Progress and Opportunities in Photochemical Enzymology of Oxidoreductases

Electron transfer engineering of artificially designed cell factory for complete biosynthesis of steroids

Biosynthesis of steroids by artificially designed cell factories often involves numerous nicotinamide adenine dinucleotide phosphate (NADPH)-dependent enzymes that mediate electron transfer reactions. However, the unclear mechanisms of electron transfer from regeneration to the final delivery to the NADPH-dependent active centers limit systematically engineering electron transfer to improve steroids production. Here, we elucidate the electron transfer mechanisms of NADPH-dependent enzymes for systematically engineer electron transfer of Saccharomyces cerevisiae, including step-by-step engineering the electron transfer residues of 7-Dehydrocholesterol reductase (DHCR7) and P450 sterol side chain cleaving enzyme (P450scc), electron transfer components for directing carbon flux, and NADPH regeneration pathways, for high-level production of the cholesterol (1.78 g/L) and pregnenolone (0.83 g/L). The electron transfer engineering (ETE) process makes the electron transfer chains shorter and more stable which significantly accelerates deprotonation and proton coupled electron transfer process. This study underscores the significance of ETE strategies in steroids biosynthesis and expands synthetic biology approaches.

Electrocatalytic HER Performance of [FeFe]-Hydrogenase Mimics Bearing M-salen Moieties (M=Zn, Ni, Fe, Mn)

The synthesis and characterization of novel compounds (5-8) as mimetics of [FeFe]-hydrogenase, combining two distinct systems capable of participating in hydrogen evolution reactions (HER): the [(μ-adt)Fe2(CO)6] fragment and M-salen complexes (salen=N,N'-bis(salicylidene)ethylenediamine) (M=Zn, Ni, Fe, Mn), is reported. These complexes were synthesized in high yields via a three-step procedure from N,N'-bis(4-R-salicylidene)ethanediamine) 4 [R=Fe2(CO)6(μ-SCH2)2NCOCH2O]. Structural analysis through spectroscopic, spectrometric, and computational (DFT) methods confirmed distorted tetrahedral and square-planar geometries for Zn-salen and Ni-salen complexes (5 and 6) respectively, while complexes Fe-salen 7 and Mn-salen 8 exhibit square-based pyramidal structures typical of Fe(III) and Mn(III) high-spin salen-complexes. Electrochemical studies revealed different reduction events for [(μ-adt)Fe2(CO)6] and M-salen moieties, but the electrocatalytic experiments in TFA demonstrated a clear cooperative effect between these components, especially at higher acid concentrations. Notably, the Ni-salen (6), Fe-salen (7), and Mn-salen (8) complexes exhibit significant reductions in overpotential, highlighting their potential for enhanced catalytic performance.

Advanced strategies in high-throughput droplet screening for enzyme engineering

Enzymes, as biocatalysts, play a cumulatively important role in environmental purification and industrial production of chemicals and pharmaceuticals. However, natural enzymes are limited by their physiological properties in practice, which need to be modified driven by requirements. Screening and isolating certain enzyme variants or ideal industrial strains with high yielding of target product enzymes is one of the main directions of enzyme engineering research. Droplet-based high-throughput screening (DHTS) technology employs massive monodisperse emulsion droplets as microreactors to achieve single strain encapsulation, as well as continuous monitoring for the inside mutant library. It can effectively sort out strains or enzymes with desired characteristics, offering a throughput of 108 events per hour. Much of the early literature focused on screening various engineered strains or designing signalling sorting strategies based on DHTS technology. However, the field of enzyme engineering lacks a comprehensive overview of advanced methods for microfluidic droplets and their cutting-edge developments in generation and manipulation. This review emphasizes the advanced strategies and frontiers of microfluidic droplet generation and manipulation facilitating enzyme engineering development. We also introduce design for various screening signals that cooperate with DHTS and devote to enzyme engineering.

A cross-metathesis approach for polymetallic [FeFe]-hydrogenase mimics

A method has been developed for synthesizing [FeFe]-H2ase mimics with diverse structures and properties, employing cross-metathesis of olefins. Vinylmetallocenes (5 and 6) and vinyl half-sandwich complexes (10 and 11) have been used as cross-metathesis partners with [FeFe]-H2ase mimics (4, 8, and 9) bearing a double bond in the moiety attached to the ADT-bridge nitrogen. Electrochemical studies of these complexes, encompassing metallocene-type (7a-b, 12a-b, and 13a-b) as well as half-sandwich derivatives (12c and 13c-d), have demonstrated that the introduction of a redox unit has a marginal impact on the reduction potential of these [FeFe]-H2ase mimics. The application of this cross-metathesis approach has allowed the synthesis of [FeFe]-H2ase mimics featuring an Ir(III) electrochemical antenna (16-18) as well as systems having an electron-donor-photosensitizer structure (ED-PS) (23). The electrocatalytic properties of these complexes have been elucidated through electrochemical studies.

Time-Resolved Infrared Spectroscopy Reveals the pH-Independence of the First Electron Transfer Step in the [FeFe] Hydrogenase Catalytic Cycle

[FeFe] hydrogenases are highly active catalysts for hydrogen conversion. Their active site has two components: a [4Fe-4S] electron relay covalently attached to the H₂ binding site and a diiron cluster ligated by CO, CN^-, and 2-azapropane-1,3-dithiolate (ADT) ligands. Reduction of the [4Fe-4S] site was proposed to be coupled with protonation of one of its cysteine ligands. Here, we used time-resolved infrared (TRIR) spectroscopy on the [FeFe] hydrogenase from Chlamydomonas reinhardtii (CrHydA1) containing a propane-1,3-dithiolate (PDT) ligand instead of the native ADT ligand. The PDT modification does not affect the electron transfer step to [4Fe-4S]_H but prevents the enzyme from proceeding further through the catalytic cycle. We show that the rate of the first electron transfer step is independent of the pH, supporting a simple electron transfer rather than a proton-coupled event. These results have important implications for our understanding of the catalytic mechanism of [FeFe] hydrogenases and highlight the utility of TRIR.

Efficient, Light-Driven Reduction of CO2 to CO by a Carbon Monoxide Dehydrogenase-CdSe/CdS Nanorod Photosystem

The solar conversion of CO₂ to low carbon fuels has been heralded as a potential solution to combat the rise in greenhouse gas emissions. Here we report the first light-driven activation of [NiFe] CODH II from Carboxydothermus hydrogenoformans for the reduction of CO₂ to CO. To accomplish this, a hybrid photosystem composed of CODH II and CdSe/CdS dot-in-rod nanocrystals was developed. By incorporating a low-potential redox mediator to assist electron transfer, quantum yields up to 19% and turnover frequencies of 9 s^-1 were achieved. These results represent a new standard in efficient CO₂ reduction by an enzyme-based photocatalytic systems. Furthermore, successful photoactivation of CODH II allows for future exploration into the enzyme's not fully understood mechanism.