Light-Driven CO2 Reduction by Co-Cytochrome b562

Abstract

The current trend in atmospheric carbon dioxide concentrations is causing increasing concerns for its environmental impacts, and spurring the developments of sustainable methods to reduce CO2 to usable molecules. We report the light-driven CO2 reduction in water in mild conditions by artificial protein catalysts based on cytochrome b562 and incorporating cobalt protoporphyrin IX as cofactor. Incorporation into the protein scaffolds enhances the intrinsic reactivity of the cobalt porphyrin toward proton reduction and CO generation. Mutations around the binding site modulate the activity of the enzyme, pointing to the possibility of further improving catalytic activity through rational design or directed evolution.

Engineering an Oxygen‐Binding Protein for Photocatalytic CO2Reductions in Water

“…Since a low oxidation state of the metal center is typically required for the reduction, we sought to take advantage of the [Ru(bpy) 3 ] 2 + photosensitizer that has been widely used to promote the photocatalytic CO 2 to CO conversion in aqueous solution by other catalysts. [18,19,33,54,55] The photoexcitation of [Ru(bpy) 3 ] 2 + to [Ru(bpy) 3 ]* 2 + can be quenched by a reductant (ascorbate), resulting in a highly reducing Ru I with a reported reduction potential (E°') of [Ru(bpy) 3 ] 2 + /1 + being À 1.26 V vs. SHE. [54,55] We mixed Mb with an excess amount of [Ru(bpy) 3 ] 2 + photosensitizer and sodium ascorbate, a sacrificial reductant, and surveyed the headspace for CO and other C 1 and C 2 short-chain hydrocarbons through GC-FID.…”

“…More importantly, the rigid 3D structure of proteins allows precise placement of multiple asymmetric non‐covalent interactions around the heme center, with minimal variation of either synthesis yield or structures. On the other hand, photocatalytic CO 2 reduction using cytochrome (cyt) b 562 to support a Cobalt(II) protoporphyrin IX (CoPPIX) cofactor has been reported, [18] but both the activity and product selectivity were moderate, probably because the work explored only the roles of primary coordination sphere in conferring and tuning the activity and selectivity, and did not take the full advantages of using protein scaffolds to modulate SCS. To address these issues, we herein report an engineered myoglobin in which the native heme is replaced with CoPPIX and demonstrate that this CoMb is capable of photocatalytically catalyzing the reduction of CO 2 to CO in the presence of [Ru(bpy) 3 ] 2+ , with up to 2000 TON and up to $${ \sim }$$ 80 % product selectivity, the highest among engineered enzymes, after optimizations by varying pH, concentrations of the enzyme and photosensitizer, as well as introducing positively charged residues (Lys or Arg) in the second sphere.…”

Rational design of photosynthetic reaction center protein maquettes

“…This is about 25 times weaker than the heme B affinity of the RC maquette, but comparable to the CoPPIX affinity of the native hemoprotein cyt. b 562 , which has a K D in the range of 8.9 µM ( Sommer et al, 2016 ) to 45 nM ( Alcala-Torano et al, 2021 ). Despite the presence of His residues at other sites in the RC maquette, none of the electron acceptors tested showed signs of binding outside the designed acceptor binding site.…”

Bioinspired and biomolecular catalysts for energy conversion and storage

“…The highest turnover numbers are for the variants that maintain the axial methionine ligand to the cobalt. This increase was proposed to be due to the soft ligand causing the metal to be more electron‐rich, stabilizing low‐valent intermediates [98]. The variability in H 2 production compared to the more consistent production of CO across the mutants is proposed to result from the mutations impacting a proton relay pathway with greater significance for H 2 production.…”

“…10) to produce an artificial protein catalyst for CO 2 reduction. The polypeptide matrix confers water solubility on the cobalt porphyrin, and provides a scaffold for mutations allowing for rational design of the primary and secondary coordination sphere [98]. This light-driven system consists of a [Ru(bpy) 3 ] 2+ photosensitizer which, when excited by light, transfers electrons to the catalyst for substrate reduction.…”