Bis-Fe(IV): NATURE'S SNIPER FOR LONG-RANGE SPECIFIC PROTEIN OXIDATION
Aimin Liu, Ph.D., Georgia Research Alliance Distinguished Scholar
Professor of Chemistry and Biochemistry, Georgia State University, Atlanta, Georgia, USA
In recent years, it has come to light that many crucial biological processes are often mediated by some of the
culprits, activated oxygen species and free radicals, which have been implicated in aging and oxidative stress.
The dual roles of these reactive species as the leading cause of non-specific cell damage and as useful
catalytic intermediates in biochemical reactions necessitate that their use in biological systems should be
highly specific and strictly controlled. In particular, radical generation and long-range electron transfer as a
means to transport oxidizing equivalents is increasingly regarded as an effective strategy for biological
systems to perform demanding redox reactions in a controlled manner. One such example is the biosynthesis
of tryptophan tryptophylquinone (TTQ), the catalytic center of methylamine dehydrogenase (MADH). TTQ is
formed through posttranslational modification catalyzed by MauG (43 kDa, monomeric), an enzyme which
employs two c-type hemes to catalyze the oxidation of a tryptophan residue (βTrp108) and a monohydroxylated tryptophan residue (βTrp57-OH) of a precursor protein, preMADH (119 kDa). The MauGcatalyzed reaction is a three-step, six-electron oxidation process (Fig. 1a). Each step requires one equivalent
of H2O2 as the oxidant. We have previously shown that an unprecedented high-valence bis-Fe(IV)
intermediate of MauG orchestrates the oxidative production of TTQ.1,2 From our most recent EPR study, a
novel tryptophan-based di-radical intermediate has been captured in the substrate protein preMADH as a
result of oxidation by the bis-Fe(IV) species.3 In this presentation, I will focus on the long-range remote
enzyme catalysis mechanism of TTQ biosynthesis with particular emphases on the chemical properties of
these two critical intermediates (Fig. 1b).
In many metalloenzymes, high-valence iron intermediates have been proposed to be involved in a rich
array of oxidation and oxygenation chemistry. However, due to their transient nature, only a small portion
have been captured and characterized. Several years ago, we reported that the MauG-catalyzed reaction
proceeds via an unprecedented high-valence bis-Fe(IV) intermediate (Fig. 1c).1 Recently, we found that upon
reacting the bis-Fe(IV) species with a stoichiometric amount of preMADH, a pair of cationic radicals, [βTrp57OH]•+ and [βTrp108]•+, are generated on preMADH as the bis-Fe(IV) species returns to the di-ferric state.3
Thus the role of the Fe(IV) ions is to extract electrons from the substrate protein, generating radical species
on it. These groundbreaking studies reveal a remarkable mechanism for the long-range catalysis which was
supported by the structure and the in crystallo catalytic activity of the MauG-preMADH complex.
Fig. 1. (a) MauG-dependent TTQ biosynthesis. Posttranslational modifications of preMADH are shown in red. (b) The enzymesubstrate complex structure confirms a remote catalytic mechanism. The two target tryptophan residues of preMADH are ca. 40 Å
from Heme5C, MauG’s “active site” for binding H2O2. (c) Chemical conversion between di-ferric and bis-Fe(IV) MauG.
References:
1.
Li, Fu, Lee, Krebs, Davidson, and Liu (2008) A catalytic di-heme bis-Fe(IV) intermediate, alternative to an
Fe(IV)=O porphyrin radical, PNAS 105, 8597-8600.
2.
Geng, Dornevil, Davidson, and Liu (2013) Tryptophan-mediated charge-resonance stabilization in the bis-Fe(IV)
redox state of MauG, PNAS 110, 9639-9644.
3.
Yukl, Liu, Krzystek, Shin, Jensen, Davidson, Wilmot, and Liu (2013) Diradical intermediate within the context of
tryptophan tryptophylquinone biosynthesis, PNAS 110, 4569-4573.
Acknowledgement
This work is supported by NIH grant NIH R01GM108988, NSF grant MCB-0843537, and Georgia Research Alliance
Distinguished Scholar Program.