Asymmetric Enzymatic Synthesis of Allylic Amines: A Sigmatropic Rearrangement Strategy.

Sigmatropic rearrangements, while rare in biology, offer opportunities for the efficient and selective synthesis of complex chemical motifs. A "P411" serine-ligated variant of cytochrome P450(BM3) has been engineered to initiate a sulfimidation/[2,3]-sigmatropic rearrangement sequence in whole E. coli cells, a non-natural function for any enzyme, providing access to enantioenriched, protected allylic amines. Five mutations in the enzyme substantially enhance its activity toward this new function, demonstrating the evolvability of the catalyst toward challenging nitrene transfer reactions. The evolved catalyst additionally performs the highly enantioselective imidation of non-allylic sulfides.

Enantioselective Enzyme-Catalyzed Aziridination Enabled by Active-Site Evolution of a Cytochrome P450.

One of the greatest challenges in protein design is creating new enzymes, something evolution does all the time, starting from existing ones. Borrowing from nature's evolutionary strategy, we have engineered a bacterial cytochrome P450 to catalyze highly enantioselective intermolecular aziridination, a synthetically useful reaction that has no natural biological counterpart. The new enzyme is fully genetically encoded, functions in vitro or in whole cells, and can be optimized rapidly to exhibit high enantioselectivity (up to 99% ee) and productivity (up to 1,000 catalytic turnovers) for intermolecular aziridination, demonstrated here with tosyl azide and substituted styrenes. This new aziridination activity highlights the remarkable ability of a natural enzyme to adapt and take on new functions. Once discovered in an evolvable enzyme, this non-natural activity was improved and its selectivity tuned through an evolutionary process of accumulating beneficial mutations.

Enzyme-controlled nitrogen-atom transfer enables regiodivergent C-H amination.

We recently demonstrated that variants of cytochrome P450BM3 (CYP102A1) catalyze the insertion of nitrogen species into benzylic C-H bonds to form new C-N bonds. An outstanding challenge in the field of C-H amination is catalyst-controlled regioselectivity. Here, we report two engineered variants of P450BM3 that provide divergent regioselectivity for C-H amination-one favoring amination of benzylic C-H bonds and the other favoring homo-benzylic C-H bonds. The two variants provide nearly identical kinetic isotope effect values (2.8-3.0), suggesting that C-H abstraction is rate-limiting. The 2.66-Å crystal structure of the most active enzyme suggests that the engineered active site can preorganize the substrate for reactivity. We hypothesize that the enzyme controls regioselectivity through localization of a single C-H bond close to the iron nitrenoid.

Enantioselective imidation of sulfides via enzyme-catalyzed intermolecular nitrogen-atom transfer.

Engineering enzymes with novel reaction modes promises to expand the applications of biocatalysis in chemical synthesis and will enhance our understanding of how enzymes acquire new functions. The insertion of nitrogen-containing functional groups into unactivated C-H bonds is not catalyzed by known enzymes but was recently demonstrated using engineered variants of cytochrome P450BM3 (CYP102A1) from Bacillus megaterium. Here, we extend this novel P450-catalyzed reaction to include intermolecular insertion of nitrogen into thioethers to form sulfimides. An examination of the reactivity of different P450BM3 variants toward a range of substrates demonstrates that electronic properties of the substrates are important in this novel enzyme-catalyzed reaction. Moreover, amino acid substitutions have a large effect on the rate and stereoselectivity of sulfimidation, demonstrating that the protein plays a key role in determining reactivity and selectivity. These results provide a stepping stone for engineering more complex nitrogen-atom-transfer reactions in P450 enzymes and developing a more comprehensive biocatalytic repertoire.

Improved cyclopropanation activity of histidine-ligated cytochrome†P450 enables the enantioselective formal synthesis of levomilnacipran.

Engineering enzymes capable of modes of activation unprecedented in nature will increase the range of industrially important molecules that can be synthesized through biocatalysis. However, low activity for a new function is often a limitation in adopting enzymes for preparative-scale synthesis, reaction with demanding substrates, or when a natural substrate is also present. By mutating the proximal ligand and other key active-site residues of the cytochrome P450 enzyme from Bacillus megaterium (P450-BM3), a highly active His-ligated variant of P450-BM3 that can be employed for the enantioselective synthesis of the levomilnacipran core was engineered. This enzyme, BM3-Hstar, catalyzes the cyclopropanation of N,N-diethyl-2-phenylacrylamide with an estimated initial rate of over 1000 turnovers per minute and can be used under aerobic conditions. Cyclopropanation activity is highly dependent on the electronic properties of the P450 proximal ligand, which can be used to tune this non-natural enzyme activity.

Expanding P450 catalytic reaction space through evolution and engineering.

Advances in protein and metabolic engineering have led to wider use of enzymes to synthesize important molecules. However, many desirable transformations are not catalyzed by any known enzyme, driving interest in understanding how new enzymes can be created. The cytochrome P450 enzyme family, whose members participate in xenobiotic metabolism and natural products biosynthesis, catalyzes an impressive range of difficult chemical reactions that continues to grow as new enzymes are characterized. Recent work has revealed that P450-derived enzymes can also catalyze useful reactions previously accessible only to synthetic chemistry. The evolution and engineering of these enzymes provides an excellent case study for how to genetically encode new chemistry and expand biology's reaction space.