page_title: Fumarate reductase (quinol)
section_title: Enzyme Mechanism
hierarchical_section_title: Fumarate reductase (quinol) / Enzyme Mechanism
caption_attribution_description: English: 1l0v labeled electron transport chain
context_page_description: Fumarate reductase (quinol) is an enzyme with systematic name succinate:quinone oxidoreductase. This enzyme catalyzes the following chemical reaction:
  fumarate + quinol succinate + quinone
  Fumarate reductase is a key enzyme induced by anaerobic growth of bacteria. By partaking in fumarate respiration, fumarate reductase performs the last step in the microbial anaerobic respiration. It is a membrane bound protein capable of oxidizing a quinone and passing the released electrons to an awaiting fumarate to be reduced. It is activated and synthesized under low oxygen conditions, when aerobic respiration cannot be performed and the cell must perform anaerobic respiration to grow. This reaction is opposite to the reaction that is catalyzed by the related complex II of the respiratory chain.
context_section_description: The reduction of fumarate in fumarate reductase is achieved via the oxidation of a quinol bound to subunit C and the resulting transfer of electrons down a chain of iron-sulfur clusters onto a waiting FAD molecule. The edge-to-edge distances between the quinol, the iron sulfur clusters, and the FAD in this enzyme do not exceed 12.5 Angstroms and can be seen on the image below. These short distances between electron receptors allow electrons to travel down the chain at a physiologically reasonable timescale. Once electrons have travelled down the iron-sulfur clusters, they pass onto the FAD molecule bound to the catalytic site of the enzyme. The final reduction of the fumarate is achieved in the active site where the asymmetrical charges from the nearby amino acids polarize the fumarate and distort its shape. Once the fumarate is no longer planar, a hydride from the bound FAD molecule in the active site attacks the double bond to reduce the fumarate. Thus, in this reaction, the fumarate serves as the terminal electron acceptor.
caption: Pathway for electron tunneling across the fumarate reductase with distances labeled in Angstroms.[1]
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