sulfide oxidation, using sulfide:quinone oxidoreductase

Definition

Target type: biologicalprocess

A sulfide oxidation process that proceeds via the reaction catalyzed by sulfide:quinone oxidoreductase. [MetaCyc:P222-PWY]

Sulfide oxidation is a crucial process in microbial metabolism, particularly in environments rich in sulfide, such as hydrothermal vents and anaerobic sediments. The enzyme sulfide:quinone oxidoreductase (SQR) plays a central role in this process, catalyzing the oxidation of sulfide (H2S) to elemental sulfur (S0).

SQR is a membrane-bound enzyme that is typically found in the periplasmic space of bacteria. The enzyme consists of two subunits: a soluble cytochrome c subunit (SQR-c) and a membrane-bound subunit (SQR-m). SQR-c is responsible for transferring electrons from sulfide to the quinone pool in the membrane, while SQR-m is responsible for binding and oxidizing sulfide.

The oxidation of sulfide by SQR occurs in a two-step process. First, sulfide binds to the active site of SQR-m, where it is oxidized to elemental sulfur. The electrons released from this oxidation are transferred to a heme group in SQR-c, which then transfers them to the quinone pool in the membrane. The quinone pool is a network of electron carriers that are essential for cellular respiration.

The reaction catalyzed by SQR is as follows:

H2S + 2Q -u003e S0 + 2QH2

where Q represents a quinone molecule and QH2 represents a reduced quinone molecule.

The sulfur produced by SQR can be further oxidized to sulfite (SO32-) or sulfate (SO42-) by other enzymes in the cell. These oxidized sulfur compounds can then be used as energy sources or as building blocks for biosynthesis.

The biological significance of sulfide oxidation by SQR is threefold:

1. **Energy production:** The oxidation of sulfide by SQR generates a proton gradient across the cell membrane. This proton gradient can be used by ATP synthase to generate ATP, the main energy currency of cells.
2. **Detoxification:** Sulfide is a toxic compound that can inhibit cellular respiration. By oxidizing sulfide to elemental sulfur, SQR detoxifies the cell from this toxic compound.
3. **Carbon fixation:** Some bacteria can use sulfide oxidation as a source of energy to fix carbon dioxide (CO2) into organic molecules. This process is known as chemolithoautotrophy.

In summary, sulfide:quinone oxidoreductase is a key enzyme in the biological oxidation of sulfide, a process that is essential for energy production, detoxification, and carbon fixation in many microorganisms.'
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