Finding the switch: turning a baeyer-villiger monooxygenase into a NADPH oxidase.

By a targeted enzyme engineering approach, we were able to create an efficient NADPH oxidase from a monooxygenase. Intriguingly, replacement of only one specific single amino acid was sufficient for such a monooxygenase-to-oxidase switch-a complete transition in enzyme activity. Pre-steady-state kinetic analysis and elucidation of the crystal structure of the C65D PAMO mutant revealed that the mutation introduces small changes near the flavin cofactor, resulting in a rapid decay of the peroxyflavin intermediate. The engineered biocatalyst was shown to be a thermostable, solvent tolerant, and effective cofactor-regenerating biocatalyst. Therefore, it represents a valuable new biocatalytic tool.

Exploring the structural basis of substrate preferences in Baeyer-Villiger monooxygenases: insight from steroid monooxygenase.

Steroid monooxygenase (STMO) from Rhodococcus rhodochrous catalyzes the Baeyer-Villiger conversion of progesterone into progesterone acetate using FAD as prosthetic group and NADPH as reducing cofactor. The enzyme shares high sequence similarity with well characterized Baeyer-Villiger monooxygenases, including phenylacetone monooxygenase and cyclohexanone monooxygenase. The comparative biochemical and structural analysis of STMO can be particularly insightful with regard to the understanding of the substrate-specificity properties of Baeyer-Villiger monooxygenases that are emerging as promising tools in biocatalytic applications and as targets for prodrug activation. The crystal structures of STMO in the native, NADP(+)-bound, and two mutant forms reveal structural details on this microbial steroid-degrading enzyme. The binding of the nicotinamide ring of NADP(+) is shifted with respect to the flavin compared with that observed in other monooxygenases of the same class. This finding fully supports the idea that NADP(H) adopts various positions during the catalytic cycle to perform its multiple functions in catalysis. The active site closely resembles that of phenylacetone monooxygenase. This observation led us to discover that STMO is capable of acting also on phenylacetone, which implies an impressive level of substrate promiscuity. The investigation of six mutants that target residues on the surface of the substrate-binding site reveals that enzymatic conversions of both progesterone and phenylacetone are largely insensitive to relatively drastic amino acid changes, with some mutants even displaying enhanced activity on progesterone. These features possibly reflect the fact that these enzymes are continuously evolving to acquire new activities, depending on the emerging availabilities of new compounds in the living environment.

Snapshots of enzymatic Baeyer-Villiger catalysis: oxygen activation and intermediate stabilization.

Baeyer-Villiger monooxygenases catalyze the oxidation of carbonylic substrates to ester or lactone products using NADPH as electron donor and molecular oxygen as oxidative reactant. Using protein engineering, kinetics, microspectrophotometry, crystallography, and intermediate analogs, we have captured several snapshots along the catalytic cycle which highlight key features in enzyme catalysis. After acting as electron donor, the enzyme-bound NADP(H) forms an H-bond with the flavin cofactor. This interaction is critical for stabilizing the oxygen-activating flavin-peroxide intermediate that results from the reaction of the reduced cofactor with oxygen. An essential active-site arginine acts as anchoring element for proper binding of the ketone substrate. Its positively charged guanidinium group can enhance the propensity of the substrate to undergo a nucleophilic attack by the flavin-peroxide intermediate. Furthermore, the arginine side chain, together with the NADP(+) ribose group, forms the niche that hosts the negatively charged Criegee intermediate that is generated upon reaction of the substrate with the flavin-peroxide. The fascinating ability of Baeyer-Villiger monooxygenases to catalyze a complex multistep catalytic reaction originates from concerted action of this Arg-NADP(H) pair and the flavin subsequently to promote flavin reduction, oxygen activation, tetrahedral intermediate formation, and product synthesis and release. The emerging picture is that these enzymes are mainly oxygen-activating and "Criegee-stabilizing" catalysts that act on any chemically suitable substrate that can diffuse into the active site, emphasizing their potential value as toolboxes for biocatalytic applications.

Beyond the Protein Matrix: Probing Cofactor Variants in a Baeyer-Villiger Oxygenation Reaction.

A general question in biochemistry is the interplay between the chemical properties of cofactors and the surrounding protein matrix. Here, the functions of NADP+ and FAD are explored by investigation of a representative monooxygenase reconstituted with chemically-modified cofactor analogues. Like pieces of a jigsaw puzzle, the enzyme active site juxtaposes the flavin and nicotinamide rings, harnessing their H-bonding and steric properties to finely construct an oxygen-reacting center that restrains the flavin-peroxide intermediate in a catalytically-competent orientation. Strikingly, the regio- and stereoselectivities of the reaction are essentially unaffected by cofactor modifications. These observations indicate a remarkable robustness of this complex multi-cofactor active site, which has implications for enzyme design based on cofactor engineering approaches.