Due to the challenges in imposing stereocontrol over free radical intermediates, the development of catalytic asymmetric radical-based transformations, particularly intermolecular ones, has remained a difficult problem in modern chemical and biological synthesis. Over the past several years, extensive efforts toward unnatural stereoselective radical biocatalysis has given rise to powerful tools to enable organic transformations whose asymmetric variants have long eluded small-molecule catalysis. Two broadly applicable strategies have guided the recent development of enzyme-catalyzed free radical processes not previously known in natural enzymology. First, by capitalizing on the innate redox activities of common coenzymes such as flavin, NAD(P)H, heme, and nonheme metallocofactors, biocatalysis researchers including our own group have repurposed natural enzymes to catalyze stereoselective radical reactions that proceed through an array of unnatural redox mechanisms (Scheme 1(A)). Second, recent work from the Hyster group and our group showed that through synergistic photobiocatalysis, radicals that are not readily accessible from coenzyme-induced redox events could be photochemically generated and efficiently intercepted by an enzyme, thereby granting access to new classes of transformations based on nonnatural one-electron processes. In particular, our studies on synergistic photoredox-pyridoxal biocatalysis demonstrated that through the use of carefully engineered photocatalystenzyme pairs, short-lived free radical intermediates formed outside the enzyme's pocket could travel into the enzyme's buried active site, thereby engaging enzymatic intermediates in a novel stereoselective C-C bond forming step. We sought to generalize synergistic photobiocatalysis to develop new classes of enzymatic radical reactions with unexplored enzyme families. In particular, we have long been interested in unlocking thiamine pyrophosphate (TPP)-dependent enzymes to perform novel unnatural radical reactions through this biocatalyst-photocatalyst synergy. As a distinct enzyme family, TPP-dependent enzymes play a crucial role in a number of metabolic pathways in all kingdoms of life. In these enzymes, the TPP cofactor serves as the essential catalytic machinery to catalyze various carbonyl activation reactions via the intermediacy of the enaminol Breslow intermediate. While almost all the biotechnologically significant and broadly applicable TPP enzymes use a two-electron mechanism to activate the carbonyl substrate, a small collection of enzymes capitalize on the potent reducing power of the Breslow intermediate (E = -0.95 V versus SCE) to access one-electron oxidized intermediates. These include TPP-dependent pyruvate:ferredoxin oxidoreductase (PFOR), pyruvate oxidase (POX) and pyruvate dehydrogenase complex (PDHC), which have evolved to catalyze consecutive single-electron transfer using [4Fe-4S] cluster, flavin adenine dinucleotide (FAD), and lipoamide, respectively, as the electron acceptor. With these thiamine redox enzymes, the conversion of the pyruvate derived enaminol into an acylthiazolium species involves a crucial TPP derived ketyl radical (Scheme 1(B)). Furthermore, recent advances from N-heterocyclic carbene (NHC) catalysis have led to a range of synthetically useful radical reactions, although the enantioselective variants of these free radical processes have remained largely underdeveloped. Inspired by these pioneering studies, we posited that through the judicious selection and engineering of substrate-promiscuous, broadly applicable TPP enzymes, we would be able to advance a new thiamine radical biocatalysis platform to develop a range of useful enzymatic reactions not presently known in the biological world. During the preparation of this manuscript, an elegant study repurposing TPPdependent benzaldehyde lyase to catalyze radical alkylation of benzaldehydes was reported by the Huang group. Based on a number of elegant synthetic organic studies, we envisioned that through the use of an appropriate alkyl radical precursor (e.g., redox-active esters), we would be able to develop an asymmetric radical alkylation of benzaldehydes and related a-ketoacid substrates using TPP-dependent enzymes. Our proposed dual photobiocatalytic cycle for asymmetric radical alkylation is detailed in Scheme 1(C). At the commencement of the TPP biocatalytic cycle, the deprotonated thiazolidene species III would engage the benzaldehyde substrate I to form the Breslow intermediate IV. In the photoredox cycle, visible light irradiation of fluorescein (PC) would produce a potent photooxidant (E * (FL*/FL ×-) = +0.77 V versus SCE). This excited-state oxidant would oxidize the enzymatically formed Breslow intermediate IV (E = -0.95 V versus SCE), furnishing a thiazolium-derived ketyl radical V. This electron transfer (ET) event between the photoredox catalyst and the enzyme would, in addition, lead to the radical anionic form of the photocatalyst (PC ×-), a potent single electron reductant that could readily reduce an N-hydroxyphthalimide ester II, leading to a transient alkyl radical VI upon mesolytic cleavage and extrusion of CO2. Based on our work on pyridoxal radical biocatalysis, we propose that this photochemically formed radical species (VI) could travel into the active site of the TPP enzyme and engage the TPP-bound radical V, allowing for stereoselective C-C bond formation through radical-radical cross-coupling. Finally, the tetrahedral intermediate VII would collapse to release the enantioenriched abranched ketone product VIII and regenerate the TPP thiazolidene species (III), thereby completing the catalytic cycles. Importantly, the key TPP-bound radical species V has been previously characterized by EPR spectroscopy and X-ray crystallography in natural TPP radical enzyme PFOR. The persistent nature of this key radical intermediate further supports the feasibility of our proposed radical-radical cross-coupling.