Streptavidin-Hosted Organocatalytic Aldol Addition

Secondary Amine Catalysis in Enzyme Design: Broadening Protein Template Diversity through Genetic Code Expansion

Secondary amines, due to their reactivity, can transform protein templates into catalytically active entities, accelerating the development of artificial enzymes. However, existing methods, predominantly reliant on modified ligands or N-terminal prolines, impose significant limitations on template selection. In this study, genetic code expansion was used to break this boundary, enabling secondary amines to be incorporated into alternative proteins and positions of choice. Pyrrolysine analogues carrying different secondary amines could be incorporated into superfolder green fluorescent protein (sfGFP), multidrug-binding LmrR and nucleotide-binding dihydrofolate reductase (DHFR). Notably, the analogue containing a D-proline moiety demonstrated both proteolytic stability and catalytic activity, conferring LmrR and DHFR with the desired transfer hydrogenation activity. While the LmrR variants were confined to the biomimetic 1-benzyl-1,4-dihydronicotinamide (BNAH) as the hydride source, the optimal DHFR variant favorably used the pro-R hydride from NADPH for stereoselective reactions (e.r. up to 92 : 8), highlighting that a switch of protein template could broaden the nucleophile option for catalysis. Owing to the cofactor compatibility, the DHFR-based secondary amine catalysis could be integrated into an enzymatic recycling scheme. This established method shows substantial potential in enzyme design, applicable from studies on enzyme evolution to the development of new biocatalysts.

An artificial nickel chlorinase based on the biotin-streptavidin technology

Herein, we report on an artificial nickel chlorinase (ANCase) resulting from anchoring a biotinylated nickel-based cofactor within streptavidin (Sav). The resulting ANCase was efficient for the chlorination of diverse C(sp3)-H bonds. Guided by the X-ray analysis of the ANCase, the activity of the artificial chlorinase could be significantly improved. This approach opens interesting perspectives for late-stage functionalization of organic intermediates as it complements biocatalytic chlorination strategies.

Artificial Metalloenzyme-Catalyzed Enantioselective Amidation via Nitrene Insertion in Unactivated C(sp3)-H Bonds

Enantioselective C-H amidation offers attractive means to assemble C-N bonds to synthesize high-added value, nitrogen-containing molecules. In recent decades, complementary enzymatic and homogeneous-catalytic strategies for C-H amidation have been reported. Herein, we report on an artificial metalloenzyme (ArM) resulting from anchoring a biotinylated Ir-complex within streptavidin (Sav). The resulting ArM catalyzes the enantioselective amidation of unactivated C(sp³)-H bonds. Chemogenetic optimization of the Ir cofactor and Sav led to significant improvement in both the activity and enantioselectivity. Up to >700 TON and 92% ee for the amidation of unactivated C(sp³)-H bonds was achieved. The single crystal X-ray analysis of the artificial nitrene insertase (ANIase) combined with quantum mechanics-molecular mechanics (QM-MM) calculations sheds light on critical second coordination sphere contacts leading to improved catalytic performance.

Going Full Circle with Organocatalysis and Biocatalysis: The Latent Potential of Cofactor Mimics in Asymmetric Synthesis

Many enzymes work in tandem with small molecule cofactors, which have inspired organocatalyst designs. Chemical modification of cofactor scaffolds has increased organocatalytic reactivity and reaction scope. This synopsis presents a selection of recent advances in the use of cofactors (native and mimics) in organocatalysis and biocatalysis. We aim to highlight the benefits of combining fundamental knowledge gained in both bio- and organo-catalysis for asymmetric biocatalysis.

Derivatives of Natural Organocatalytic Cofactors and Artificial Organocatalytic Cofactors as Catalysts in Enzymes

Catalytically active non-metal cofactors in enzymes carry out a variety of different reactions. The efforts to develop derivatives of naturally occurring cofactors such as flavins or pyridoxal phosphate and the advances to design new, non-natural cofactors are reviewed here. We report the status quo for enzymes harboring organocatalysts as derivatives of natural cofactors or as artificial ones and their application in the asymmetric synthesis of various compounds.

An Artificial Metalloenzyme Based on a Copper Heteroscorpionate Enables sp3 C-H Functionalization via Intramolecular Carbene Insertion

The selective functionalization of sp³ C–H bonds is a versatile tool for the diversification of organic compounds. Combining attractive features of homogeneous and enzymatic catalysts, artificial metalloenzymes offer an ideal means to selectively modify these inert motifs. Herein, we report on a copper(I) heteroscorpionate complex embedded within streptavidin that catalyzes the intramolecular insertion of a carbene into sp³ C–H bonds. Target residues for genetic optimization of the artificial metalloenzyme were identified by quantum mechanics/molecular mechanics simulations. Double-saturation mutagenesis yielded detailed insight on the contribution of individual amino acids on the activity and the selectivity of the artificial metalloenzyme. Mutagenesis at a third position afforded a set of artificial metalloenzymes that catalyze the enantio- and regioselective formation of β- and γ-lactams with high turnovers and promising enantioselectivities.

The role of streptavidin and its variants in catalysis by biotinylated secondary amines

Here, we combine the use of host screening, protein crystallography and QM/MM molecular dynamics simulations to investigate how the protein structure affects iminium catalysis by biotinylated secondary amines in a model 1,4 conjugate addition reaction. Monomeric streptavidin (M-Sav) lacks a quaternary structure and the solvent-exposed reaction site resulted in poor product conversion in the model reaction with low enantio- and regioselectivities. These parameters were much improved when the tetrameric host T-Sav was used; indeed, residues at the symmetrical subunit interface were proven to be critical for catalysis through a mutagenesis study. The use of QM/MM simulations and the asymmetric dimeric variant D-Sav revealed that both Lys121 residues which are located in the hosting and neighboring subunits play a critical role in controlling the stereoselectivity and reactivity. Lastly, the D-Sav template, though providing a lower conversion than that of the symmetric tetrameric counterpart, is likely a better starting point for future protein engineering because each surrounding residue within the asymmetric scaffold can be refined for secondary amine catalysis.

Transfer hydrogenations catalyzed by streptavidin-hosted secondary amine organocatalysts

Here, the streptavidin-biotin technology was applied to enable organocatalytic transfer hydrogenation. By introducing a biotin-tethered pyrrolidine (1) to the tetrameric streptavidin (T-Sav), the resulting hybrid catalyst was able to mediate hydride transfer from dihydro-benzylnicotinamide (BNAH) to α,β-unsaturated aldehydes. Hydrogenation of cinnamaldehyde and some of its aryl-substituted analogues was found to be nearly quantitative. Kinetic measurements revealed that the T-Sav:1 assembly possesses enzyme-like behavior, whereas isotope effect analysis, performed by QM/MM simulations, illustrated that the step of hydride transfer is at least partially rate-limiting. These results have proven the concept that T-Sav can be used to host secondary amine-catalyzed transfer hydrogenations.

Evolution of strept(avidin)-based artificial metalloenzymes in organometallic catalysis

Artificial metalloenzymes have been recently established as efficient alternatives to traditional transition metal catalysts. The presence of a secondary coordination sphere in artificial metalloenzymes makes them advantageous over transition metal catalysts, which rely essentially on their first coordination sphere to exhibit their catalytic activity. Recent developments on streptavidin- and avidin-based artificial metalloenzymes have made them highly chemically and genetically evolved for selective organometallic transformations. In this review, we discuss the chemo-genetic optimization of streptavidin- and avidin-based artificial metalloenzymes for the enhancement of their catalytic activities towards a wide range of synthetic transformations. Considering the high impact in vivo applications of artificial metalloenzymes, their catalytic efficacies to promote abiological reactions in intracellular as well as periplasmic environment are also discussed. Overall, this review can provide an insight to readers regarding the design and systematic optimization of strept(avidin)-based artificial metalloenzymes for specific reactions.