Asymmetric reductive amination of ketones catalysed by amine dehydrogenases
Amine dehydrogenases (AmDHs) are NADH/NADPH dependent enzymes that catalyze the reversible conversion of ketones and aldehydes into enantiomerically pure amines at the sole expense of ammonia and a hydride source. The latter is required for cofactor recycling and can usually be formate or glucose. A second enzyme such as a formate dehydrogenase (FDH) or a glucose dehydrogenase (GDH) catalyzes the cofactor recycling. The AmDH-FDH (or GDH) system offers elevated atom economy, as the buffer of the reaction (HCOONH4) directly provides the source of nitrogen and the reducing equivalents. Thus, only catalytic amount of NAD(P)H is needed while carbonate and water are the by-products.
Fourteen members of AmDHs are currently available. They have been created either by enzyme engineering starting from L-Amino acid dehydrogenases (L-AADHs) or discovered as native (nat-AmDHs) using metagenomic data. To date, the substrate scope of these AmDHs already covers a significant range of structurally diverse carbonyl compounds, thus giving access to approximately eighty α-chiral amines in high optical purity. This section describes the detailed procedure for the preparation and application of four engineered AmDHs: 1) Bb-PhAmDH engineered from Bacillus badius phenylalanine dehydrogenase; 2) cFL1-AmDH, a chimera obtained by domain shuffling from two first generation AmDH variants; 3) Rs-PheAmDH engineered from Rhodococcus sp. M4 phenylalanine dehydrogenase; and 4) LE-AmDH-v1 engineered from Geobacillus sterothemophilus ε-deaminating L-Lysine dehydrogenase. The enzymes can be applied in tandem with FDH from Candida boidinii for the reductive amination of carbonyl compounds. Overall, these AmDHs can give access to R-configured amines.