger, J. Steinreiber, H. Weber, M. Schꢁrmann,
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Communications
[14] C. L. Kielkopf, S. K. Burley, Biochemistry 2002, 41, 11711.
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