Hoodwinking Cytochrome P450BM3 into Hydroxylating Non-Native Substrates by Exploiting Its Substrate Misrecognition.

Abstract

Bacterial cytochrome P450s (P450s) are at the focus of attention as potential biocatalysts for applications in green synthetic chemistry, as they possess high activity for the hydroxylation of inert substrate C-H bonds. The high activity of bacterial P450s, such as P450BM3, is chiefly due to their high substrate specificity, and consequently, the catalytic activity of P450BM3 toward non-native substrates is very low, limiting the utility of bacterial P450s as biocatalysts. To enable oxidation of non-native substrates by P450BM3 without any mutagenesis, we have developed a series of "decoy molecules", inert dummy substrates, with structures that resemble those of the native substrates. Decoy molecules fool P450BM3 into generating the active species, so-called Compound I, enabling the catalytic oxidation of non-native substrates other than fatty acids. Perfluorinated carboxylic acids (PFCs) serve as decoy molecules to initiate the activation of molecular oxygen in the same manner as long-alkyl-chain fatty acids, due to their structural similarity, and induce the generation of Compound I, but, unlike the native substrates, PFCs are not oxidizable by Compound I, allowing the hydroxylation of non-native substrates, such as gaseous alkanes and benzene. The catalytic activity for non-native substrate hydroxylation was significantly enhanced by employing second generation decoy molecules, PFCs modified with amino acids (PFC-amino acids). Cocrystals of P450BM3 with PFC9-Trp revealed clear electron density in the fatty-acid-binding channel that was readily assigned to PFC9-Trp. The alkyl chain terminus of PFC9-Trp does not reach the active site owing to multiple hydrogen bonding interactions between the carboxyl and carbonyl groups of PFC9-Trp and amino acids located at the entrance of the substrate binding channel of P450BM3 that fix it in place. The remaining space above the heme after binding of PFC9-Trp can be utilized to accommodate non-native substrates. Further developments revealed that third generation decoy molecules, N-acyl amino acids, such as pelargonoyl-l-phenylalanine (C9-Phe), can serve as decoy molecules, indicating that the rationale "fluorination is required for decoy molecule function" can be safely discarded. Diverse carboxylic acids including dipeptides could now be exploited as building blocks, and a library of decoy molecules possessing diverse structures was prepared. Among the third-generation decoy molecules examined N-enanthyl-l-proline modified with l-phenylalanine (C7-Pro-Phe) afforded the maximum turnover rate for benzene hydroxylation. The structural diversity of third-generation decoy molecules was also utilized to control the stereoselectivity of hydroxylation for the benzylic hydroxylation of Indane, showing that decoy molecules can alter stereoselectivity. As both the catalytic activity and enantioselectivity are dependent upon the structure of the decoy molecules, their design allows us to regulate reactions catalyzed by wild-type enzymes. Furthermore, decoy molecules can also activate intracellular P450BM3, allowing the use of E. coli expressing wild-type P450BM3 as an efficient whole-cell bioreactor. It should be noted that Mn-substituted full-length P450BM3 (Mn-P450BM3) is also active for the hydroxylation of propane in which the regioselectivity diverged from that of Fe-P450BM3. The results summarized in this Account represent good examples of how the reactive properties of P450BM3 can be controlled for the monooxygenation of non-native substrates in vitro as well as in vivo to expand the potential of P450BM3.