Separation of xylose and glucose using an integrated membrane system for enzymatic cofactor regeneration and downstream purification

Enzymatic cofactor regeneration systems: A new perspective on efficiency assessment

Nowadays, the specificity of enzymatic processes makes them more and more important every year, and their usage on an industrial scale seems to be necessary. Enzymatic cofactors, however, play a crucial part in the prospective applications of enzymes, because they are indispensable for conducting highly effective biocatalytic activities. Due to the relatively high cost of these compounds and their consumption during the processes carried out, it has become crucial to develop systems for cofactor regeneration. Therefore, in this review, an attempt was made to summarize current knowledge on enzymatic regeneration methods, which are characterized by high specificity, non-toxicity and reported to be highly efficient. The regeneration of cofactors, such as nicotinamide dinucleotides, coenzyme A, adenosine 5'-triphosphate and flavin nucleotides, which are necessary for the proper functioning of a large number of enzymes, is discussed, as well as potential directions for further development of these systems are highlighted. This review discusses a range of highly effective cofactor regeneration systems along with the productive synthesis of many useful chemicals, including the simultaneous renewal of several cofactors at the same time. Additionally, the impact of the enzyme immobilization process on improving the stability and the potential for multiple uses of the developed cofactor regeneration systems was also presented. Moreover, an attempt was made to emphasize the importance of the presented research, as well as the identification of research gaps, which mainly result from the lack of available literature on this topic.

Bioelectrocatalysis with a palladium membrane reactor

Enzyme catalysis is used to generate approximately 50,000 tons of value-added chemical products per year. Nearly a quarter of this production requires a stoichiometric cofactor such as NAD⁺/NADH. Given that NADH is expensive, it would be beneficial to regenerate it in a way that does not interfere with the enzymatic reaction. Water electrolysis could provide the proton and electron equivalent necessary to electrocatalytically convert NAD⁺ to NADH. However, this form of electrocatalytic NADH regeneration is challenged by the formation of inactive NAD₂ dimers, the use of high overpotentials or mediators, and the long-term electrochemical instability of the enzyme during electrolysis. Here, we show a means of overcoming these challenges by using a bioelectrocatalytic palladium membrane reactor for electrochemical NADH regeneration from NAD⁺. This achievement is possible because the membrane reactor regenerates NADH through reaction of hydride with NAD⁺ in a compartment separated from the electrolysis compartment by a hydrogen-permselective Pd membrane. This separation of the enzymatic and electrolytic processes bypasses radical-induced NAD⁺ degradation and enables the operator to optimize conditions for the enzymatic reaction independent of the water electrolysis. This architecture, which mechanistic studies reveal utilizes hydride sourced from water, provides an opportunity for enzyme catalysis to be driven by clean electricity where the major waste product is oxygen gas.

Advances in electrochemical cofactor regeneration: enzymatic and non-enzymatic approaches

Nicotinamide adenine dinucleotide(NAD(P)H) is a metabolically interconnected redox cofactor serving as a hydride source for the majority of oxidoreductases, and consequently constituting a significant cost factor for bioprocessing. Much research has been devoted to the development of efficient, affordable, and sustainable methods for the regeneration of these cofactors through chemical, electrochemical, and photochemical approaches. However, the enzymatic approach using formate dehydrogenase is still the most abundantly employed in industrial applications, even though it suffers from system complexity and product purity issues. In this review, we summarize non-enzymatic and enzymatic electrochemical approaches for cofactor regeneration, then discuss recent developments to solve major issues. Issues discussed include Rh-catalyst mediated enzyme mutual inactivation, electron-transfer rates, catalyst sustainability, product selectivity and simplifying product purification. Recently reported remedies are discussed, such as heterogeneous metal catalysts generating H⁺ as the sole byproduct or high activity and stability redox-polymer immobilized enzymatic systems for sustainable organic synthesis.

Sharpening Nanofiltration: Strategies for Enhanced Membrane Selectivity

Nanofiltration plays an increasingly large role in many industrial applications, such as water treatment (e.g., desalination, water softening, and fluoride removal) and resource recovery (e.g., alkaline earth metals). Energy consumption and benefits of nanofiltration processes are directly determined by the selectivity of the nanofiltration membranes, which is largely governed by pore-size distribution and Donnan effects. During operation, the separation performance of unmodified nanofiltration membranes will also be impacted (deleteriously) upon unavoidable membrane fouling. Many efforts, therefore, have been directed toward enhancing the selectivity of nanofiltration membranes, which can be classified into membrane fabrication method improvement and process intensification. This review summarizes recent developments in the field and provides guidance for potential future approaches to improve the selectivity of nanofiltration membranes.

Optimization of operating parameters for xylose reductase separation through ultrafiltration membrane using response surface methodology

The application of the xylose reductase (XR) enzyme in the development of biotechnology demands an efficient and large scale enzyme separation technique. The aim of this present work was to optimize xylose reductase (XR) purification process through ultrafiltration membrane (UF) technology using Central composite design (CCD) of response surface methods (RSM). The three effective parameters analyzed were filtration time (0-100), transmembrane pressure (TMP) (1-1.6 bar), cross flow velocity (CFV) (0.52-1.2 cm/s-1) and its combined effect to obtain high flux with less possibility of membrane fouling. Experimental studies revealed that the best range for optimization process for filtration time, operational transmembrane pressure and cross flow velocity was 30 min, 1.4 bars and 1.06 cm/s, respectively as these conditions yielded the highest membrane permeability (56.03 Lm-2h-1 bar-1) and xylitol content (15.49 g/l). According to the analysis of variance (ANOVA), the p-value (<0.0001) indicated the designed model was highly significant. The error percentage between the actual and predicted value for membrane permeability and xylitol amount (2.21 % and 4.85 % respectively), which both were found to be close to the predicted values. The verification experiments gave membrane actual permeability of 57.3 Lm-2h-1 bar-1 and 16.29 g/l of xylitol production, thus indicating that the successfully developed model to predict the response.

Bioconversion of xylose to xylonic acid via co-immobilized dehydrogenases for conjunct cofactor regeneration

Enzymatic cofactor-dependent conversion of monosaccharides can be used in the bioproduction of value-added compounds. In this study, we demonstrate co-immobilization of xylose dehydrogenase (XDH, EC 1.1.1.175) and alcohol dehydrogenase (ADH, EC 1.1.1.1) using magnetite-silica core-shell particles for simultaneous conversion of xylose into xylonic acid (XA) and in situ cofactor regeneration. The reaction conditions were optimized by factorial design, and were found to be: XDH:ADH ratio 2:1, temperature 25 °C, pH 7, and process duration 60 min. Under these conditions enzymatic production of xylonic acid exceeded 4.1 mM and was more than 25% higher than in the case of a free enzymes system. Moreover, the pH and temperature tolerance as well as the thermo- and storage stability of the co-immobilized enzymes were significantly enhanced. Co-immobilized XDH and ADH make it possible to obtain higher xylonic acid concentration over broad ranges of pH (6-8) and temperature (15-35 °C) as compared to free enzymes, and retained over 60% of their initial activity after 20 days of storage. In addition, the half-life of the co-immobilized system was 4.5 times longer, and the inactivation constant (k_D = 0.0141 1/min) four times smaller, than those of the free biocatalysts (k_D = 0.0046 1/min). Furthermore, after five reaction cycles, immobilized XDH and ADH retained over 65% of their initial properties, with a final biocatalytic productivity of 1.65 mM of xylonic acid per 1 U of co-immobilized XDH. The results demonstrate the advantages of the use of co-immobilized enzymes over a free enzyme system in terms of enhanced activity and stability.