Probing Bis‐Fe^IV MauG: Experimental Evidence for the Long‐Range Charge‐Resonance Model

Abstract

The biosynthesis of tryptophan tryptophylquinone, a protein-derived cofactor, involves a longrange reaction mediated by a bis-Fe(IV) intermediate of a di-heme enzyme, MauG. Recently, a unique charge-resonance (CR) phenomenon was discovered in this intermediate, and a biological, long-distance CR model was proposed. This model suggests that the chemical nature of the bisFe(IV) species is not as simple as it appears; rather, it is composed of a collection of resonance structures in a dynamic equilibrium. Here, we experimentally evaluated the proposed CR model by introducing small molecules to, and measuring the temperature dependence of, bis-Fe(IV) MauG. Spectroscopic evidence was presented to demonstrate that the selected compounds increase the decay rate of the bis-Fe(IV) species via disrupting the equilibrium of the resonance structures that constitutes the proposed CR model. The results support this new CR model and bring a fresh concept to the classical CR theory. Keywords charge resonance; electronic structure; heme proteins; high-valence iron; near-infrared Since its first documentation by Brocklehurst and Badgers in 1968, [1] charge-resonance (CR) phenomena have been actively researched by organic chemists. [2] In a typical CR event, one-electron oxidation of an aromatic compound generates a cation radical which spontaneously associates with its neutral parent molecule or another molecule of the cation radical to form non-covalent "sandwich-like" dimeric complexes. The former scenario stabilizes an odd number of spin/charge in a mixed-valence species, (Π) 2•+ , and is classified as Type I CR; the latter one stabilizes an even number of spin/charge in a di-cation diradical, (Π 2 •+ ), and is classified as Type II CR. [3] Notably, unique electronic absorption bands in the near-infrared (NIR) region arise from resonance stabilization of spin/charge in the CR complexes and are thereby termed as CR bands (see Figure S1 for an MO-diagram illustration). [4][5][6][7] CR complexes represent the simplest intermolecular units that carry delocalized spin/charge. Investigation of these phenomena may provide the chemical basisCorrespondence to: Aimin Liu, Feradical@gsu.edu. Supporting information for this article is given via a link at the end of the document. HHS Public AccessAuthor manuscript Author ManuscriptAuthor Manuscript Author ManuscriptAuthor Manuscript for electron transfer (ET), conductivity, and ferromagnetism in many organic materials and metalloporphyrin complexes.Like many classical chemical models adopted by nature, the utilization of CR in biological systems to transiently stabilize spin/charge was first suggested in a pair of chlorophyll molecules, known as the "special pair", in bacterial photosynthetic reaction centers. [8][9] Recently, a second example was revealed from a di-heme enzyme, MauG. [3] MauG is the terminal enzyme in the biogenesis pathway of a protein-derived cofactor, tryptophan tryptophylquinone (TTQ), [10] which is the catalytic center of methylamine dehydrogenase (MADH). [1...

Capturing a bis-Fe(IV) State in Methylosinus trichosporium OB3b MbnH

“…Plotting the decay rate as a function of substrate concentration yields a hyperbolic shaped curve (Figure S3). This activation energy is significantly less than the ∼76 kJ/mol (18.6 kcal/mol) reported for MauG, consistent with the 7-fold more rapid decay of the high-valent state in MbnH back to the diferric state than in MauG (0.002 s –1 ) and 60-fold more rapid decay of the bis -Fe­(IV) state in BthA (0.014 min –1 ) . Although all three proteins share similar cofactor structures, differences in the lifetimes of the bis -Fe­(IV) state may arise from the differences in redox potentials across each system (Table S1).…”

“…22 This feature, which does not form in the absence of H 2 O 2 , is consistent with the formation of a bis-Fe(IV) state involving the two hemes and the intervening tryptophan. 15,16 To further characterize this species, we monitored the decay of this nIR feature after the addition of varying quantities of H 2 O 2 . The addition of at least 3 equivalents of H 2 O 2 is required for the development of maximum intensity, and the feature persists for tens of seconds at room temperature (Figure 2A).…”

“…This long lifetime is thought to arise from electron transfer between the porphyrin ring of the HS, low-potential heme 1 and the low-spin (LS), high-potential, and six-coordinate heme 2, mediated by a tryptophan residue between the two hemes. Heme 2 is axially ligated by tyrosine and histidine, a coordination motif proposed to be critical to accessing its Fe­(IV) oxidation state in the bis -Fe­(IV) complex. − Canonical bC c Ps feature a Met–His ligation of heme 2 and are unable to form bis -Fe­(IV) species. Instead, they use their diheme active site to reduce H 2 O 2 to water by oxidizing a substrate protein .…”