A path to perpetual chemical synthesis via photocatalytic cofactor regeneration

Abstract

Harnessing the power of the Sun for perpetual chemical synthesis is one of the most sustainable ways to reduce the carbon footprint in the chemical industry. In this regard, the natural photosynthetic machinery offers key insights into the sustainable production of chemical entities in a ceaseless manner. The natural process of photosynthesis couples light harvesting to produce cofactor molecules, which then participate in enzyme-driven dark cycles for continuous biocatalytic transformations. At the core of photosynthetic machinery is the constant regeneration and consumption of cofactors, which sustain the metabolic cycles continuously. Consequently, coupling the unique powers of photocatalysis and biocatalysis through cofactor shuttling emerges as an excellent opportunity for the ceaseless production of fine chemicals. The present Perspective highlights the design principles for integrating photocatalytically regenerated cofactors with natural enzymatic cycles for various chemical transformations. Further, we examine the existing limitations of the integrated system and highlight the efforts to alleviate them. Finally, we highlight the possibilities of incorporating ideas from different research fields, from material science to synthetic biology to organometallic chemistry, to develop robust cofactor-dependent photobiocatalytic systems for the perpetual synthesis of chemicals.

Fig. 1. Mimicking natural photosynthesis with artificial light-harvesting components for the perpetual synthesis of chemicals. Schematic representation of light and dark cycles in (a) natural photosynthesis and (b) semi-artificial photosynthesis using a photocatalyst and enzyme. In natural photosynthesis, NADPH is regenerated in the light cycle, which then participates in the dark cycle (Calvin cycle) for the ceaseless production of sugars from CO₂. In semi-artificial photosynthesis, a photocatalyst is employed to regenerate NAD(P)H, which is consumed by oxidoreductase enzymes in the dark cycle. A continuous regeneration and consumption of NAD(P)H in light and dark cycles, respectively, will allow the perpetual synthesis of fine chemicals in semi-artificial systems.

Fig. 2. Molecular structures of (a) nicotinamide adenine dinucleotide cofactors and (b) products of artificial regeneration of NAD(P)⁺.

Fig. 3. Coupling artificial light-harvesting components with biological enzymes for perpetual synthesis of value-added chemicals. (a) Schematics of continuous synthesis of l-lactate from pyruvate via NADH shuttling between photocatalytic and enzymatic cycles. The NADH regenerated by carbon nitride mesoporous spheres (CNMS) in the light cycle participates in the dark cycle driven by l-lactate dehydrogenase to convert pyruvate into l-lactate. (b) Plot showing the stoichiometric formation of l-lactate from pyruvate with an initial NAD⁺: pyruvate ratio of 1 : 5, enabled by continuous shuttling of NADH cofactors. Reprinted and adapted with permission from ref. . Copyright 2014 The Royal Society of Chemistry.

Fig. 4. (a) Schematics showing the integration of a semi-artificial photosynthetic system based on InP QDs and ADH enzyme for butanol production. (b) Sequential light and dark cycles reveal the continuous production of butanol. The difference in slopes in the light and dark cycles corresponds to different rates of butanol formation in light and dark cycles, respectively. (c) Simultaneous light and dark cycles lead to butanol formation beyond the stoichiometric limit of initially added NAD⁺, confirming the in situ shuttling of NADH cofactor between multiple photocatalytic and enzymatic cycles. Adapted with permission from ref. . Copyright 2024 American Chemical Society.

Fig. 5. (a) Schematics of thylakoid membrane-inspired capsule (TMC) and cofactor shuttling module for propionaldehyde conversion. The spatial separation of photocatalytic and enzymatic centers was achieved through a porous titanium dioxide (p-TiO₂) membrane. The photocatalytic unit comprising of CdS QD and Rh-mediator was immobilized on the inner membrane of p-TiO₂, while silica-encapsulated ADH enzyme was immobilized on the outer membrane to carry out light-independent biocatalytic conversion of propionaldehyde. (b) Rate of propionaldehyde conversion in the integrated system as a function of the distance between photocatalytic and enzymatic reaction centers compared to the non-integrated system consisting of free ADH enzyme. (c) Apparent shuttling number (ASN) defining the degree of NAD⁺/NADH shuttling process as a function of distance between photocatalyst and enzyme active centers in the integrated system. A higher value of ASN for the integrated system confirms promoted cofactor shuttling in closely spaced catalytic centers. Adapted with permission from ref. . Copyright 2022 American Chemical Society.

Fig. 6. Comparison of photocatalytic systems integrated without and with Rh-M for cofactor regeneration.

Fig. 7. Integration of Rh-M with various light-harvesting components. (a) Schematic showing the photo-enzymatic conversion of CO₂ to formic acid using Rh-M integrated and FDH encapsulated NU-1006 MOF. (b) NADH regeneration by Rh-M integrated NU-1006 MOF compared with the physical mixture of NU-1006 and freely diffusing Rh-M shows an enhanced charge transfer rate. Adapted with permission from ref. . Copyright 2020 American Chemical Society. (c) Schematic representation of core–shell MOF for integration of Rh-M with the light-harvesting component. (d) Plot showing the 1,4-NADH regeneration activity for multiple cycles, which confirms negligible loss of Rh-M from the surface of core–shell MOF. Adapted with permission from ref. . Copyright 2020 American Chemical Society. (e) Schematics of the photo-enzymatic system for methanol production with Rh-M embedded in a porous TiO₂ over graphitic carbon nitride (GCN) to spatially separate it from the ADH enzyme. Such a strategy prevented aggregation-induced deactivation of the ADH enzyme. Adapted with permission from ref. . Copyright 2021 American Chemical Society. (f) Schematics showing the photoregeneration of 1,4-NADH with Rh-M modified CdS nanorods. The carboxylate groups of bipyridine moiety in Rh-M coordinate with surface Cd²⁺ and displace parent thiolate ligands. Adapted with permission from ref. . Copyright 2024 American Chemical Society.

Fig. 8. Various electron mediators for regioselective regeneration of 1,4-NADH. (a) Schematics showing 1,4-NADH regeneration using iridium-pyridine-2-sulfonamidate catalyst with phosphonic acid as the hydride source. Here, R = H, 4-CF₃, or 6-NH₂, out of which R = 6-NH₂ yields the highest TOF of 3731 h⁻¹ at 313 K. Adapted with permission from ref. . Copyright 2020 American Chemical Society. (b) Schematic showing the mechanism of NADH regeneration using [Cp*Ir(R′-pica)Cl] (pica = R′-picolinamidate, R′ = H and Me) electron mediator. A rapid equilibrium was established between 1,4-NADH and 1,6-NADH in 91 : 9 ratio. Adapted with permission from ref. . Copyright 2024 American Chemical Society. (c) Structures of earth-abundant cobaloximes complexes for 1,4-NADH regeneration in presence of eosin Y as the light-harvesting moiety. The regenerated 1,4-NADH was coupled to enzymatic dark cycle to reduce CO₂ to formic acid. Adapted with permission from ref. . Copyright 2012 American Chemical Society. (d) Selective regeneration of 1,4-BNAH, a synthetic analogue of 1,4-NADH, in presence of cobalt diamine–dioxime complexes under visible light. Adapted with permission from ref. Copyright 2020 The Royal Society of Chemistry.

Fig. 9. Compartmentalization of light and dark cycles. (a) Schematic representation of protection strategies to achieve compartmentalization in photobiocatalytic systems. Passive protection involves building a protective shell around the enzyme, whereas the light-harvesting component is isolated with a barrier in active passivation. (b) Schematics showing the functionally compartmental photocatalyst-enzyme hybrid system for formic acid production from CO₂, wherein formate dehydrogenase enzyme was encapsulated inside the MAF-7 metal–organic framework to avoid its deactivation. Adapted with permission from ref. . Copyright 2020 American Chemical Society. (c) Schematic illustration of polymer micelle-vesicle core–shell structure coupled with glucose 1-dehydrogenase. The shell comprising a hydrophilic poly(ethylene glycol) layer acts as a shield between enzymes and ROS generated during the photoexcitation process to avoid enzyme deactivation from ROS. Adapted with permission from ref. . Copyright 2022 American Chemical Society. (d) A scheme showing the photoenzymatic system in microdroplets for producing chiral alcohols. A spatial arrangement of enzymes at the air–water interface and photocatalyst within the microdroplet leads to microscale compartmentalization. Adapted with permission from ref. . Copyright 2022 The Royal Society of Chemistry. (e) Schematic illustration of Pickering droplet interface to compartmentalize photocatalytic and biocatalytic cycles. The photocatalyst and enzymes are spatially isolated in different mediums of the Pickering emulsion, which restricts their direct interaction. The presence of mesoporous silica allows the selective transfer of reaction intermediates and blocks charge transfer to enzymes to prevent their deactivation. Adapted with permission from ref. . Copyright 2024 American Chemical Society.

Fig. 10. (a) Structure of artificial transfer hydrogenase (ATHase) and its further assembly inside streptavidin. (b) Coupling of biotinylated-ATHase with alcohol dehydrogenase enzyme (ADH) for conversion of linear aminoalcohol into a cyclic amine via an imine. Adapted with permission from ref. . Copyright 2016 American Chemical Society. (c) Schematics of metal-ion catalyzed reduction of keto-acid in the presence of NADH without needing an oxidoreductase enzyme. Here, metal ions such as Al³⁺/Fe^2+/3+ mimic the role of enzymes by pre-organizing and activating the substrates. Adapted with permission from ref. . Copyright 2024 Elsevier Inc. (d) Structure of nicotinamide cofactors, oxidized (left) and reduced (right). In artificial cofactor, β-d-ribose oxygen(x) is replaced by methylene group to form 2,3-dihydroxy cyclopentane ring yielding carba-NADP/H. (e) Thermal decomposition studies clearly show the increased lifetime of artificial carbon-NADP/H at elevated temperatures compared to NADP/H. Adapted with permission from ref. . Copyright 2021 Wiley VCH.

Vanshika Jain

Pramod P. Pillai