Coenzymatic properties of low molecular-weight and macromolecular N6-derivatives of NAD+ and NADP+ with dehydrogenases of interest for organic synthesis

Cofactor and Process Engineering for Nicotinamide Recycling and Retention in Intensified Biocatalysis

There is currently considerable interest in the intensification of biocatalytic processes to reduce the cost of goods for biocatalytically produced chemicals, including pharmaceuticals and advanced pharmaceutical intermediates. Continuous-flow biocatalysis shows considerable promise as a method for process intensification; however, the reliance of some reactions on the use of diffusible cofactors (such as the nicotinamide cofactors) has proven to be a technical barrier for key enzyme classes. This minireview covers attempts to overcome this limitation, including the cofactor recapture and recycling retention of chemically modified cofactors. For the latter, we also consider the state of science for cofactor modification, a field reinvigorated by the current interest in continuous-flow biocatalysis.

Physiological Role of Glutamate Dehydrogenase in Cancer Cells

NH 4 + increased growth rates and final densities of several human metastatic cancer cells. To assess whether glutamate dehydrogenase (GDH) in cancer cells may catalyze the reverse reaction of NH 4 + fixation, its covalent regulation and kinetic parameters were determined under near-physiological conditions. Increased total protein and phosphorylation were attained in NH 4 + -supplemented metastatic cells, but total cell GDH activity was unchanged. Higher V _max values for the GDH reverse reaction vs. forward reaction in both isolated hepatoma (HepM) and liver mitochondria [rat liver mitochondria (RLM)] favored an NH 4 + -fixing role. GDH sigmoidal kinetics with NH 4 + , ADP, and leucine fitted to Hill equation showed n _H values of 2 to 3. However, the K _0.5 values for NH 4 + were over 20 mM, questioning the physiological relevance of the GDH reverse reaction, because intracellular NH 4 + in tumors is 1 to 5 mM. In contrast, data fitting to the Monod-Wyman-Changeux (MWC) model revealed lower K _m values for NH 4 + , of 6 to 12 mM. In silico analysis made with MWC equation, and using physiological concentrations of substrates and modulators, predicted GDH N-fixing activity in cancer cells. Therefore, together with its thermodynamic feasibility, GDH may reach rates for its reverse, NH 4 + -fixing reaction that are compatible with an anabolic role for supporting growth of cancer cells.

Activity of select dehydrogenases with sepharose-immobilized N(6)-carboxymethyl-NAD

N⁶-carboxymethyl-NAD (N⁶-CM-NAD) can be used to immobilize NAD onto a substrate containing terminal primary amines. We previously immobilized N⁶-CM-NAD onto sepharose beads and showed that Thermotoga maritima glycerol dehydrogenase could use the immobilized cofactor with cofactor recycling. We now show that Saccharomyces cerevisiae alcohol dehydrogenase, rabbit muscle L-lactate dehydrogenase (type XI), bovine liver L-glutamic dehydrogenase (type III), Leuconostoc mesenteroides glucose-6-phosphate dehydro-genase, and Thermotoga maritima mannitol dehydrogenase are active with soluble N⁶-CM-NAD. The products of all enzymes but 6-phospho-D-glucono-1,5-lactone were formed when sepharose-immobilized N⁶-CM-NAD was recycled by T. maritima glycerol dehydrogenase, indicating that N⁶-immobilized NAD is suitable for use by a variety of different dehydrogenases. Observations of the enzyme active sites suggest that steric hindrance plays a greater role in limiting or allowing activity with the modified cofactor than do polarity and charge of the residues surrounding the N⁶-amine group on NAD.

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.

Structure/activity relationship of adenine-modified NAD derivatives with respect to porcine heart lactate dehydrogenase isozyme H4 simulated with molecular mechanics

Using a significantly simplified modification procedure, four charged analogues of the coenzyme NAD, N(1)- and N6-(2-hydroxy-3-trimethylammoniumpropyl)-NAD, N(1)- and N6-(3-sulfopropyl)-NAD were prepared. The kinetic parameters of these derivatives and N(1)-(2-aminoethyl)-NAD, N6-(2-aminoethyl)-NAD and tricyclic 1,N6-ethanoadenine-NAD, all with alterations to the adenine moiety, were determined for porcine heart lactate dehydrogenase isoenzyme H4. The coenzyme activity depends on both position and charge of the introduced groups. Modification of the N6-position leads to a 25-250-fold increase of the kcat/Km value compared to the related N(1) derivative. The kcat/Km value for 1,N6-ethanoadenine-NAD is in the range between that of N(1)-(2-aminoethyl)-NAD and N6-(2-aminoethyl)-NAD. In the case of both N(1) and N6 functionalization, the Km values increase from (3-sulfopropyl)-NAD, with a negatively charged substituent at the adenine, over (2-amino-ethyl)-NAD to (2-hydroxy-3-trimethylammoniumpropyl)-NAD with an uncharged and positively charged substituent, respectively, at the adenine. All N6 derivatives are analogues like NAD with respect to Km and/or Vmax and kcat/Km. The conformation of NAD and its derivatives was calculated and their interaction in the active site of lactate dehydrogenase was simulated using the molecular mechanics program AMBER. The significant differences in activity in correlation to porcine heart lactate dehydrogenase isoenzyme H4 could be rationalized by modelling the three-dimensional structure of the NAD site.