Question

All of the following starements are correct except A. reactive oxygen species (oxygen radicals) result when there is a concerted addition of four electrons at a time to \(\mathrm{O}_{2}\) B. superoxide anion \(\left(\mathrm{O}_{2}^{-}\right)\) and hydroxyl radical (.OH) are two forms of reactive oxygen. C. superoxide dismutase is a naturally occurring enzyme that protects against damage by converting \(\mathrm{O}_{2}^{-}\) to \(\mathrm{H}_{2} \mathrm{O}_{2}\) D. reactive oxygen species damage phospholipids, proteins, and nucleic acids. E. glutathione protects against \(\mathrm{H}_{2} \mathrm{O}_{2}\) by reducing it to water.

Short answer

Answer: Reactive oxygen species (oxygen radicals) result when there is a concerted addition of four electrons at a time to O₂.

Step-by-step solution

Review Statement A

Reactive oxygen species (oxygen radicals) are indeed formed when there is an addition of electrons to molecular oxygen. However, the important note here is that reactive species can form from a single, double, or triple electron addition, not from the concerted addition of four electrons at a time. Therefore, statement A is incorrect.

Review Statement B

Superoxide anion \(\left(\mathrm{O}_{2}^{-}\right)\) and hydroxyl radical (.OH) are reactive oxygen species, as they contain oxygen and can react with other molecules. This statement is correct.

Review Statement C

Superoxide dismutase is a naturally occurring enzyme that protects against damage caused by reactive oxygen species. Specifically, it does so by converting \(\mathrm{O}_{2}^{-}\) (superoxide anion) to \(\mathrm{H}_{2} \mathrm{O}_{2}\) (hydrogen peroxide). This statement is correct.

Review Statement D

Reactive oxygen species can indeed cause damage to biological molecules like phospholipids, proteins, and nucleic acids due to their high reactivity. This statement is correct.

Review Statement E

Glutathione is an antioxidant that protects against reactive oxygen species and their damaging effects on cells. Specifically, glutathione helps in reducing hydrogen peroxide (\(\mathrm{H}_{2} \mathrm{O}_{2}\)) to water, thereby neutralizing its harmful effects. This statement is correct. Since Statement A is incorrect while all other statements are correct, the answer is: All of the following statements are correct except (A).

Key concepts

Superoxide Dismutase

Superoxide dismutase (SOD) plays a crucial role in our body's defense system against the damaging effects of reactive oxygen species (ROS). At its core, SOD is an enzyme that mediates the transformation of the particularly reactive molecule superoxide anion (\(O_2^−\)) into less reactive species. If we were to visualize it step by step, SOD catalyzes the conversion of two molecules of the superoxide anion into molecular oxygen (\(O_2\)) and hydrogen peroxide (\(H_2O_2\)).

This process is vital because the superoxide anion, a byproduct of oxygen metabolism, is harmful to cellular components. Acting like a guardian, SOD efficiently and rapidly converts this dangerous molecule into hydrogen peroxide, which is still reactive but less harmful. Hydrogen peroxide is then further broken down into water and oxygen by other antioxidant mechanisms, notably catalase and the glutathione system, which will be discussed next.

Glutathione Antioxidant Mechanism

Imagine glutathione as a diligent janitor for our cells, cleaning up the potentially harmful substances. Glutathione, often abbreviated as GSH, is a powerful antioxidant found in almost every cell of the body. Its antioxidant mechanism revolves around its ability to donate electrons to reactive species like hydrogen peroxide (\(H_2O_2\)), thus converting them into harmless byproducts such as water (\(H_2O\)).

Glutathione peroxidase, an enzyme that contains selenium, aids in this critical conversion process. When GSH gives away an electron, it becomes oxidized to form glutathione disulfide (\(GSSG\)). Another enzyme, glutathione reductase, helps recycle \(GSSG\) back to its reduced form, using the reducing power of nicotinamide adenine dinucleotide phosphate (NADPH). This cycle allows glutathione to be ready for action again, keeping the balance within the cell and thus safeguarding it from oxidative stress.

Reactive Oxygen Species Damage

Reactive oxygen species, or ROS, are not always the villains they're often portrayed to be. In small amounts, ROS play essential roles in cell signaling and homeostasis. However, when their levels rise significantly, ROS become harmful agents that can inflict severe damage on cellular structures. This damage occurs through a process called oxidative stress, where ROS such as hydroxyl radicals (.OH) and superoxide anions (\(O_2^−\)) start attacking important biological molecules.

For example, ROS can lead to lipid peroxidation, where they damage the cell membrane's phospholipids, compromising cell integrity. They can also modify amino acids in proteins, leading to a loss of enzyme activity, and induce mutations by reacting with nucleic acids like DNA. This vast scope of damage highlights why our body relies heavily on a complex system of antioxidants, including the superoxide dismutase and glutathione mechanisms, providing a counterbalance to these potential threats.