Enzymes Sulfoxidize Toxic Organosulfur

Biological Strategy

Cyclocybe aegerita

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Modify Reactivity With Water

Living organisms do chemistry in water, but not all biomolecules, or parts of biomolecules, are water soluble, so organisms devise clever strategies to overcome, or leverage, this mismatch. For example, the overall size of a typical enzyme is much larger than the reactive site where the enzyme’s main function takes place. One role of the enzyme’s large outer structure is to shield the non-polar reactive site from water molecules.

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Chemically Break Down Organic Compounds

The vast majority of biochemical assembly and breakdown processes–even by the most complex organisms–occur within cells. In fact, cells are able to perform hundreds, even thousands of chemical transformations at the same time under life-friendly conditions (ambient temperature and pressure in an aqueous environment). Part of the reason that decomposition reactions (chemical breakdown) can occur under such mild conditions is because most often, they occur in a stepwise, enzyme-mediated fashion, sipping or releasing small amounts of energy at each step. For example, the breakdown of glucose to pyruvate–and the release of energy–occurs in the 10-step enzyme catalyzed process of glycolysis.

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Chemically Assemble Organic Compounds

Part of the reason that synthesis reactions (chemical assembly) can occur under such mild conditions as ambient temperature and pressure in water is because most often, they occur in a stepwise, enzyme-mediated fashion, sipping or releasing small amounts of energy at each step. For example, the synthesis of glucose from carbon dioxide in the Calvin cycle is a 15-step process, each step regulated by a different enzyme.

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Catalyze Chemical Breakdown

Life depends upon the building up and breaking down of biological molecules. Catalysts, in the form of proteins or RNA, play an important role by dramatically increasing the rate of a chemical transformation––without being consumed in the reaction. The regulatory role that catalysts play in complex biochemical cascades is one reason so many simultaneous chemical transformations can occur inside living cells in water at ambient conditions. For example, consider the 10-enzyme catalytic breakdown and transformation of glucose to pyruvate in the glycolysis metabolic pathway.

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Basidiomycetes (Club fungi)

Division Basidiomycota (“club fungi”): Mushrooms, smuts, rusts

Basidiomycota are the iconic group of organisms that comprises up to one-third of all fungi: 16 classes, 1,589 genera, and about 30,000 species. Basidiomycota is the second largest group of fungi next to Ascomycota, the sac fungi known as molds, yeasts, and powdery mildews. Basidiomycota hold a worldwide importance in food, culture, folklore, and medicine. Most club fungi reproduce through spores, while a small group can reproduce asexually. While many mushrooms are foraged and eaten by people, some can produce deadly toxins and hallucinogens. Psilocybin mushrooms produce hallucinogenic toxins which hold some potential for the treatment of mental health disorders.

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Enzymes produced by the mushroom-forming fungus Agrocybe aegerita break down toxic organosulfur compounds by catalyzing the oxidization of the sulfur-containing portion of the molecule.

Dibenzothiophene (DBT) is an organosulfur compound present in heavier fossil fuels like crude oil, coal tar, and coal. Petroleum spills and combustion of these fuels release the compound into the environment. Like many similar chemicals, DBT is toxic to life and some organisms have evolved methods for partially breaking it down into more degradable compounds. The fungus Agrocybe aegerita produces an enzyme called aromatic peroxygenase which it secretes into its external environment. This enzyme uses hydrogen peroxide to the oxidation of the sulfur-containing portion of DBY. Though the mechanism has yet to be thoroughly established, the active site on the enzyme seems to stimulate the formation of an epoxide adjacent to the sulfur which then rapidly and spontaneously hydrolyzes into hydroxy-DBTs. The products vary depending on the random way in which water reacts with the transient epoxide form but all are more easily broken down further by spontaneous or enzymatic reactions.

Last Updated August 23, 2016

References

Journal article

Conversion of dibenzothiophene by the mushrooms Agrocybe aegerita and Coprinellus radians and their extracellular peroxygenases

Appl Microbiol Biotechnol | 27/11/2008 | Elizabet Aranda, Matthias Kinne, Martin Kluge, René Ullrich, Martin Hofrichter

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