A Tailor-Made Deazaflavin-Mediated Recycling System for Artificial Nicotinamide Cofactor Biomimetics

Nicotinamide adenine dinucleotide (NAD) and its 2'-phosphorylated form NADP are crucial cofactors for a large array of biocatalytically important redox enzymes. Their high cost and relatively poor stability, however, make them less attractive electron mediators for industrial processes. Nicotinamide cofactor biomimetics (NCBs) are easily synthesized, are inexpensive, and are also generally more stable than their natural counterparts. A bottleneck for the application of these artificial hydride carriers is the lack of efficient cofactor recycling methods. Therefore, we engineered the thermostable F420:NADPH oxidoreductase from Thermobifida fusca (Tfu-FNO), by structure-inspired site-directed mutagenesis, to accommodate the unnatural N1 substituents of eight NCBs. The extraordinarily low redox potential of the natural cofactor F420H2 was then exploited to reduce these NCBs. Wild-type enzyme had detectable activity toward all selected NCBs, with K m values in the millimolar range and k cat values ranging from 0.09 to 1.4 min-1. Saturation mutagenesis at positions Gly-29 and Pro-89 resulted in mutants with up to 139 times higher catalytic efficiencies. Mutant G29W showed a k cat value of 4.2 s-1 toward 1-benzyl-3-acetylpyridine (BAP+), which is similar to the k cat value for the natural substrate NADP+. The best Tfu-FNO variants for a specific NCB were then used for the recycling of catalytic amounts of these nicotinamides in conversion experiments with the thermostable ene-reductase from Thermus scotoductus (TsOYE). We were able to fully convert 10 mM ketoisophorone with BAP+ within 16 h, using F420 or its artificial biomimetic FOP (FO-2'-phosphate) as an efficient electron mediator and glucose-6-phosphate as an electron donor. The generated toolbox of thermostable and NCB-dependent Tfu-FNO variants offers powerful cofactor regeneration biocatalysts for the reduction of several artificial nicotinamide biomimetics at both ambient and high temperatures. In fact, to our knowledge, this enzymatic method seems to be the best-performing NCB-recycling system for BNAH and BAPH thus far.

An Artificial Cofactor Catalyzing the Baylis-Hillman Reaction with Designed Streptavidin as Protein Host*

An artificial cofactor based on an organocatalyst embedded in a protein has been used to conduct the Baylis-Hillman reaction in a buffered system. As protein host, we chose streptavidin, as it can be easily crystallized and thereby supports the design process. The protein host around the cofactor was rationally designed on the basis of high-resolution crystal structures obtained after each variation of the amino acid sequence. Additionally, DFT-calculated intermediates and transition states were used to rationalize the observed activity. Finally, repeated cycles of structure determination and redesign led to a system with an up to one order of magnitude increase in activity over the bare cofactor and to the most active proteinogenic catalyst for the Baylis-Hillman reaction known today.

Design and Evaluation of Artificial Hybrid Photoredox Biocatalysts

A pair of 9-mesityl-10-phenyl acridinium (Mes-Acr⁺ ) photoredox catalysts were synthesized with an iodoacetamide handle for cysteine bioconjugation. Covalently tethering of the synthetic Mes-Acr⁺ cofactors with a small panel of thermostable protein scaffolds resulted in 12 new artificial enzymes. The unique chemical and structural environment of the protein hosts had a measurable effect on the photophysical properties and photocatalytic activity of the cofactors. The constructed Mes-Acr⁺ hybrid enzymes were found to be active photoinduced electron-transfer catalysts, controllably oxidizing a variety of aryl sulfides when irradiated with visible light, and possessed activities that correlated with the photophysical characterization data. Their catalytic performance was found to depend on multiple factors including the Mes-Acr⁺ cofactor, the protein scaffold, the location of cofactor immobilization, and the substrate. This work provides a framework toward adapting synthetic photoredox catalysts into artificial cofactors and includes important considerations for future bioengineering efforts.