Synthesis of l-homophenylalanine with immobilized enzymes

The N-Acetyl Amino Acid Racemases (NAAARs); Native and evolved biocatalysts applied to the synthesis of canonical and non-canonical amino acids

Amino acids are one of the most important synthons employed in the biotechnology, pharmaceutical and agrochemical industries for the preparation of active agents. Recently, the emerging use of these compounds as tools for protein engineering, has also been reported. Numerous chemo- and biocatalytic strategies have been developed for the stereoselective synthesis of these compounds. One of the most efficient processes is the enzymatic dynamic kinetic resolution of N-acylated derivatives, where an N-acyl amino acid racemase (NAAAR) is coupled with an enantioselective, hydrolytic enzyme (aminoacylase), and used to convert a racemic mixture of starting materials to enantiopure products. Here we provide a brief overview of the structure and mechanism of NAAAR. We will also review the applications of this class of biocatalyst, as well as discussing the various strategies employed to obtain an efficient system for the synthesis of optically pure canonical and non-canonical amino acids.

N-succinylamino acid racemases: Enzymatic properties and biotechnological applications

The N-succinylamino acid racemase/o-succinylbenzoate synthase (NSAR/OSBS) subfamily from the enolase superfamily contains different enzymes showing promiscuous N-substituted-amino acid racemase (NxAR) activity. These enzymes were originally named as N-acylamino acid racemases because of their industrial application. Nonetheless, they are pivotal in several enzymatic cascades due to their versatility to catalyze a wide substrate spectrum, allowing the production of optically pure d- or l-amino acids from cheap precursors. These compounds are of paramount economic interest, since they are used as food additives, in the pharmaceutical and cosmetics industries and/or as chiral synthons in organic synthesis. Despite its economic importance, the discovery of new N-succinylamino acid racemases has become elusive, since classical sequence-based annotation methods proved ineffective in their identification, due to a high sequence similarity among the members of the enolase superfamily. During the last decade, deeper investigations into different members of the NSAR/OSBS subfamily have shed light on the classification and identification of NSAR enzymes with NxAR activity of biotechnological potential. This review aims to gather the dispersed information on NSAR/OSBS members showing NxAR activity over recent decades, focusing on their biotechnological applications and providing practical advice to identify new enzymes.

Toward a cell-free hydantoinase process: screening for expression optimization and one-step purification as well as immobilization of hydantoinase and carbamoylase

The hydantoinase process is applied for the industrial synthesis of optically pure amino acids via whole cell biocatalysis, providing a simple and well-established method to obtain the catalyst. Nevertheless, whole cell approaches also bear disadvantages like intracellular degradation reactions, transport limitations as well as low substrate solubility. In this work the hydantoinase and carbamoylase from Arthrobacter crystallopoietes DSM 20117 were investigated with respect to their applicability in a cell-free hydantoinase process. Both enzymes were heterologously expressed in Escherichia coli BL21DE3. Cultivation and induction of the hydantoinase under oxygen deficiency resulted in markedly higher specific activities and a further increase in expression was achieved by codon-optimization. Further expression conditions of the hydantoinase were tested using the microbioreactor system BioLector^®, which showed a positive effect upon the addition of 3% ethanol to the cultivation medium. Additionally, the hydantoinase and carbamoylase were successfully purified by immobilized metal ion affinity using Ni Sepharose beads as well as by functionalized magnetic beads, while the latter method was clearly more effective with respect to recovery and purification factor. Immobilization of both enzymes via functionalized magnetic beads directly from the crude cell extract was successful and resulted in specific activities that turned out to be much higher than those of the purified free enzymes.

Enhancement of biocatalytic efficiency by increasing substrate loading: enzymatic preparation of L-homophenylalanine

Enantiomerically pure L-homophenylalanine (L-HPA) is a key building block for the synthesis of angiotensin-converting enzyme inhibitors and other chiral pharmaceuticals. Among the processes developed for the L-HPA production, biocatalytic synthesis employing phenylalanine dehydrogenase has been proven as the most promising route. However, similar to other dehydrogenase-catalyzed reactions, the viability of this process is markedly affected by insufficient substrate loading and high costs of the indispensable cofactors. In the present work, a highly efficient and economic biocatalytic process for L-HPA was established by coupling genetically modified phenylalanine dehydrogenase and formate dehydrogenase. Combination of fed-batch substrate addition and a continuous product removal greatly increased substrate loading and cofactor utilization. After systemic optimization, 40 g (0.22 mol) of keto acid substrate was transformed to L-HPA within 24 h and a total of 0.2 mM NAD(+) was reused effectively in eight cycles of fed-batch operation, consequently giving an average substrate concentration of 510 mM and a productivity of 84.1 g l(-1) day(-1) for L-HPA. The present study provides an efficient and feasible enzymatic process for the production of L-HPA and a general solution for the increase of substrate loading.

Effect of molecular mobility on coupled enzymatic reactions involving cofactor regeneration using nanoparticle-attached enzymes

Cofactor-dependent multi-step enzymatic reactions generally require dynamic interactions among cofactor, enzyme and substrate molecules. Maintaining such molecular interactions can be quite challenging especially when the catalysts are tethered to solid state supports for heterogeneous catalysis for either biosynthesis or biosensing. The current work examines the effects of the pattern of immobilization, which presumably impacts molecular interactions on the surface of solid supports, on the reaction kinetics of a multienzymic system including glutamate dehydrogenase, glucose dehydrogenase and cofactor NAD(H). Interestingly, particle collision due to Brownian motion of nanoparticles successfully enabled the coupled reactions involving a regeneration cycle of NAD(H) even when the enzymes and cofactor were immobilized separately onto superparamagnetic nanoparticles (124 nm). The impact of particle motion and collision was evident in that the overall reaction rate was increased by over 100% by applying a moderate alternating magnetic field (500 Hz, 17 Gs), or using additional spacers, both of which could improve the mobility of the immobilized catalysts. We further observed that integrated immobilization, which allowed the cofactor to be placed in the molecular vicinity of enzymes on the same nanoparticles, could enhance the reaction rate by 1.8 fold. These results demonstrated the feasibility in manipulating molecular interactions among immobilized catalyst components by using nanoscale fabrication for efficient multienzymic biosynthesis.