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.