Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 105, Issue 25, Pages 8597-8600Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0801643105
Keywords
ferryl; posttranslational modification; spectroscopy; tryptophan tryptophylquinone (TTQ); free radical
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Funding
- NIGMS NIH HHS [GM069618, GM41574, R01 GM041574, R37 GM041574, R01 GM069618] Funding Source: Medline
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High-valent iron species are powerful oxidizing agents in chemical and biological catalysis. The best characterized form of an Fe(V) equivalent described in biological systems is the combination of a b-type heme with Fe(IV)=O and a porphyrin or amino acid cation radical (termed Compound 1). This work describes an alternative natural mechanism to store two oxidizing equivalents above the ferric state for biological oxidation reactions. MauG is an enzyme that utilizes two covalently bound c-type hemes to catalyze the biosynthesis of the protein-derived cofactor tryptophan tryptophylquinone. Its natural substrate is a morohydroxylated tryptophan residue present in a 119-kDa precursor protein. An EPR-silent di-heme reaction intermediate of MauG was trapped. Mossbauer spectroscopy revealed the presence of two distinct Fe(IV) species. One is consistent with an Fe(IV)=O (ferryl) species (delta = 0.06 mm/s, Delta E-Q = 1.70 mm/s). The other is assigned to an Fe(IV) heme species with two axial ligands from protein (delta = 0.17 mmls, Delta E-Q = 2.54 mm/s), which has never before been described in nature. This bis-Fe(IV) intermediate is remarkably stable but readily reacts with its native substrate. These findings broaden our views of how proteins can stabilize a highly reactive oxidizing species and the scope of enzyme-catalyzed posttranslational modifications.
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