1. Introduction
D-enantiomers of amino acids are rarely applied for protein synthesis
during evolution. In recent years, D-amino acids have been increasingly
used as intermediates to produce pharmaceuticals and fine chemicals
[1]. D-amino acid can be environmentally produced by a one-step
reaction using D-amino acid dehydrogenase (DAADHs) [2] . Several
NAD(P)+-dependent L-amino acid dehydrogenases have
been reported for the synthesis of L-amino acids while D-amino acid
dehydrogenases are less for D-amino acids [3]. Different from
L-amino acids, D-amino acid are recognized as the “unnatural” amino
acids, which means D-amino acids have unique potential in
pharmaceuticals and fine chemicals while DAADHs are more likely unstable
and with low catalytic efficiency [4]. Therefore, researchers long
for obtaining DAADHs with high stability so that DAADHs can be widely
applied in industrial production. NADP+-dependent
D-amino acid dehydrogenase (DAADH) from Ureibacillus
thermosphaericus , which is meso-diaminopimelate dehydrogenase, for
synthesis of D-leucine and D-isoleucine through site-directed
mutagenesis was reported [5].
Akita et al. [6] reported an NADPH-dependent DAADH mutated from
meso-diaminopimelate dehydrogenase for synthesis of D-branched-chain
amino acids with high yields and optical purity. Meso-diaminopimelate
dehydrogenase has been reported to be used for the synthesis of D-amino
acid from 2-keto acids by one-step reductive amination [7]. Cui et
al. [8] reported biosynthesis of D-phenylalanine by tri-enzymatic
cascade from Proteus vulgaris meso-diaminopimelate dehydrogenase with
96.3 % conversion rate and > 99 % enantioselectivity on a
3 L scale. However, DAADHs are not sufficiently stable [5].
Immobilization of enzymes would be a good way to solve the problem.
Metal-Organic Frameworks (MOFs) have extremely high specific surface
area, abundant porosity, extraordinary functionality, and relatively
high stability [9]. Immobilization of enzyme into metal–organic
frameworks (MOFs) is performed through a one-step and facile method.
Liang et al. [10] reported peptide-induced super-assembly of MOFs
for programmed multi-enzyme cascade reactions, which showed 7.3-fold and
4.4-fold catalytic activity enhancements for the two-enzyme and
three-enzyme cascade reactions, respectively. Sha et al. [11]
reported that they stabilized enzyme cytochrome c by encapsulating it in
a hierarchical mesoporous zirconium-based MOF, NU-1000 against
denaturing organic solvents. Immobilized cytochrome c has significantly
enhanced activity compared to the native enzyme. Gascón et al. [12]
studied the use of a metal–organic framework (MOF) as a support for thein situ immobilization of alcohol dehydrogenase with enhancement
of stability.
Peptides are applied as linkers for fusion enzymes. Peptides linkers
would be beneficial to improve catalytic efficiency and stability of
enzyme. Liu et al. [13] reported that a collagen-like polypeptide
(CLP) and an elastin-like polypeptide (ELP) were fused to D-amino acid
oxidase (DAAO).The catalytic efficiency of DAAO-CLP-ELP was 1.7-fold
that of DAAO. Wang et al. [14] reported the MOF immobilized
L-phenylalanine dehydrogenase. The
stability of immobilized enzyme with peptide linker was greatly enhanced
at 70-80 ℃ and pH 10-11. Du et al. [15] reported that an
elastin-like polypeptide (ELP) was fused to D-amino acid oxidases
(DAAO). ELP–DAAO exhibited a better solubility in aqueous solutions
than DAAO, and its enzymatic activity is about 1.6 times than of DAAO.
Adam et al. [16] reported that the role of peptide linker properties
was investigated for fusions of a leucine zipper immobilization domain
to a chimeric amine dehydrogenase or a formate dehydrogenase, which
aimed at providing a linker library. Song et al. [17] combined a
zwitterionic peptide with iron metal-organic framework to develop a
sensitive electrochemical enzyme sensing platform for T4 polynucleotide
kinase detection.
(RTHRK)4 is conductive peptide from Cytochrome P450,
which can facilitate electron transfer. [18]
Designing should be considered
[19] so that (RTHRK)4 can be connected appropriately
to the N-terminus of DAADH. Immobilization of DAADH fromUreibacillus thermosphaericus by encapsulating them within MOF
was proposed. Then, the hybrid materials of 2-methylimidazole zinc salt
(ZIF-8) with reduced graphene oxide (RGO), such as ZIF-8/RGO and
ZIF-8/RGO/Ni, were applied for enzyme immobilization. Catalytic
activity, thermostability, reuse stability and morphological
characterization of immobilized enzyme were investigated. Mechanism of
immobilization of DAADH with peptide linker in ZIF-8 was investigated by
multi-level interactions and kinetic study. Furthermore, in situimmobilization by ZIF-8/RGO/Ni was studied.