Non-Heme iron enzymes

Chapter 5: The Role of Metals in Life

Non-Heme Iron Enzymes

• Crystal structure of soluble methane monooxygenase

• Dinuclear Non-Heme Iron Enzymes

  • Hemerythrin: Functions in O2 transport.

  • Other Examples:

  • Ribonucleotide Reductase: Involved in DNA synthesis.

  • Soluble Methane Monooxygenase (sMMO): Enzyme key to methane oxidation.

Hydrocarbon Metabolism

• Methane Oxidation: Considered the most demanding step in hydrocarbon metabolism.

  • Methane Monooxygenase (MMO)

  • Catalyzes the oxidation reaction:
    $ext{CH}<em>4 + ext{O}</em>2 + ext{NAD(P)} ext{H} + ext{H}^+ <br>ightarrow ext{CH}<em>3 ext{OH} + ext{NAD(P)}^+ + ext{H}</em>2 ext{O}$

  • Notable since Cyt. P450 can hydroxylate C-H bonds but works poorly with methane.

  • Bond Strength:

    • Methane has the strongest C-H bond among hydrocarbons, at approximately 105 kcal/mol, making it difficult to oxidize.

  • Chemical Feedstock:

    • Despite the abundance of methane in natural gas, it is not currently utilized as a chemical feedstock.

Methane Availability and Applications

• Economic Value of Methane:

  • Methane could serve as an ideal feedstock in the future, especially as oil resources deplete.

  • Notable Methane Transformations:

  • Methane can be converted into other forms like:

    • $ext{CH}_3 ext{OH}$ (Methanol)

    • Long chain alkanes such as $ext{C}<em>n ext{H}</em>{2n+2}$

• Types of Methane Monooxygenase:

  • Soluble MMO (sMMO): An iron-based enzyme.

  • Particulate MMO (pMMO): A membrane-bound copper enzyme, less understood.

  • Occurrence: Found in methanotropic bacteria that thrive between aerobic and anaerobic conditions.

  • Highly Researched Organic:

  • sMMO from Methylococcus capsulatus (Bath), identified in geothermal springs in Bath, England.

Methane Monooxygenase Mechanisms

• Functions of MMO:

  • Capable of oxidizing methane and related hydrocarbons via the previously mentioned reaction.

Proposed Mechanisms for MMO

• High-Valent Iron Model Complexes:

  • Introduced the concept of Fe(IV)=O.

  • Mononuclear Complexes:

  • Various mononuclear complexes have been characterized (Que and coworkers, Acc. Chem. Res. 2007).

  • Dinuclear Complexes:

  • Less frequent but noteworthy: e.g., $[ ext{Fe}<em>2( ext{μ-O})</em>2(5- ext{Et}<em>3- ext{TPA})</em>2]^{3+}$ representing an intermediate in the reaction.

  • Discussion on reactivity suggests that the ‘Open’ high-spin configuration is more reactive than the ‘Closed’, diamond core configuration.

Mononuclear Non-Heme Iron Enzymes

• Characteristics:

  • Encompasses a large family of enzymes that participate heavily in biosynthetic pathways.

  • Primarily activates O2 to generate reactive species necessary for catalysis:

  • Particularly, $ ext{Fe(IV)=O}$ species serves as catalytic intermediates (Solomon et al., Chem. Rev. 2000).

• a-Ketoglutarate-Dependent Hydroxylases:

  • Utilize $ ext{α-ketoglutarate (α-KG)}$ as a cofactor.

  • Generate high-spin $ ext{Fe(IV)-oxo}$ intermediates.

  • Function via a rebound mechanism to hydroxylate C-H bonds, leading to alcohol formation (R-OH).

• Biosynthetic Involvement:

  • Essential in various biosynthetic pathways, enhancing the understanding of enzymatic action.

Detoxification via Mononuclear Non-Heme Iron Enzymes

• Focus on Technical Applications:

  • Notably used in the detoxification of contaminated soil.

  • Rieske-Type Dioxygenases:

  • Catalyze the dioxygenation (cis-dihydroxylation) of non-activated aromatic compounds such as benzene, toluene, naphthalene, and benzoates.

• Key Studies on Naphthalene Dioxygenase (NDO):

  • Identified as a homotrimer comprising identical subunits arranged in a head-to-tail structure.

  • The Rieske center from one subunit exchanges electrons with the non-heme iron center of an adjacent subunit, facilitating catalytic control.

• Active Site Characteristics:

  • NDO's active site features a five-coordinate structure that binds O2 substrates efficiently.

Mechanisms of Action in NDO

• Oxygen Adduct Formation:

  • Elucidated through substrate model systems (Karlsson et al., Science 2003).

• Intra- and Extradiol Dioxygenases:

  • Both classes cleave catechols, resulting in ring-opened products.

• Mechanistic Insights:

  • Intradiol dioxygenases utilize a ferric form for substrate activation.

  • Extradiol dioxygenases rely on substrate binding to modify the coordination number of ferrous iron from six-coordinate to five-coordinate, allowing O2 access.

Biotechnological Applications

• Integration of Dioxygenases:

  • A combination of Rieske dioxygenases and intra/extradiol dioxygenases enables effective degradation of aromatic contaminants from soil, including compounds like benzene, thus promoting environmental health and remediation strategies.