Stabilization of formate dehydrogenase from Candida boidinii through liposome-assisted complexation with cofactors

Facile preparation of aminosilane-alginate hybrid beads for enzyme immobilization: Kinetics and equilibrium studies

A simple, facile and potential platform for enzyme immobilization using alginate-based beads has been demonstrated by simultaneous gelation and modification of alginate using calcium chloride (CaCl₂) and 3-aminopropyltriethoxysilane (APTES). In this method, sodium alginate solution containing enzyme was simply dripped into a crosslinker solution containing CaCl₂ and APTES, leading to the formation of APTES-alginate hybrid beads (AP-beads). The optical observation, FT-IR analysis and amino group measurements provided evidence that APTES was successfully adsorbed to the alginate chain via electrostatic interaction. On the assumption that the binding of Ca²⁺ ion to polymannuronate residues of alginate via bidentate bridging coordination is competitive with APTES, the equilibrium isotherm and kinetics for the adsorption of APTES to AP-beads was found to follow extended Langmuir isotherm in binary system. Formate dehydrogenase (FDH) as a model enzyme was successfully immobilized in AP-beads and the immobilization yield of ca. 100% could be achieved under optimal conditions of CaCl₂ and APTES concentrations in crosslinker solution. Furthermore, the AP-beads were reused efficiently for 9 cycles without loss of FDH activity. The above results demonstrated that AP-beads were effective support for enzyme immobilization.

Identification of catalysis, substrate, and coenzyme binding sites and improvement catalytic efficiency of formate dehydrogenase from Candida boidinii

Formate dehydrogenases (FDHs) are continually used for the cofactor regeneration in biocatalysis and biotransformation with hiring NAD(P)H-dependent oxidoreductases. Major weaknesses of most native FDHs are their low activity and operational stability in the catalytic reaction. In this work, the FDH from Candida boidinii (CboFDH) was engineered in order to gain an enzyme with high activity and better operational stability. Through comparing and analyzing its spatial structure with other FDHs, the catalysis, substrate, and coenzyme binding sites of the CboFDH were identified. To improve its performance, amino acids, which concentrated on the enzyme active site or in the conserved NAD(+) and substrate binding motif, were mutated. The mutant V120S had the highest catalytic efficiency (k cat/K m ) with COONH4 as it enhanced the catalytic velocity (k cat) and k cat/K m 3.48-fold and 1.60-fold, respectively, than that of the wild type. And, the double-mutant V120S-N187D had the highest k cat/K m with NAD(+) as it displayed an approximately 1.50-fold increase in k cat/K m . The mutants showed higher catalytic efficiency than other reported FDHs, suggesting that the mutation has achieved good results. The single and double mutants exhibited higher thermostability than the wild type. The structure-function relationship of single and double mutants was analyzed by homology models and site parsing. Asymmetric synthesis of L-tert-leucine was executed to evaluate the ability of cofactor regeneration of the mutants with about 100 % conversion rates. This work provides a helpful theoretical reference for the evolution of an enzyme in vitro and promotion of the industrial production of chiral compounds, e.g., amino acid and chiral amine.

Liposomes as chaperone mimics with controllable affinity toward heat-denatured formate dehydrogenase from Candida boidinii

Chaperone machinery in living systems can catch denatured enzymes and induce their reactivation. Chaperone mimics are beneficial for applying enzymatic reactions in vitro. In this work, the affinity between liposomes and thermally denatured enzymes was controlled to stabilize the enzyme activity. The model enzyme is formate dehydrogenase from Candida boidinii (CbFDH) which is a homodimer and negatively charged in the phosphate buffer solution (pH 7.2) used. The activity of free CbFDH readily decreased at 58 °C following the first-order kinetics with the half-life t1/2 of 27 min. The turbidity measurements showed that the denatured enzyme molecules formed aggregates. The liposomes composed of zwitterionic phosphatidylcholines (PCs) stabilized the CbFDH activity at 58 °C, as revealed with six different PCs. The PC liposomes were indicated to bind to the aggregate-prone enzyme molecules, allowing reactivation at 25 °C. The cofactor β-reduced nicotinamide adenine dinucleotide (NADH) also stabilized the enzyme activity. The affinity between liposomes and denatured CbFDH could be modulated by incorporating cationic 1,2-dioleoyloxy-3-trimethylammonium propane chloride (DOTAP) in PC membranes. The t1/2 values significantly increased in the presence of liposomes ([lipid] = 1.5 mM) composed of PC and DOTAP at the mole fraction f(D) of 0.1. On the other hand, the DOTAP-rich liposomes (f(D) ≥ 0.7) showed strong affinity toward denatured CbFDH, accelerating its deactivation. The liposomes with low charge density function as chaperone mimics that can efficiently catch the denatured enzymes without interfering with their intramolecular interaction for reactivation.

Preparation of liposome-coupled NADH and evaluation of its affinity toward formate dehydrogenase based on deactivation kinetics of the enzyme

β-Reduced nicotinamide adenine dinucleotide (NADH) has been immobilized onto the surface of liposome containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE). Amino groups of NADH were coupled to POPE via glutaraldehyde (GA) or poly(ethylene glycol) dialdehyde (PEG-ALD2). Formate dehydrogenase from Candida boidinii (CbFDH) was anchored on NADH through bioaffinity, where 5 NADH molecules on the liposome were associated with one CbFDH molecule. We evaluated the affinity between CbFDH and NADH present in various conditions based on of the first-order deactivation constant k(d) of the enzyme at 60°C. The kd value observed with the liposome-coupled NADH was apparently smaller than that with liposome alone, indicating the thermostability of the NADH-CbFDH complex on the liposome surface. On the other hand, free NADH showed the strongest affinity toward CbFDH. This can be recognized by considering that the affinity between CbFDH and liposome-coupled NADH is relatively weakened by the formation of chemical linkage between them. PEG-ALD2 provided a smaller k(d) value than GA. This bulkier PEG-ALD2 may cause a similar situation to NADH alone by shielding the effect of liposomes.

Thermal stabilization of formaldehyde dehydrogenase by encapsulation in liposomes with nicotinamide adenine dinucleotide

The thermal stability of formaldehyde dehydrogenase (FaDH) from Pseudomonas sp. was examined and controlled by encapsulation in liposomes with β-reduced nicotinamide adenine dinucleotide (NADH). The activity of 4.8 μg/mL free FaDH at pH 8.5 in catalyzing the oxidation of 50mM formaldehyde was highly dependent on temperature so that the activity at 60 °C was 27 times larger than that at 25 °C. Thermal stability of the FaDH activity was examined with and without liposomes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Rapid deactivation of free FaDH was observed at 60 °C because of its dissociation into two subunits. The rate of dissociative deactivation of POPC liposome-encapsulated FaDH was smaller than that of the free enzyme. The liposomal FaDH was however progressively deactivated for the incubation period of 60 min eventually leading to complete loss of its activity. The free FaDH and NADH molecules were revealed to form the thermostable binary complex. The thermal stability of POPC liposome-encapsulated FaDH and NADH system was significantly higher than the liposomal enzyme without cofactor. The above results clearly show that NADH is a key molecule that controls the activity and stability of FaDH in liposomes at high temperatures.