Emerging Structural and Functional Diversity in Proteins With Dioxygen-Reactive Dinuclear Transition Metal Cofactors

Enzymes that harbor dinuclear transition-metal cofactors with histidine and carboxylate ligands enable important biochemical processes in all three domains of life. In many cases, the cofactors promote redox reactions involving a form of dioxygen. A functional paradigm for these enzymes emerged from early studies on the β subunit of class I-a ribonucleotide reductase, soluble methane monooxygenase, and stearoyl-acyl carrier protein Δ9-desaturase. It comprises 10 key features. (1) A diiron cluster held within (2) a conserved, ferritin-like four-helix bundle reacts with (3) O2 (4) from the diferrous (Fe2II/II) oxidation state. (5) A peroxo-Fe2III/III intermediate is produced, and it (6) converts to a high-valent (Fe2IV/IV or Fe2III/IV) complex, which (7) effects a one- or two-electron oxidation reaction that (8) cleaves at least one strong CH or OH bond by hydrogen atom (H) transfer (HAT). In the process, (9) the cofactor is converted to a stable, oxo- or hydroxo-bridged Fe2III/III form, which (10) must be recycled by a reducing protein back to the O2-reactive Fe2II/II state for subsequent events. More recent studies have expanded this original paradigm in each of its 10 features. (1) Ferritin-like proteins have been shown to use manganese-iron and dimanganese cofactors, and (2) O2-activating diiron clusters have been found in different protein architectures. In some cases, (3) a partially reduced dioxygen species, rather than O2 itself, reacts with the dimetal cofactor. (4) A new structural and functional subclass has been described, in which O2 is activated by a mixed-valent (Fe2II/III) cofactor via (5) a superoxo-Fe2III/III intermediate that (6) directly abstracts a H to initiate (7) a four-electron oxidation (10) requiring no additional reducing system. In other enzymes functioning in the canonical Fe2II/II manifold, (6) peroxide-level complexes themselves, rather than high-valent successors, initiate transformations, and (7) novel zero- and four-electron-oxidations that cleave (8) strong CC and NH bonds, respectively, have been described. (9) In some cases, the Fe2II/II cluster is regenerated directly, and, in others, (10) the Fe2III/III cluster may disassemble rather than being recycled in situ. Here, we summarize these recent studies, highlighting the specific structural and mechanistic variations that enable the newly recognized outcomes to expand the known repertoire of the functional class.