Emerging artificial metalloenzymes for asymmetric hydrogenation reactions

Accessing iridium Cp* as a cofactor for artificial metalloenzymes

By introducing new-to-nature transformations, artificial metalloenzymes hold great potential for expanding the biosynthetic toolbox. The chemistry of an active cofactor in these enzymes is highly dependent on how the holoprotein is assembled, potentially limiting the choice of organometallic complexes amenable to incorporation and ability of the protein structure to influence the metal centre. We have previously reported a method utilising ligand exchange as a means to introduce ruthenium-arene fragments into a four-helix bundle protein. In this work we expand the scope of this method to incorporate an iridium pentamethylcyclopentadienyl fragment into a four-helix bundle, yielding an artificial metalloenzyme with improved transfer hydrogenation properties, highlighting that understanding ligand exchange reactions is important for speciation control.

Binding a C12-appended rhenium-(Bispyridine) carbonyl complex to β-Lactoglobulin: Effects of pH & cysteine modification on calyx affinity

Due to its commercial availability and well-defined structure, the interaction between bovine protein β-lactoglobulin (βLG) and a wide variety of non-native ligands - including transition metal complexes - has been explored, but its application as an artificial metalloenzyme scaffold is limited. This protein is hypothesized to transport fatty acids and other nutrients during juvenile development, and it binds hydrophobic ligands inside a binding pocket constructed upon an 8-stranded β-barrel, called the 'calyx'. Herein, we compare the binding behavior of two rhenium(anthracene-bispyridine) ('Anth-py2') tricarbonyl complexes, one with a 12‑carbon chain appended to the ligand scaffold ('C12Anth-py2') to βLG. We investigate (i) how calyx-binding specificity is affected by pH (which controls βLG structure at the entrance to the calyx) and (ii) modification of a free cysteine residue located in a putative second binding site of βLG (SMe-βLG). The binding affinities of [Re(C12Anth-py2)(CO)3(solv)]+ (ReC12) and [Re(Anth-py2)(CO)3(solv)]+ (ReCH) for βLG at pH 7.3 were similar at 36 ± 2 μM and 43 ± 1 μM, respectively. The KD of ReC12 decreased by ∼13 μM at pH 6.1 due to a well-known conformational change (Tanford transition) at the entrance to the calyx; the KD value was not significantly affected by Cys121 modification, indicating β-barrel calyx binding specificity. In contrast, ReCH experienced a decrease in KD in response to blocking the second binding (SMe-βLG), but was also unaffected by pH. The results show an increase in binding affinity and specificity as a result of targeted ligand design and utilization of native protein characteristics. The findings will inform and improve the design of future βLG-derived ArMs.

Artificial Small Molecules as Cofactors and Biomacromolecular Building Blocks in Synthetic Biology: Design, Synthesis, Applications, and Challenges

Enzymes are essential catalysts for various chemical reactions in biological systems and often rely on metal ions or cofactors to stabilize their structure or perform functions. Improving enzyme performance has always been an important direction of protein engineering. In recent years, various artificial small molecules have been successfully used in enzyme engineering. The types of enzymatic reactions and metabolic pathways in cells can be expanded by the incorporation of these artificial small molecules either as cofactors or as building blocks of proteins and nucleic acids, which greatly promotes the development and application of biotechnology. In this review, we summarized research on artificial small molecules including biological metal cluster mimics, coenzyme analogs (mNADs), designer cofactors, non-natural nucleotides (XNAs), and non-natural amino acids (nnAAs), focusing on their design, synthesis, and applications as well as the current challenges in synthetic biology.

Characterization of the interaction of metal-protoporphyrins photosensitizers with β- lactoglobulin

We investigated the interaction of a series of metal-protoporphyrins (PPIXs) with bovine β- lactoglobulin (BLG) using a combination of optical spectroscopy and computational simulations. Unlike other studies, the simulations were not merely used to rationalize the experimental data but were employed to refine the experimental data itself. The study was carried out at two pH values, 5 and 9, where BLG is known to have different conformation dictated by the so-called Tanford transition which occurs near pH 7.5. The transition is postulated to regulate access to the interior binding cavity of the protein, thus the pH variation was used as a parameter to investigate whether PPIXs access the central cavity of BLG. The results of our study show that indeed binding increases significantly at alkaline pH, however, the increased affinity is not due to the accessibility of the central cavity. Instead, binding appears to be determined by the tendency of PPIXs to form large inhomogeneous aggregates at acidic pH which hinders interactions with proteins. The binding site determined through a combination of experimental and computational methods is located at the interface between two BLG monomers where the long α-helix segment of the protein face each other. This region is rich in positively charged Lys residues that interact with the propionic acid chains of the protoporphyrins. Establishing the modality of binding between protoporphyrins and BLG would have important consequences for the use of BLG:PPIX complexes in applications such as artificial photoreceptors, artificial metallo-enzymes, delivery of photosensitizers for phototherapy and even solar energy conversion.

Fungus-Based MnO/Porous Carbon Nanohybrid as Efficient Laccase Mimic for Oxygen Reduction Catalysis and Hydroquinone Detection

Developing efficient laccase-mimicking nanozymes via a facile and sustainable strategy is intriguing in environmental sensing and fuel cells. In our work, a MnO/porous carbon (MnO/PC) nanohybrid based on fungus was synthesized via a facile carbonization route. The nanohybrid was found to possess excellent laccase-mimicking activity using 2,2′-azinobis (3-ethylbenzthiazoline-6-sulfonic acid) diammonium salt (ABTS) as the substrate. Compared with the natural laccase and reported nanozymes, the MnO/PC nanozyme had much lower K_m value. Furthermore, the electrochemical results show that the MnO/PC nanozyme had high electrocatalytic activity toward the oxygen reduction reaction (ORR) when it was modified on the electrode. The hybrid nanozyme could catalyze the four-electron ORR, similar to natural laccase. Moreover, hydroquinone (HQ) induced the reduction of oxABTS and caused the green color to fade, which provided colorimetric detection of HQ. A desirable linear relationship (0–50 μM) and detection limit (0.5 μM) were obtained. Our work opens a simple and sustainable avenue to develop a carbon–metal hybrid nanozyme in environment and energy applications.