Retention and regeneration of native NAD(H) in noncharged ultrafiltration membrane reactors: application to L-lactate and gluconate production

Sequential Co-immobilization of Enzymes in Metal-Organic Frameworks for Efficient Biocatalytic Conversion of Adsorbed CO2 to Formate

The main challenges in multienzymatic cascade reactions for CO₂ reduction are the low CO₂ solubility in water, the adjustment of substrate channeling, and the regeneration of co-factor. In this study, metal-organic frameworks (MOFs) were prepared as adsorbents for the storage of CO₂ and at the same time as solid supports for the sequential co-immobilization of multienzymes via a layer-by-layer self-assembly approach. Amine-functionalized MIL-101(Cr) was synthesized for the adsorption of CO₂. Using amine-MIL-101(Cr) as the core, two HKUST-1 layers were then fabricated for the immobilization of three enzymes chosen for the reduction of CO₂ to formate. Carbonic anhydrase was encapsulated in the inner HKUST-1 layer and hydrated the released CO₂ to HCO3-. Bicarbonate ions then migrated directly to the outer HKUST-1 shell containing formate dehydrogenase and were converted to formate. Glutamate dehydrogenase on the outer MOF layer achieved the regeneration of co-factor. Compared with free enzymes in solution using the bubbled CO₂ as substrate, the immobilized enzymes using stored CO₂ as substrate exhibited 13.1-times higher of formate production due to the enhanced substrate concentration. The sequential immobilization of enzymes also facilitated the channeling of substrate and eventually enabled higher catalytic efficiency with a co-factor-based formate yield of 179.8%. The immobilized enzymes showed good operational stability and reusability with a cofactor cumulative formate yield of 1077.7% after 10 cycles of reusing.

Tethering of nicotinamide adenine dinucleotide inside hollow nanofibers for high-yield synthesis of methanol from carbon dioxide catalyzed by coencapsulated multienzymes

Enzymatic conversion of carbon dioxide (CO2) to fuel or chemicals is appealing, but is limited by lack of efficient technology for regeneration and reuse of expensive cofactors. Here we show that cationic polyelectrolyte-doped hollow nanofibers, which can be fabricated via a facile coaxial electrospinning technology, provide an ideal scaffold for assembly of cofactor and multienzymes capable of synthesizing methanol from CO2 through a cascade multistep reaction involving cofactor regeneration. Cofactor and four enzymes including formate, formaldehyde, alcohol, and glutamate dehydrogenases were in situ coencapsulated inside the lumen of hollow nanofibers by involving them in the core-phase solution for coaxial electrospinning, in which cationic polyelectrolyte was predissolved. The polyelectrolyte penetrating across the shell of the hollow nanofibers enabled efficient tethering and retention of cofactor inside the lumen via ion-exchange interactions between oppositely charged polyelectrolytes and cofactor. With carbonic anhydrase assembled on the outer surface of the hollow nanofibers for accelerating hydration of CO2, these five-enzymes-cofactor catalyst system exhibited high activity for methanol synthesis. Compared with methanol yield of only 36.17% using free enzymes and cofactor, the hollow nanofiber-supported system afforded a high value up to 103.2%, the highest reported value so far. It was believed that the linear polyelectrolytes acted as spacers to enhance the shuttling of cofactor between enzymes that were coencapsulated within near vicinity, thus improving the efficiency of the system. The immobilized system showed good stability in reusing. About 80% of its original productivity was retained after 10 reusing cycles, with a cofactor-based cumulative methanol yield reached 940.5%.

A novel method for preparation of MNP@CS-tethered coenzyme for coupled oxidoreductase system

The immobilized cofactor NAD(H) is easily recovered from the reaction bulk, which is essential for repeated use of NAD(H) in the bioprocess catalyzed by NAD(H)-dependent oxidoreductase. Here, a magnetic nanoparticle platform was designed to immobilize both of the NADH and the NAD(+). The design was based on chitosan-coated magnetic nanoparticles (MNP@CS) which was activated by the EDC/NHS with the aid of azelaic acid as spacer. Interestingly, the succinimide group at the end of spacer arm catalyzed direct coupling of a carboxyl-terminal to the 6-amino group of the adenine residue of NAD(H). Our results indicated that 150 μmol NADH and 50 μmol NAD(+) was effectively attached to 1g MNP@CS at 25°C in 120 min and the prepared MNP@CS-NAD(H) showed good activity according to the coupling reaction of benzyl alcohol and acetaldehyde catalyzed by alcohol dehydrogenase.

Nanoparticle-supported multi-enzyme biocatalysis with in situ cofactor regeneration

Although there have been a long history of studying and using immobilized enzymes, little has been reported regarding the nature of immobilized cofactors. Herein we report that cofactor NAD(H) covalently attached to silica nanoparticles successfully coordinated with particle-immobilized enzymes and enabled multistep biotransformations. Specifically, silica nanoparticle-attached glutamate dehydrogenase (GLDH), lactate dehydrogenase (LDH) and NAD(H) were prepared and applied to catalyze the coupled reactions for production of alpha-ketoglutarate and lactate with the cofactor regenerated within the reaction cycle. It appeared that particle-particle collision driven by Brownian motion of the nanoparticles provided effective interactions among the catalytic components, and thus realized a dynamic shuttling of the particle-supported cofactor between the two enzymes to keep the reaction cycles continuing. Total turnover numbers (TTNs) as high as 20,000h(-1) were observed for the cofactor. It appeared to us that the use of particle-attached cofactor promises a new biochemical processing strategy for cofactor-dependent biotransformations.

Multi-scale features in recent development of enzymic biocatalyst systems

Functional relation among elements of different size scales in a system is probably a main challenge across the areas of the science of engineering ever since their emergence. Multi-scale time and size correlation for description and prediction of complex systems, however, has been systematically examined only recently with the aid of new computational tools. In the pursuit of efficient and sustainable chemical processing technologies, people have seen a growing emphasis on synthetic biotechnology in recent R&D efforts. In particular, industrial enzyme technologies are attracting enormous attention. Having been traditionally developed for food and detergent applications, industrial enzyme technologies are being re-examined and tested to their limits to keep abreast of the challenges in drug, biochemical, and the emerging biorenewable energy industries. Toward that, enzymes are required to function in non-conventional conditions, such as organic solvents, extreme pH, and temperatures; they also have to compete against alternative chemical technologies in terms of costs and efficiency. Accordingly, enzymic biocatalyst systems are being tackled dynamically at all size levels through efforts ranging from molecular level protein engineering and modification, nanoscale structure fabrication, and microenvironment manipulation to the construction of microchip devices and macroscopic industrial bioreactors and devices. These efforts are probably still on a case-to-case trial basis without much consideration of cross-scale correlations. Discovering, understanding, and controlling of the common features that relate functions of biocatalysts at different size scales may eventually be realized in future.

Development of a new bioprocess for production of 1,3-propanediol I.: Modeling of glycerol bioconversion to 1,3-propanediol with Klebsiella pneumoniae enzymes

Glycerol is a renewable resource for it is formed as a byproduct during biodiesel production. Because of its large volume production, it seems to be a good idea to develop a technology that converts this waste into products of high value, for example, to 1,3-propanediol (1,3-PD). We suggested an enzymatic bioconversion in a membrane reactor in which the NAD coenzyme can be regenerated, and three key enzymes are retained by a 10-kDa ultrafilter membrane. Unfortunately, some byproducts also formed during successful glycerol to 1,3-PD bioconversion runs, as we used crude enzyme solution of Klebsiella pneumoniae. To study the possibilities to avoid this byproduct formation, we built a mathematical description of this system. The model was also used for simulation bioconversions of high glycerol concentration with and without elimination of byproduct formation and of continuous operation.

Cofactor regeneration for sustainable enzymatic biosynthesis

Oxidoreductases are attractive catalysts for biosynthesis of chiral compounds and polymers, construction of biosensors, and degradation of environmental pollutants. Their practical applications, however, can be quite challenging since they often require cofactors such as nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). These cofactors are generally expensive. Efficient regeneration of cofactors is therefore critical to the economic viability of industrial-scale biotransformations using oxidoreductases. The chemistry of cofactor regeneration is well known nowadays. The challenge is mostly regarding how to achieve the regeneration with immobilized enzyme systems which are preferred for industrial processes to facilitate the recovery and continuous use of the catalysts. This has become a great hurdle for the industrialization of many promising enzymatic processes, and as a result, most of the biotransformations involving cofactors have been traditionally performed with living cells in industry. Accompanying the rapidly growing interest in industrial biotechnology, immobilized enzyme biocatalyst systems with cofactor regeneration have been the focus for many studies reported since the late 1990s. The current paper reviews the methods of cofactor retention for development of sustainable and regenerative biocatalysts as revealed in these recent studies, with the intent to complement other reviewing articles that are mostly regeneration chemistry-oriented. We classify in this paper the methods of sustainable cofactor regeneration into two categories, namely membrane entrapment and solid-attachment of cofactors.