Activation of sp3-hydridized carbon-hydrogen (C-H) bonds and respective functionalization are central and highly demanding challenges in Nature and also in the chemical industry, due to the high thermodynamic barrier, required selectivity of oxidation, and catalyst stability. The selective C-H activation has been pursued for more than 70 years in all areas of catalysis - homogeneous, heterogeneous and biological - and is considered to be a Holy Grail of chemistry. Nature solves this by the evolution of enzymes containing either iron or copper that use activated oxygen species for selective oxidation of C-H bonds. A central iron containing enzyme is the cytochrome P450 (CYP), a monooxygenase with a heme cofactor. These proteins oxidize steroids, fatty acids, and xenobiotics, and are vital for the clearance of various compounds, as well as for hormone synthesis and breakdown. Hence, CYPs have become essential biocatalysts for industrial production of critical chemicals such as pharmaceuticals and drug metabolites, steroids and antibiotics. CYPs have severe drawbacks: membrane-bound and require complex and expensive machinery for regeneration. An important class of copper containing enzymes was discovered in the past decade by the NMBU’s research team. LPMOs are involved in the oxidative depolymerization of cellulose and chitin. A common feature of CYPs and LPMOs is an active site that provides confinement bringing the oxygen species together with the C-H bond in a "closed" environment. This allows for a specificity with respect to which C-H bond of the substrate is oxidized. In CYPs, confinement results from a narrow active site buried inside a protein core. In LPMOs, this occurs when the enzyme complexes with the polymeric substrate. Key differences include LPMOs being easy to produce in large scale, small, robust, and rigid water-soluble proteins with a plethora of electron donors. CYPs are membrane bound and require a redox partner such as the cytochrome P450 reductase in combination with NADPH. Moreover, LPMOs have an active site architecture enabling a million-fold higher efficiency in their peroxygenase mechanism versus those of CYPs. This includes conserved amino acids able to “cage” and mediate the conversion of H2O2 to the copper-oxyl intermediate preventing oxidative damage to the enzyme, contrasting CYPs that lack a residue near the active site that can participate in general acid−base catalysis important for the formation of compound I in H2O2-dependent oxidation.
In the NewCat project, we explore to take advantage of creating the necessary tunnel for confinement by immobilizing LPMOs on nanomaterial scaffolds to produce nano-hybrid catalysts that can accommodate substrates of abundance and low-value, or an intermediate in a reaction chain, to yield products of high-value. This new catalytic system is expected to have a number of advantages and unique features: (i) enzymes immobilized on nanomaterial scaffolds are more robust, stable, and recoverable than free entities; (ii) reaction speed-up due to high/efficient enzyme concentration; (iii) the high oxidative power provides opportunities to design a versatile technology for valorization of several classes of natural compounds, including components of the highly available lignin; and (iv) environmentally beneficent processes are possible, such as organic and plastic polymers waste decomposition.
