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

Fatty acid photodecarboxylase derived from Chlorella variabilis NC64A (CvFAP) 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_n) into their corresponding biohydrocarbons (C_n-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 CvFAP 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 CvFAP 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 CvFAP 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 CvFAP platform for the kinetic resolution (KR) of α-functionalized carboxylic acids. This successful application establishes CvFAP as a pivotal enantioselective catalyst and highlights FRISM as a powerful strategy for enhancing CvFAP's performance. This CvFAP-catalyzed KR platform was further extended to enable stereodivergent access to chiral secondary alcohols with tailor-made R or S 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 CvFAP variant's differential interaction with geometric isomers of cis/trans double bonds to selectively eliminate trans-fatty acids, thereby mitigating their adverse effects. Moreover, the engineered CvFAP 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 CvFAP 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 CvFAP'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 CvFAP modification. This is followed by a section on our FRISM strategy and its pivotal role in optimizing CvFAP'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 CvFAP's potential, and the concepts and strategies discussed herein are expected to advance the broader field of organic synthesis.