Amine Transaminase from<i>Exophiala Xenobiotica</i>—Crystal Structure and Engineering of a Fold IV Transaminase that Naturally Converts Biaryl Ketones

Amine transaminases are frequently used for the production of chiral amines starting from prochiral ketones. These amines can be applied as active pharmaceutical ingredients or drug precursors. However, there are still limitations to the use of amine transaminases when it comes to bulky ketone substrates, such as biaryl ketones. Using data mining, an (R)-selective amine transaminase from Exophiala xenobiotica was identified which naturally converts biaryl ketone substrates to the corresponding amines with up to 85% conversion and excellent enantioselectivity (>99% ee). Its protein crystal structure was obtained with a resolution of 1.52 Å, which enabled us to explain this interesting substrate acceptance. Structure-guided protein engineering resulted in a quintuple variant with increased stability. Moreover, the amino acid exchange T273S increased the activity and broadened the substrate scope, enabling conversions of various biaryl ketones with up to >99%. A preparative biotransformation of 1-(4-(pyridin-3-yl)phenyl)ethenone at 75 mM (15 g/L) resulted in 96% of isolated yield of the respective amine.