Iron catalysis for in situ regeneration of oxidized cofactors by activation and reduction of molecular oxygen: A synthetic metalloporphyrin as a biomimetic NAD(P)H oxidase

Article abstract of DOI:10.1002/anie.201004101

(Figure Presented) An enzyme substitute: A synthetic Fe^III porphyrin was used as a catalyst for the activation and reduction of O ₂ into H₂O with the cofactor NAD(P)H in aqueous solution. The catalyst is compatible with different preparative enzymatic oxidation reactions. Thus, a new method is provided for the in situ regeneration of the oxidized cofactor NAD(P)⁺ with help from a nonenzymatic, synthetic catalyst (see scheme).

Full text of DOI:10.1002/anie.201004101

DOI: 10.1002/anie.201004101
Metalloporphyrins
Iron Catalysis for In Situ Regeneration of Oxidized Cofactors by
Activation and Reduction of Molecular Oxygen: A Synthetic
Metalloporphyrin as a Biomimetic NAD(P)H Oxidase**
Harald Maid, Philipp Bꢀhm, Stefan M. Huber, Walter Bauer, Werner Hummel, Dr. Norbert Jux,
and Harald Grꢀger*
A major challenge in biomimetic catalysis is the development
of synthetic low-molecular-weight compounds that are able to
mimic the catalytic function of enzymes.[1a] Thus, biomimetic
redox enzymes should, on the one hand, be able to function as
a catalyst in water and on the other hand accept cofactors, in
particular NADH and NADPH (NAD(P) = nicotinamide
adenine dinucleotide (phosphate)) and their oxidized forms
NAD+ and NADP+, respectively, as co-substrates. The in situ
recycling of the expensive cofactors, a process carried out
mostly by means of biotransformations, is considered a key
Scheme 1. Concept of the NAD(P)H-oxidase-catalyzed or biomimetic
in situ cofactor regeneration of NAD(P)+.
technique for conducting enzymatic redox reactions in an
attractive fashion.[1b] For the reduction mode of the cofactor
regeneration (to regenerate the reduced forms NADH and
NADPH) Steckhan et al. developed a “biomimetic formate
dehydrogenase” for the regeneration of NAD(P)H by
oxidation of formic acid into carbon dioxide by using a
suitable rhodium complex.[2] For the oxidation mode of the
cofactor regeneration, NAD(P)H oxidases[3] as natural cata-
lysts have been applied as well as chemoenzymatic,[4] electro-
chemical,[5] and biomimetic[6] catalyst systems. However, the
biomimetic catalysts developed so far produce undesired
hydrogen peroxide as a by-product instead of (preferably)
water.[6] To the best of our knowledge no biomimetic catalyst
for the regeneration of NAD(P)+ from NAD(P)H by
activation and reduction of molecular oxygen into water,
similar to the mode of action of a NAD(P)H oxidase, is
known. The mode of action of such a water-producing
NAD(P)H oxidase is depicted in Scheme 1. In addition only
a few synthetically suitable NAD(P)H oxidases, which serve
as key tools for the “oxidative cofactor regeneration”, are
known. However, in part these enzymes show a lack of
stability under process conditions, different preferences for
the two cofactors NADH and NADPH, and the production of
unwanted hydrogen peroxide (instead of water) as a by-
product.[1b,3] Additionally, from this perspective the availabil-
ity of an “artificial” biomimetic, water-producing NAD(P)H
oxidase would be desirable and a valuable alternative to
NAD(P)H-oxidase-type enzymes in preparative syntheses.
Herein we report the application of a synthetic, water-
soluble iron(III) porphyrin as an artificial, biomimetic water-
producing NAD(P)H oxidase. In analogy to enzymes, the
metalloporphyrin is suitable for the in situ regeneration of
both cofactors NAD+ and NADP+ by activation and reduc-
tion of molecular oxygen, and is also compatible with
different preparative enzymatic oxidative reactions. Further-
more, to the best of our knowledge, this represents the first
application of a synthetic metalloporphyrin as a catalyst for
the activation and reduction of molecular oxygen into water
by means of a natural cofactor in aqueous solution. Addi-
tionally a novel alternative is presented for carrying out
enzymatic oxidation reactions under in situ regeneration of
the oxidized cofactor NAD(P)+ by means of a non-enzymatic,
synthetic catalyst serving as an “artificial enzyme mimic”.
At the beginning of our work we searched for a low-
molecular-weight and water-soluble metal complex that
accepts the natural cofactors NAD(P)H as a hydride donor
for the activation of molecular oxygen, and would thereby be
able to reduce oxygen into water while simultaneously being
recycled as a catalyst. As the FeIII porphyrin subunit, located
in the active site of monooxygenases, exhibits comparable
characteristics in the initial steps of monohydroxylation
(though here a one-electron transfer involving a further
cofactor takes place),[7] we focused our preliminary screening
on low-molecular-weight FeIII complexes having a water-
[*] Dr. H. Maid, P. Bꢀhm, Dr. S. M. Huber, Prof. Dr. W. Bauer,
Priv.-Doz. D. N. Jux, Prof. Dr. H. Grꢀger
Department of Chemistry and Pharmacy
University of Erlangen-Nꢁrnberg
Henkestrasse 42, 91054 Erlangen (Germany)
E-mail: harald.groeger@chemie.uni-erlangen.de
Prof. Dr. W. Hummel
Institute of Molecular Enzyme Technology at the
Heinrich-Heine-University of Dꢁsseldorf, Research Centre Jꢁlich
Stetternicher Forst, 52426 Jꢁlich (Germany)
[**] We thank Evonik-Degussa GmbH, Amano Enzymes Inc., and
Oriental Yeast Company Ltd. Japan for chemicals and the Deutsche
Forschungsgemeinschaft (DFG) for generous support within the
Sonderforschungsbereich 583 “Redoxaktive Metallkomplexe” (Teil-
projekt B7).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004101.
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Communications
soluble porphyrin ligand.[8,9] A set of highly water-soluble
meso-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) as
a
porphyrin ligands was recently developed by Jux et al.[10] The
FeIII complexes 1–3, which are based on such porphyrin
derivatives having a fourfold and eightfold positively charged
or eightfold negatively charged substitution pattern, were
used in a spectrophotometric study to evaluate their suit-
ability as catalysts for the oxidation of the cofactor NADH in
the presence of molecular oxygen with formation of the
oxidized form NAD+ (water or hydrogen peroxide as by-
product). However, none of the water-soluble metallopor-
phyrin complexes 1–3 were suitable for the oxidative trans-
formation of the cofactor NADH into NAD+ in aqueous
medium (Figure 1). A potential reason for this result could
arise from hindered access of the charged cofactor to the iron
center because of the sterically demanding (charged) sub-
stituents in the iron porphyrin complexes 1–3.
ligand. At first we used spectrophotometry to study the
suitability of the iron TSPP complex 4 as a catalyst for the
oxidation of the cofactor NADH in the presence of molecular
oxygen with formation of its oxidized form NAD+ (and water
or hydrogen peroxide as a by-product). Here, the cofactor
NADH was successfully accepted as a substrate in aqueous
reaction medium and transformed into NAD+ by oxidation
(Figure 1). The activity with respect to the amount of catalyst
4 (in mg) was determined to be 6.3 UmgÀ1. Thus, this
“biomimetic specific activity” is in an interesting activity
range for synthetic applications. Furthermore, 4 is also
suitable for the regeneration of the cofactor NADP+ through
oxidation of NADPH. For this transformation a lower, but
still synthetically attractive specific activity of 3.7 UmgÀ1
(59% compared to the activity when using NADH) was
obtained. Converting these values into a turn over frequency
(TOF), which is more common for chemocatalysts, gave
0.11 sÀ1 for the oxidation of NADH and 0.06 sÀ1 for the
oxidation of NADPH. To the best of our knowledge this is the
first example demonstrating the suitability of a metal-
containing porphyrin complex as a catalyst for the oxidation
of the cofactors NADH as well as NADPH with molecular
oxygen in an aqueous reaction medium.[12]
As an alternative we tested the structurally simplified
iron(III) complex 4,[11] containing the water-soluble, fourfold
Given the encouraging results of the activity studies on
the iron TSPP complex 4, a catalytic biomimetic cofactor
oxidation for the in situ regeneration of the cofactor, in
combination with a biotransformation in aqueous reaction
medium was performed. Accordingly, the cofactor was used in
catalytic amounts, thus fulfilling the prerequisite for a
“biomimetic NAD(P)H oxidase” (see Scheme 1). For the
coupled biotransformation reaction the enzymatic oxidation
of d-glucose (5a) into d-gluconolactone (6a) in the presence
of a glucose dehydrogenase was chosen. In this reaction the
oxidized form of the cofactor NAD(P)+ is required and
transformed into its reduced form. Subsequently, the in situ
formed d-gluconolactone (6a) is hydrolyzed into d-gluconic
acid, which is neutralized by the addition of a solution of
sodium hydroxide with the formation of the sodium salt of d-
gluconic acid[13] (7a), thereby maintaining the pH value at
pH 7.0. This synthetic principle based on the use of metal-
loporphyrin 4 as a “biomimetic NAD(P)H oxidase” is
depicted in Table 1.
In the presence of 2 mol% of 4 and 2 mol% of the
cofactor NADH, the cofactor regeneration proceeded suc-
cessfully and by using a glucose dehydrogenase from Bacillus
sp. the desired sodium salt of d-gluconic acid (7a) was
obtained with a (product-related) conversion of greater than
95% (Table 1, entry 1). Using the oxidized form of this
cofactor, NAD+, also led to a conversion of greater than 95%
as well as when using the cofactor NADP+ (entries 2 and 3).
The corresponding turn over numbers (TONs) for the
reactions described in entries 1–3 are each in the range of
48–50. Subsequently, additional biotransformations were
carried out using other monosaccharides to evaluate the
developed method for the in situ cofactor regeneration of
NAD(P)+ for these reactions. We were pleased to find that
analogously d-mannonic acid (7b) and d-xylonic acid (7c)
were obtained with good to excellent (product-related)
Figure 1. Activities of the iron(III) porphyrins 1–4 for the oxidation of
NAD(P)H; the activities of 6.3 UmgÀ1 and 3.7 UmgÀ1 obtained for
catalyst 4 correspond to TOFs of 0.11 sÀ1 and 0.06 sÀ1, respectively.
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Table 1: Iron(III) porphyrin catalyzed in situ cofactor regeneration for the
enzymatic oxidation of monosaccharides 5.
Scheme 2. Iron(III) porphyrin catalyzed in situ cofactor regeneration
for the enzymatic oxidation of cyclooctanol (8).
Entry[a] Substrate
R1
R2
R3
Cofactor
t
Conv.
[h] [%][b]
1
2
3
4
5
d-glucose (5a)
OH
OH
OH
H
H
H
H
CH2OH NADH
CH2OH NAD+
24
24
>95
>95
>95
73
d-glucose (5a)
d-glucose (5a)
d-mannose (5b)
d-xylose (5c)
CH2OH NADP+ 24
OH CH2OH NADP+ 113
OH
H
H
NADP+ 66
>95
[a] For experimental protocols, see the Supporting Information. [b] The
conversion corresponds to the formation of 7 (product-related con-
version); the formation of side products in the enzymatic oxidation of 5
was not observed and the hydrolysis of 6 into 7 proceeds quantitatively.
conversions of 73% and greater than 95%, respectively, and
TONs of 37 and 48–50, respectively (entries 4 and 5).
The need for the presence of all involved components
(molecular oxygen, glucose dehydrogenase, and 4) for the
catalytic process was additionally confirmed by experiments
in which one component had been omitted; none of these
experiments showed any activity.[14] An interesting question
concerns the type of by-product formed in the reduction of
molecular oxygen. In the previous biomimetic synthesis and
also in part when using NAD(P)H oxidases, the disadvanta-
geous by-product hydrogen peroxide is formed, which causes
deactivation of the involved enzymes and cofactors. Accord-
ingly such reactions proceed successfully only in the presence
of a catalase, which is used to decompose the hydrogen
peroxide formed in situ.[6] The formation of hydrogen perox-
ide as a by-product would be also generally conceivable in our
reactions. With test reactions being negative with respect to
the presence of hydrogen peroxide (see the Supporting
Information), and successful biotransformations proceeding
without the addition of a catalase, however, a four-electron
transfer on molecular oxygen yielding the attractive co-
product water can be assumed for our reaction.
Scheme 3. Postulated reaction mechanism of the iron(III) porphyrin
catalyzed in situ cofactor regeneration of NAD(P)+.
Starting from the metalloporphyrin 4 and the reduced form of
the cofactor, an NAD(P)H iron(III) hydride complex (10) is
formed[16] and subsequently transformed into the iron hydro-
peroxo complex 12 by coordination and reduction of molec-
ular oxygen. This step might proceed by homolytic cleavage
of the Fe–H bond in the formation of the complex 11, which
consists of an FeII species and a hydroperoxyl radical. After
protonation and elimination of water (analogous to the
mechanism of P450-monooxygenases) the formed iron
peroxo complex 12 is transformed into a FeIV oxo species of
type 14.[17] The final step consists of the regeneration of the
iron(III) porphyrin 4 and simultaneous formation of water as
a by-product.
Furthermore, the metalloporphyrin 4, which was success-
fully identified as a biomimetic NADH oxidase, turned out to
be highly suitable for the cofactor regeneration of other
oxidative reactions. This is exemplified for the alcohol
dehydrogenase catalyzed oxidation of cyclooctanol (8) into
the corresponding ketone[15] (Scheme 2). Thus, in the pres-
ence of an alcohol dehydrogenase, 2 mol% of the catalyst 4,
and cofactor NAD+ the oxidation to cyclooctanone (9)
proceeded with an excellent conversion of greater than
95%, and after simple isolation by extraction the desired
product 9 was obtained in 93% yield.
In conclusion, we have reported the first application of a
synthetic metalloporphyrin, namely the complex 4, as a
catalyst for the activation and reduction of molecular oxygen
into water by means of a natural cofactor NAD(P)H in
aqueous solution. Furthermore the synthetic and water-
soluble iron(III) porphyrin 4 proved to be an artificial
biomimetic NAD(P)H oxidase, which is compatible with
different types of preparative enzymatic oxidations. Thus, a
novel non-enzymatic, synthetic catalyst-based technology has
been found for the in situ regeneration of the oxidized
cofactor NAD(P)+ in synthetic enzymatic oxidations. As a
result of the catalytic efficiency and ready availability of the
metalloporphyrin 4, this biomimetic recycling of the cofactors
The catalytic mechanism of the reaction involving the
iron(III) porphyrin 4 is proposed as follows (Scheme 3):
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Communications
NAD+ and NADP+ in water can be regarded as an alternative
to enzymatic cofactor regenerations with NAD(P)H oxidases.
[10] J.-E. Jee, S. Eigler, N. Jux, A. Zahl, R. van Eldik, Inorg. Chem.
2007, 46, 3336 – 3352.
[11] a) Pioneering work in synthesis and characterization of iron
TSPP complex 4: E. B. Fleischer, J. M. Palmer, T. S. Srivastava,
A. Chatterjee, J. Am. Chem. Soc. 1971, 93, 3162 – 3167; b) The
iron TSPP complex 4 is commercially available from TriPorTech
GmbH, Germany (product abbreviation: TSPP-FeIII; product
number: TPT00142617).
Received: July 5, 2010
Revised: October 1, 2010
Published online: February 8, 2011
Keywords: cofactors · enzyme catalysis · iron · iron catalysis ·
[12] Hitherto existing applications of metalloporphyrin complexes in
oxidation reactions were either based on non-natural nicotina-
mide derivatives serving as NADH analogues or proceeded
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Battioni, D. Mansuy, C. R. Chim. 2000, 3, 751 – 755; c) G.
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[13] The development of synthetic methods for the production of
sugar acids is of fundamental interest because of their versatile
applications. For example, the industrial chemical d-gluconic
acid, which is biotechnologically produced starting from d-
glucose on a 100000 ton scale per year, is widely used in the food,
metal, and textile industry. For a recently developed highly
efficient chemocatalytic alternative, see: a) A. Mirescu, U.
Prꢁße, Catal. Commun. 2006, 7, 11 – 17; b) C. Baatz, U. Prꢁße,
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[17] The formation of related iron(III) hydroperoxyl, iron(III)
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was also described in connection with the reaction mechanisms
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oxygenases and nonheme-containing monooxygenases; for
selected contributions, see: a) reference [7]; b) L. Que, Jr., J.
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