Recycling of NAD+ crosslinked to albumin or hemoglobin immobilized with multienzyme systems in artificial cells

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.

Artificial cells with emphasis on bioencapsulation in biotechnology

The most common use of artificial cells is for bioencapsulation of biologically active materials. Each artificial cell can contain combinations of materials. The permeability, composition and shape of an artificial cell membrane can be varied using different types of synthetic or biological materials. These possible variations in contents and membranes allow for large variations in the properties and functions of artificial cells. Artificial cells containing adsorbents have been a routine form of treatment in hemoperfusion for patients. This includes acute poisoning, high blood aluminum and iron, and supplement to dialysis in kidney failure. Artificial red blood cell substitutes based on modified hemoglobin are already in Phase I and Phase II clinical trials in patients. Artificial cell encapsulated cell cultures are being studied for the treatment of diabetes, liver failure, gene therapy and other conditions. Research on artificial cells containing enzymes includes their use for treatment in hereditary enzyme deficiency diseases and other diseases. Recent demonstration of extensive enterorecirculation of amino acids in the intestine has allowed oral administration to deplete specific amino acids. One example is phenylketonuria, an inborn error or metabolism resulting in high systemic phenylalanine levels. Preliminary clinical studies in patients using bioencapsulation of cells or enzymes have started. Artificial cells containing complex enzyme systems convert wastes like urea and ammonia into essential amino acids. Artificial cells are being used for the production of monoclonal antibodies, interferon and other biotechnological products. Other areas of biotechnological uses include drug delivery, and other areas of biotechnology, chemical engineering and medicine.

Artificial cells in immobilization biotechnology

Artificial cells contain biologically active materials. Artificial cells containing adsorbents have been a routine form of treatment in hemoperfusion for patients. This includes acute poisoning, high blood aluminum and iron, and supplement to dialysis in kidney failure. Artificial cells are being tested for use as red blood cell substitutes. Artificial cells encapsulated cell culture are being tested in animals for the treatment of diabetes and liver failure. A novel 2 step method has prevented xenograft rejection. Artificial cells containing enzymes are being studied for treatment in hereditary enzyme deficiency diseases and other diseases. Recent demonstration of extensive enterorecirculation of amino acids in the intestine has allowed its oral administration to deplete specific amino acids. Artificial cells containing complex enzyme system convert wastes like urea and ammonia into essential amino acids. Artificial cell is being used for the production of monoclonal antibodies, interferons and other biotechnological products. It is also being investigated for drug delivery, and for use in other applications in biotechnology, chemical engineering and medicine.

Conversion of ammonia or urea into essential amino acids, L-leucine, L-valine, and L-isoleucine using artificial cells containing an immobilized multienzyme system and dextran-NAD. L-lactic dehydrogenase for coenzyme recycling

A multienzyme system consisting of leucine dehydrogenase (EC 1.4.1.9), L-lactic dehydrogenase (EC 1.1.1.27), urease (EC 3.5.1.5), and dextran-NAD+ was microencapsulated within artificial cells. This system could convert ammonia and urea into essential amino acids, L-leucine, L-valine, and L-isoleucine. L-lactate acted as a cosubstrate for the regeneration of dextran-NADH. Greater concentrations of L-lactate favored the higher conversion ratios. The effects of ammonium salts and urea on reaction rate were also studied. The relative reaction rates in ammonium salts solutions were 44.6-78.8% of those in urea solutions. More than 90% of the original activity was retained when artificial cells were kept at 4 degrees C for 6 wk.

Recycling of NAD(P) by multienzyme systems immobilized by microencapsulation in artificial cells

Multistep enzyme systems can be immobilized in solution within semipermeable microcapsules. With the ability to recycle cofactors, a number of potentially useful systems have been made possible. Furthermore NAD+ can be retained inside the microcapsules by two approaches. (1) NAD+ can be linked to macromolecules such as dextran or polyethyleneimine. However, in this form, there are significant increases in steric hindrance and diffusion restrictions. (2) "Artificial cells" consisting of lipid-polyamide membrane microcapsules containing multienzyme systems, cofactors, and substrates can retain NAD+ in the free form. Analogous to the intracellular environments of red blood cells, free NAD+ in solution inside the microcapsules is effectively recycled by the multistep enzyme systems which are also in solution. Enzymes in the microcapsules are in high concentrations and in close proximity to one another. Any number and any concentration of different enzyme systems can be microencapsulated all within one artificial cell, within the limit of solubility of the total amount of enzymes. Products of sequential reactions inside the microcapsules are at much higher concentrations than outside. All these factors result in an optimal intracellular environment for multistep enzyme reactions. External substrates in the form of lipophilic or small hydrophilic molecules can equilibrate across the membrane to participate as initial substrates in the multistep reactions in the microcapsules. A number of potential applications are possible using this approach. The lipid-polyamide membrane artificial cell can also be used in basic research as a biochemical cell model for the simpler types of biological cells such as erythrocytes.