Research progress of the decarboxylation mechanisms of fatty acids to generate aliphatic hydrocarbons catalyzed by iron-containing enzymes

With the progress of human society, the consumption of energy materials is increasing in particularly the fossil fuels, which accounts for 84.7% of the total global consumption in 2018. Thus, excessive exploitation of fossil fuels not only brings about an increasingly serious energy crisis, but also leads to a series of environmental problems, such as the air quality deterioration and soil acidification, which are not conducive to the sustainable development of society. Since the 21st Annual Conference of the China Association for Science and Technology in 2019, “renewable synthetic fuels” has become one of the focuses of attention, and it also puts forward higher requirements for the research and development of renewable energy. About 60 years ago, many plants, insects, microorganisms and microalgae were found to metabolize hydrocarbons. In 2010, Schirmer’s groups recognized the alkane biosynthesis pathways of marine chlorobacteria, which aroused great interest in hydrocarbon biosynthesis. Recently, more proteases to catalyze the synthesis of hydrocarbons have been successively discovered, such as aldehyde deformylase (ADO, CYP4G and CER), fatty acid decarboxylase (UndA, UndB and OleT), polyketone synthase (Ols) and thiolytic enzyme complex system (OleABCD). In the past decades, many studies have been conducted to explore the biosynthesis of hydrocarbon, and useful information regarding the reaction mechanism has been obtained, including the kinetic rate constants and the crystal structures of the enzymes. In addition, on the basis of crystal structure, quantum chemistry method (QM) and molecular dynamics simulations (MD) were also used to explore the reaction mechanism of fatty acid decarboxylation. Since most enzymes involved in the hydrocarbon biosynthesis are iron-containing enzymes, which employ the Fe-coordinate dioxygen to trigger the reaction. In general, the whole reaction cycle contains the binding of substrates and dioxygen, and decarboxylation usually involves the hydrogen atom abstraction and breaking of C−C bond. During the reaction, complex electron transfer takes place among the Fe center, O2 and substrate, and many factors were found to affect the catalytic activity. According to the previous experimental and theoretical studies, different enzymes show different activity and substrate scope. In general, the catalytic efficiency is still very low for most enzymes, and thus, understanding the mechanism of these catalytic reactions is undoubtedly an important basis for engineering optimization and all applications, and can also provide ideas for the design and synthesis of biomimetic catalysts. In this paper, we review the research progress of five representative iron enzymes that catalyze the biosynthesis of aliphatic hydrocarbon, including OleT, UndA, UndB, ADO and CYP4G, and discuss the future development and application of enzyme catalysis in renewable synthetic fuels. This paper may offer some ideas and guidelines for sustainable energy supply in the future.