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Questions and Answers
Which type of metal sites in metalloenzymes are particularly vulnerable to irreversible reactions with oxygen?
What makes unsaturated lipids in membranes highly susceptible to free radical autoxidation?
Why are membranes of early organisms highly susceptible to direct reactions with oxygen?
What is a characteristic of superoxide that makes it damaging to cells?
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What type of molecules can trap peroxy radical intermediates and prevent free radical autoxidation?
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Why is oxygen more likely to react with components of the cell in membranes rather than in the aqueous environment of the cell?
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What is the primary reason why superoxide is not considered a strong oxidant?
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What was the observation that led to the discovery of the chemical reactions responsible for superoxide's toxicity?
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What is the role of labile iron-sulfur clusters in the toxicity of superoxide?
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What is the characteristic of superoxide that makes it a selective oxidant?
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What is the result of the reaction between superoxide and labile iron-sulfur clusters?
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What is the current understanding of the targets of superoxide?
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Study Notes
Vulnerability of Metalloenzymes to Oxygen
- Solvent-exposed low-potential metal sites in metalloenzymes are susceptible to irreversible reactions with O2.
- Iron-sulfur clusters are particularly labile, reacting irreversibly with superoxide, hydrogen peroxide, and O2 itself.
Vulnerability of Membranes to Oxygen
- Membranes of early organisms may have been highly susceptible to direct reactions with O2, especially if they contained unsaturated fatty acids and no lipid-soluble antioxidant molecules.
- O2 is more soluble in the nonpolar environment of the membrane interior than in the aqueous environment of the cell.
Autoxidation of Unsaturated Lipids
- Unsaturated lipids are extremely susceptible to free radical autoxidation via a radical chain mechanism.
- The presence of a chain-breaking antioxidant molecule, such as α-tocopherol, can trap the peroxy radical intermediates and prevent autoxidation.
Superoxide and Its Effects
- All living cells contain components that are highly reducing and capable of reacting rapidly with O2 to produce superoxide.
- Superoxide can react with and damage vulnerable components of the cell, including labile iron-sulfur clusters.
- Superoxide has the thermodynamic capacity to be a strong oxidant, but is generally not reactive with common components of cells such as peptides, carbohydrates, nucleic acids, or lipids.
- Labile iron-sulfur clusters in their reduced states are rapidly and irreversibly oxidized by reaction with superoxide.
- Superoxide is a selective oxidant, highly reactive with some essential sites and therefore highly toxic.
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Description
This quiz covers the reactions of metal sites in metalloenzymes and membranes with oxygen, including irreversible reactions with superoxide and hydrogen peroxide. It explores the susceptibility of early organisms' membranes to direct reactions with oxygen.
