Light has received increased attention for various chemical reactions but also in combination with biocatalytic reactions. Because currently only a few enzymatic reactions are known, which per se require light, most transformations involving light and a biocatalyst exploit light either for providing the cosubstrate or cofactor in an appropriate redox state for the biotransformation. In selected cases, a promiscuous activity of known enzymes in the presence of light could be induced. In other approaches, light-induced chemical reactions have been combined with a biocatalytic step, or light-induced biocatalytic reactions were combined with chemical reactions in a linear cascade. Finally, enzymes with a light switchable moiety have been investigated to turn off/on or tune the actual reaction. This Review gives an overview of the various approaches for using light in biocatalysis.

Biocatalysis, using enzymes for organic synthesis, has emerged as powerful tool for the synthesis of active pharmaceutical ingredients (APIs). The first industrial biocatalytic processes launched in the first half of the last century exploited whole-cell microorganisms where the specific enzyme at work was not known. In the meantime, novel molecular biology methods, such as efficient gene sequencing and synthesis, triggered breakthroughs in directed evolution for the rapid development of process-stable enzymes with broad substrate scope and good selectivities tailored for specific substrates. To date, enzymes are employed to enable shorter, more efficient, and more sustainable alternative routes toward (established) small molecule APIs, and are additionally used to perform standard reactions in API synthesis more efficiently. Herein, large-scale synthetic routes containing biocatalytic key steps toward >130 APIs of approved drugs and drug candidates are compared with the corresponding chemical protocols (if available) regarding the steps, reaction conditions, and scale. The review is structured according to the functional group formed in the reaction.

European Union's Horizon 2020 program Marie Skłodowska-Curie (764920) for funding. We are grateful to the industrial affiliates of the Centre of Excellence for Biocatalysis (CoEBio3) for a studentship to T.M. Scheme 6. AaeUPO-catalyzed hydroxylation of 15 using Au-TiO 2 .Scheme 7. Aryl iodide and sulfinic acid salt cross-coupling model reaction used for reaction optimization.

By using structure-guided directed evolution, the substrate scope of the FeII and α-ketoglutarate dependent halogenase Wi-WelO15 from Westiella intricata HT-29-1 was engineered to enable chemo-, regio-, and diastereoselective chlorination of unactivated C­(sp3)–H bonds using NaCl as chlorine source. While FeII dependent enzymes are often oxygen sensitive, variants of this halogenase could be screened in lysates under aerobic conditions. The developed biocatalysts offer a route to mild, late-stage chlorination on milligram scale of non-natural hapalindoles containing a ketone instead of an isonitrile functionality, thereby unlocking them for preparative biocatalysis.

Podophyllotoxin is probably the most prominent representative of lignan natural products. Deoxy‐, epi ‐, and podophyllotoxin, which are all precursors to frequently used chemotherapeutic agents, were prepared by a stereodivergent biotransformation and a biocatalytic kinetic resolution of the corresponding dibenzylbutyrolactones with the same 2‐oxoglutarate‐dependent dioxygenase. The reaction can be conducted on 2 g scale, and the enzyme allows tailoring of the initial, “natural” structure and thus transforms various non‐natural derivatives. Depending on the substitution pattern, the enzyme performs an oxidative C−C bond formation by C−H activation or hydroxylation at the benzylic position prone to ring closure.

Controlling the selectivity of a chemical reaction with external stimuli is common in thermal processes, but rare in visible-light photocatalysis. Here we show that the redox potential of a carbon nitride photocatalyst (CN-OA-m) can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials. This tuning was the key to realizing photo-chemo-enzymatic cascades that give either the (S)-or the (R)-enantiomer of phenylethanol. In combination with an unspecific peroxygenase from Agrocybe aegerita, green light irradiation of CN-OA-m led to the enantioselective hydroxylation of ethylbenzene to (R)-1-phenylethanol (99 % ee). In contrast, blue light irradiation triggered the photocatalytic oxidation of ethylbenzene to acetophenone, which in turn was enantioselectively reduced with an alcohol dehydrogenase from Rhodococcus ruber to form (S)-1-phenylethanol (93 % ee).

Photobiocatalysis is an alternative approach in synthesis that has received much attention in the recent years. Due to the youth of the topic, only few reactor systems are commercially available. To allow a parallel parameter‐screening approach as often used in the optimization of biocatalytic processes, a photoreactor was developed that can illuminate up to 24 samples at well‐defined reaction conditions. The device‘s optical features and temperature regulation have been thoroughly characterized and its application was demonstrated in four examples, specifically three photobiocatalytic and one photocatalytic process: (i) Light‐dependent decarboxylation using a photodecarboxylase; (ii) Reduction of protochlorophyllide using a protochlorophyllide oxidoreductase; (iii) Photosynthetic oxygen production performed by cyanobacteria; and (iv) (−)‐Riboflavin‐catalyzed (E/Z)‐isomerization of cinnamic acid derivatives.

Biocatalysis is increasingly used in combination with light to develop new and more sustainable synthetic methods. Thereby, mostly a chemical photocatalyst harvesting the light energy is combined with an established enzymatic reaction, thus the biocatalyst itself does not require the light for its specific reaction. Here we expand the library of an enzyme which requires light for its natural reaction, namely the light‐dependent protochlorophyllide oxidoreductase (LPOR). This enzyme catalyzes the NADPH‐dependent reduction of a C=C in a N‐heterocycle. Out of five LPORs identified by sequence search, four were found to be well expressible in E. coli and active. Investigating the light intensity, which is an important parameter describing energy input and subsequently may enable fast reaction, it turned out that the four LPORs can stand the maximum light intensity reachable with the equipment used (1450 μmol photons m−2 s−1). However, the natural substrate and product were degraded at these conditions, allowing only 15 % of the maximum input (211 μmol photons m−2 s−1). Furthermore, the LPORs accepted seven different water miscible solvents with a solvent content of up to 20 % v/v and were active at a pH from 6 to 10. While all LPORs known to date are exclusively NADPH dependent, two LPORs identified here were active also with NADH. The cofactor selectivity could be pinned to three amino acid residues, which interestingly do not directly bind to the cofactor.