AMINE OXIDASES (AOs)
Amine oxidases have been a key area of research in the Turner group for more than a decade. They oxidise amines to imines (or iminium ions) using molecular oxygen. The oxidation is enantioselective and addition of an amine oxidase to a racemic amine alongside a non-selective chemical reducing agent results in deracemisation to yield, after several cycles of selective-oxidation/non-selective reduction, a single enantiomer. The cofactor for amine oxidases is Flavin Adenine Dinucleotide (FAD) which when re-oxidised by O2 releases hydrogen peroxide. This detail is key as it has allowed us to develop a colony based assay in which turnover of substrate results in production of peroxide which can be detected using a peroxidase enzyme and a dye. This has enabled a program of directed evolution and semi-rational structure-based design to be applied to monoamine oxidase from Aspergillus niger(MAO-N), resulting in a range of variants with differing substrate scopes and increased activities. MAO-N variants have been used to synthesise key moieties in the APIs Solifenacin and Levocetirizine and have been applied in the deracemsiation of the natural products eleagnine and leptaflorin. Cyclohexyl Amine Oxidase (CHAO) and variants offer a differing substrate scope to MAO-N. We have applied CHAO in the deracemisation of 2-substituted tetrahydroquinolines. MAO-N and CHAO are (S)-selective AOs and thus recent research has been in the development of an (R)-selective AO. We have begun the evolution of 6-hydroxy-D-nicotine oxidase (6-HDNO) which has shown (R)-selectivity for all amines tested thus far, and have applied this in combination with imine reductases for a purely enzymatic deracemisation.