Selective Hydroxylation of Alkanes Catalyzed by Cytochrome P450 Enzymes

The selective oxyfunctionalization of unactivated C-H bonds is one of long-standing issues and current topics in synthetic chemistry. One of the major synthetic targets for these reactions is the direct and selective hydroxylation of alkanes to alcohols, however, which faces many severe challenges in controlling chemoselectivity, regioselectivity and stereoselectivity. In nature, the oxidative metalloenzymes is capable of selectively catalyzing the insertion of oxygen into inert C-H bonds of alkanes, such as methane monooxygenases (MMO), soluble butane monooxygenases (sBMO), fungal peroxygenases and Cytochrome P450 monooxygenases (P450s). Among them, P450s that catalyze a variety of oxygenation reactions have attracted special attentions because of some intrinsic advantages. P450s are widely distributed in plants, animals and microorganisms and over 41000 sequences of P450 genes have been named from various databases, which enhances the potentials of P450s in developing the oxidative biocatalysts. In addition, compared with MMOs, P450s that have smaller molecule weight (similar to 45 kDa) are simple and amenable to recombinant expression and engineering. Herein, we reviewed the recent progress of alkanes hydroxylation by P450 enzymes either in its natural forms or engineered variants, as well as chemical activated systems. The related background and the catalytic mechanism of P450s for alkanes hydroxylation were firstly discussed. The representative examples by natural P450s mainly from CYP153, CYP52 and other P450 families were then outlined. The strategies of rational design and directed evolution on P450s engineering were then summarized focusing on the native/non-native alkane substrates. Three unusual strategies, including substrate engineering, decoy molecule, and dual-functional small molecule co-catalysis, were also discussed on their applications for activating P450s to hydroxylate non-native small alkanes. Finally, we perspective the challenges and solutions that faced by P450 enzymes in the development of new biocatalytic systems toward selective hydroxylation of alkanes. In conclusion, cytochrome P450 enzymes in both of their native and modified form are promising biocatalysts for alkanes hydroxylation and need further be investigated to gain the practical industrial applications.