Investigation of a New Type I Baeyer-Villiger Monooxygenase from Amycolatopsis thermoflava Revealed High Thermodynamic but Limited Kinetic Stability

Baeyer-Villiger monooxygenases (BVMOs) are remarkable biocatalysts, but, due to their low stability, their application in industry is hampered. Thus, there is a high demand to expand on the diversity and increase the stability of this class of enzyme. Starting from a known thermostable BVMO sequence from Thermocrispum municipale (TmCHMO), a novel BVMO from Amycolaptosis thermoflava (BVMOFlava ), which was successfully expressed in Escherichia coli BL21(DE3), was identified. The activity and stability of the purified enzyme was investigated and the substrate profile for structurally different cyclohexanones and cyclobutanones was assigned. The enzyme showed a lower activity than that of cyclohexanone monooxygenase (CHMOAcineto ) from Acinetobacter sp., as the prototype BVMO, but indicated higher kinetic stability by showing a twofold longer half-life at 30 °C. The thermodynamic stability, as represented by the melting temperature, resulted in a Tm value of 53.1 °C for BVMOFlava , which was comparable to the Tm of TmCHMO (ΔTm =1 °C) and significantly higher than the Tm value for CHMOAcineto ((ΔTm =14.6 °C)). A strong deviation between the thermodynamic and kinetic stabilities of BVMOFlava was observed; this might have a major impact on future enzyme discovery for BVMOs and their synthetic applications.

Morpholine-based buffers activate aerobic photobiocatalysis via spin correlated ion pair formation

The use of enzymes for synthetic applications is a powerful and environmentally-benign approach to increase molecular complexity. Oxidoreductases selectively introduce oxygen and hydrogen atoms into myriad substrates, catalyzing the synthesis of chemical and pharmaceutical building blocks for chemical production. However, broader application of this class of enzymes is limited by the requirements of expensive cofactors and low operational stability. Herein, we show that morpholine-based buffers, especially 3-(N-morpholino)propanesulfonic acid (MOPS), promote photoinduced flavoenzyme-catalyzed asymmetric redox transformations by regenerating the flavin cofactor via sacrificial electron donation and by increasing the operational stability of flavin-dependent oxidoreductases. The stabilization of the active forms of flavin by MOPS via formation of the spin correlated ion pair ³[flavin˙^--MOPS˙⁺] ensemble reduces the formation of hydrogen peroxide, circumventing the oxygen dilemma under aerobic conditions detrimental to fragile enzymes.

Boosting photobioredox catalysis by morpholine electron donors under aerobic conditions

Morpholine derivatives expand the applicability of photobiocatalysis towards stabilization of flavin-based bio- and photocatalysts.