Tunable Catalytic Performance and New-to-Nature Reactions of Fatty Acid Photodecarboxylase

ConspectusFatty acid photodecarboxylase derived from <i>Chlorella variabilis</i> NC64A (<i>Cv</i>FAP) is a rare natural photoenzyme that has attracted considerable interest since its initial discovery. Its appeal lies in its unique blue-light-driven capacity to valorize (waste) fatty acids (C_<i>n</i>) into their corresponding biohydrocarbons (C_<i>n</i>-1). These products exhibit high calorific value and drop-in compatibility with existing combustion engines, offering a sustainable approach to addressing the ongoing energy and environmental crisis. However, the practical application of <i>Cv</i>FAP is severely impeded by critical limitations, such as its stringent substrate scope, susceptibility to photoinactivation, poor recyclability, and restricted catalytic versatility. Over the past decade, leading groups have dedicated considerable effort to modifying <i>Cv</i>FAP to broaden its synthetic utility. These studies have pursued objectives ranging from improving catalytic efficiency for biohydrocarbons, achieving stereocontrol for chiral molecules, and most ambitiously, developing new-to-nature C-C bond-forming activities. Fundamental to these efforts has been mechanistic investigation, which provides the insights for rational enzyme redesign.Our work has advanced <i>Cv</i>FAP research from expanding its native function to fundamentally reprogramming its catalytic identity. In 2019, we employed our "focused rational iterative site-specific mutagenesis" (FRISM) strategy to construct the first engineered <i>Cv</i>FAP platform for the kinetic resolution (<i>KR</i>) of α-functionalized carboxylic acids. This successful application establishes <i>Cv</i>FAP as a pivotal enantioselective catalyst and highlights FRISM as a powerful strategy for enhancing <i>Cv</i>FAP's performance. This <i>Cv</i>FAP-catalyzed <i>KR</i> platform was further extended to enable stereodivergent access to chiral secondary alcohols with tailor-made <i>R</i> or <i>S</i> configurations on an optional basis via the decarboxylation of oxalates and oxamic acids with γ-chiral centers. In a distinct approach, we exploited our engineered <i>Cv</i>FAP variant's differential interaction with geometric isomers of <i>cis/trans</i> double bonds to selectively eliminate <i>trans</i>-fatty acids, thereby mitigating their adverse effects. Moreover, the engineered <i>Cv</i>FAP was utilized to synthesize hydrocarbons of tunable chain length as well as deuterated molecules via a redirected decarboxylation cycle. Going beyond these applications, we fundamentally repurposed <i>Cv</i>FAP as a dehalogenase that operates via a reductive single electron transfer (SET)-initiated mechanism to efficiently synthesize chiral tetralones.Unlike previous reviews that mainly focused on <i>Cv</i>FAP's role in biohydrocarbon production, this Account presents advances with a broader scope, encompassing applications in asymmetric synthesis and the mechanistic insights guiding its engineering. We begin with an overview of naturally occurring photoenzymes and key outcomes in <i>Cv</i>FAP modification. This is followed by a section on our FRISM strategy and its pivotal role in optimizing <i>Cv</i>FAP's performance. The contributions of other leading groups, including those of Beisson, Hollmann, Yang, and collaborators, are integrated throughout our discussion. Finally, we identify outstanding challenges and opportunities to fully realize <i>Cv</i>FAP's potential, and the concepts and strategies discussed herein are expected to advance the broader field of organic synthesis.