The recent increase of interest in photocatalysis spread to biocatalysis and triggered a rush for the development of light-dependent enzyme-mediated or enzyme-coupled processes. After several years of intense research on photobiocatalysis, it is time to evaluate the state of the field in a structured manner. In this Perspective, we suggest to group photobiocatalysis into distinct disciplines and provide principal guidelines and standards for the reporting of photobiocatalytic research results as well as advice on performing photobiocatalytic reactions. Overall, we assess that the field contributes to the diversity of biocatalytic reactions while offering the selectivity of enzymes to photocatalysis. We foresee that the ongoing excitement for light-dependent enzymatic processes will lead to the discovery of novel photobiocatalytic mechanisms to complement biocatalysis with new bond-forming reactions and will provide additional innovative strategies to utilize light as a possible benign energy source.

The use of neat reaction media, that is the avoidance of additional solvents, is the simplest and the most efficient approach to follow in biocatalysis. Here, we show that unspecific peroxygenase from Agrocybe aegerita (AaeUPO) can hydroxylate the neat model substrate cyclohexane. H₂O₂ was photocatalytically generated in situ by nitrogen-doped carbon nanodots (N−CNDs) and UV LED illumination. AaeUPO entrapment in alginate beads increased enzyme stability and facilitated the reaction in neat cyclohexane. N−CNDs absorption in beads containing AaeUPO created a 2-in-1 heterogeneous photobiocatalyst that was active for up to seven days under reaction conditions and produced cyclohexanol, 2.5 mM. To increase productivity, the bead size and the photocatalyst-to-enzyme ratio have been identified as promising targets for optimisation.

Peroxidases and peroxygenases are promising classes of enzymes for biocatalysis because of their ability to carry out one-electron oxidation reactions and stereoselective oxyfunctionalizations. However, industrial application is limited, as the major drawback is the sensitivity toward the required peroxide substrates. Herein, we report a novel biocatalysis approach to circumvent this shortcoming: in situ production of H₂O₂ by dielectric barrier discharge plasma. The discharge plasma can be controlled to produce hydrogen peroxide at desired rates, yielding desired concentrations. Using horseradish peroxidase, it is demonstrated that hydrogen peroxide produced by plasma treatment can drive the enzymatic oxidation of model substrates. Fungal peroxygenase is then employed to convert ethylbenzene to (R)-1-phenylethanol with an ee of >96 % using plasma-generated hydrogen peroxide. As direct treatment of the reaction solution with plasma results in reduced enzyme activity, the use of plasma-treated liquid and protection strategies are investigated to increase total turnover. Technical plasmas present a noninvasive means to drive peroxide-based biotransformations.

En route to a bio-based chemical industry, the conversion of fatty acids into building blocks is of particular interest. Enzymatic routes, occurring under mild conditions and excelling by intrinsic selectivity, are particularly attractive. Here we report photoenzymatic cascade reactions to transform unsaturated fatty acids into enantiomerically pure secondary fatty alcohols. In a first step the C=C-double bond is stereoselectively hydrated using oleate hydratases from Lactobacillus reuteri or Stenotrophomonas maltophilia. Also, dihydroxylation mediated by the 5,8-diol synthase from Aspergillus nidulans is demonstrated. The second step comprises decarboxylation of the intermediate hydroxy acids by the photoactivated decarboxylase from Chlorella variabilis NC64A. A broad range of (poly)unsaturated fatty acids can be transformed into enantiomerically pure fatty alcohols in a simple one-pot approach.

Oxidoreductases belong to the most-applied industrial enzymes. Nevertheless, they need external electrons whose supply is often costly and challenging. Recycling of the electron donors NADH or NADPH requires the use of additional enzymes and sacrificial substrates. Interestingly, several oxidoreductases accept hydrogen peroxide as electron donor. While being inexpensive, this reagent often reduces the stability of enzymes. A solution to this problem is the in situ generation of the cofactor. The continuous supply of the cofactor at low concentration drives the reaction without impairing enzyme stability. This paper demonstrates a method for the light-catalyzed in situ generation of hydrogen peroxide with the example of the heme-dependent fatty acid decarboxylase OleT_JE. The fatty acid decarboxylase OleT_JE was discovered due to its unique ability to produce long-chain 1-alkenes from fatty acids, a hitherto unknown enzymatic reaction. 1-alkenes are widely used additives for plasticizers and lubricants. OleT_JE has been shown to accept electrons from hydrogen peroxide for the oxidative decarboxylation. While addition of hydrogen peroxide damages the enzyme and results in low yields, in situ generation of the cofactor circumvents this problem. The photobiocatalytic system shows clear advantages regarding enzyme activity and yield, resulting in a simple and efficient system for fatty acid decarboxylation.

A recombinant enoate reductase was expressed in cyanobacteria and used for the light-catalyzed, enantioselective reduction of C=C bonds. The coupling of oxidoreductases to natural photosynthesis allows asymmetric syntheses fueled by the oxidation of water. Bypassing the addition of sacrificial cosubstrates as electron donors significantly improves the atom efficiency and avoids the formation of undesired side products. Crucial factors for product formation are the availability of NADPH and the amount of active enzyme in the cells. The efficiency of the reaction is comparable to typical whole-cell biotransformations in E. coli. Under optimized conditions, a solution of 100 mg prochiral 2-methylmaleimide was reduced to optically pure 2-methylsuccinimide (99 % ee, 80 % yield of isolated product). High product yields and excellent optical purities demonstrate the synthetic usefulness of light-catalyzed whole-cell biotransformations using recombinant cyanobacteria.