Explaining Operational Instability of Amine Transaminases: Substrate-Induced Inactivation Mechanism and Influence of Quaternary Structure on Enzyme-Cofactor Intermediate Stability.

The insufficient operational stability of amine transaminases (ATA) constitutes a limiting factor for high productivity in chiral amine synthesis. In this work, we investigated the operational stability of a tetrameric ATA with 92% sequence identity to a Pseudomonas sp. transaminase and compared it to the two commonly used dimeric ATAs from Chromobacterium violaceum and Vibrio fluvialis. In the presence of substrate, all three ATAs featured reduced stability in comparison to their resting stability, but the tetramer showed slower inactivation rates than the dimeric ATAs. Kinetic and thermodynamic analysis revealed an amine donor induced inactivation mechanism involving accumulation of the less stable aminated enzyme–cofactor intermediate. Dissociation of the enzyme–PMP complex forms the unstable apoenzyme, which can rapidly unfold. Crystal structure analysis shed light on the structure–function relationship suggesting that the cofactor–ring binding element is stabilized in the quaternary structure conferr...