Phenylalanine Dehydrogenase Catalyzed Reductive Amination of 6-(1′,3′-Dioxolan-2′-YL)-2-Keto-Hexanoic Acid to 6-(1′,3′-Dioxolan-2′-YL)-2S-Aminohexanoic Acid with Nadh Regeneration and Enzyme and Cofactor Retention

Abstract

Two systems were analyzed that make it possible to reuse enzymes and cofactors in the reductive amination of the lithium salt of 6-(1′,3′-dioxolan-2′-yl)-2-keto-hexanoic acid (compound 1) to 6-(1′,3′-dioxolan-2′-yl)-2S-aminohexanoic acid, lithium salt (compound 2) catalyzed by phenylalanine dehydrogenase (PDH) with in situ NADH regeneration catalyzed by formate dehydrogenase (FDH). First, both enzymes and PEG-NADH, a mass-enlarged cofactor, were encapsulated within semipermeable microcapsules. These microcapsules catalyzed the biotransformation in the absence of external NAD. About 20 mg of compound 2 was formed per mL microcapsule per day. Substrate diffusion into the microcapsules was not rate-limiting. PEG(35,000)—NADH, but not PEG(20,000)—NADH, was retained within the microcapsules. The activity of the encapsulated enzyme was about 15–25% of the activity of the free enzyme. If saturating free NAD was added to microcapsules, the activity increased about threefold. The microcapsules could be reused, and lost about half of their activity after eight days of continuous use at 40°C. The stability of FDH limited the reusability of microcapsules. As an alternative system, enzymes and PEG-NADH were used in a membrane reactor. Preliminary results showed the production of about 1 g of product per day in a 10 mL reactor over a period of ten days at ambient temperature without loss of activity.