Discovery and structural characterization of a thermostable bacterial monoamine oxidase

Monoamine oxidases (MAOs) are pivotal regulators of neurotransmitters in mammals, while microbial MAOs have been shown to be valuable biocatalysts for enantioselective synthesis of pharmaceutical compounds or precursors thereof. To extend the knowledge of how MAOs function at the molecular level and in order to provide more biocatalytic tools, we set out to identify and study a robust bacterial variant: a MAO from the thermophile Thermoanaerobacterales bacterium (MAO_Tb ). MAO_Tb is highly thermostable with melting temperatures above 73 °C and is well expressed in Escherichia coli. Substrate screening revealed that the oxidase is most efficient with n-alkylamines with n-heptylamine being the best substrate. Presteady-state kinetic analysis shows that reduced MAO_Tb rapidly reacts with molecular oxygen, confirming that it is a bona fide oxidase. The crystal structure of MAO_Tb was resolved at 1.5 Å and showed an exceptionally high similarity with the two human MAOs, MAO A and MAO B. The active site of MAO_Tb resembles mostly the architecture of human MAO A, including the cysteinyl protein-FAD linkage. Yet, the bacterial MAO lacks a C-terminal extension found in human MAOs, which explains why it is expressed and purified as a soluble protein, while the mammalian counterparts are anchored to the membrane through an α-helix. MAO_Tb also displays a slightly different active site access tunnel, which may explain the specificity toward long aliphatic amines. Being an easy-to-express, thermostable enzyme, for which a high-resolution structure was elucidated, this bacterial MAO may develop into a valuable biocatalyst for synthetic chemistry or biosensing.