Reactive oxygen species

ROSreactive oxygenoxygen radicalsreactive oxygen species (ROS)oxidative damageoxidative stressReactive oxygen intermediatereactive oxygen molecules(ROS)active oxygen species
Reactive oxygen species (ROS) are chemically reactive chemical species containing oxygen.wikipedia
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Hydrogen peroxide

H 2 O 2 H2O2HO
Dismutation of superoxide produces hydrogen peroxide (H 2 O 2 ):
Concentrated hydrogen peroxide, or "high-test peroxide", is a reactive oxygen species and has been used as a propellant in rocketry.

Oxidative stress

oxidative damageoxidant stressoxidative
Cumulatively, this is known as oxidative stress.
Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage.

Alpha-oxygen

Examples include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.
Alpha-oxygen (α-O) is a reactive oxygen species formed from an oxygen-atom abstraction (OAT) from nitrous oxide (N 2 O) by alpha-Iron (α-Fe) catalysts.

Superoxide

superoxide anionsuperoxide radicalsuperoxides
Examples include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. In normal conditions, the oxygen is reduced to produce water; however, in about 0.1–2% of electrons passing through the chain (this number derives from studies in isolated mitochondria, though the exact rate in live organisms is yet to be fully agreed upon), oxygen is instead prematurely and incompletely reduced to give the superoxide radical, most well documented for Complex I and Complex III.
The reactive oxygen anion superoxide is particularly important as the product of the one-electron reduction of dioxygen O 2, which occurs widely in nature.

Oxidative phosphorylation

ATP generationmitochondrial oxidative phosphorylationoxidative
The process of ATP production in the mitochondria, called oxidative phosphorylation, involves the transport of protons (hydrogen ions) across the inner mitochondrial membrane by means of the electron transport chain.
Although oxidative phosphorylation is a vital part of metabolism, it produces reactive oxygen species such as superoxide and hydrogen peroxide, which lead to propagation of free radicals, damaging cells and contributing to disease and, possibly, aging (senescence).

Catalase

catalase testcatalase-negativeCAT
Catalase, which is concentrated in peroxisomes located next to mitochondria, reacts with the hydrogen peroxide to catalyze the formation of water and oxygen.
It is a very important enzyme in protecting the cell from oxidative damage by reactive oxygen species (ROS).

Peroxisome

peroxisomesperoxisomalperoxisomal beta-oxidation pathway
Catalase, which is concentrated in peroxisomes located next to mitochondria, reacts with the hydrogen peroxide to catalyze the formation of water and oxygen.
They perform key roles in lipid metabolism and the conversion of reactive oxygen species.

Glutathione

glutathione metabolismglutathionylGSH
Glutathione peroxidase reduces hydrogen peroxide by transferring the energy of the reactive peroxides to a very small sulfur-containing protein called glutathione.
Glutathione is capable of preventing damage to important cellular components caused by reactive oxygen species such as free radicals, peroxides, lipid peroxides, and heavy metals.

Ionizing radiation

ionising radiationradiationnuclear radiation
ROS are also generated by exogenous sources such as ionizing radiation.
To some extent, visible light and also ultraviolet A (UVA) which is closest to visible energies, have been proven to result in formation of reactive oxygen species in skin, which cause indirect damage since these are electronically excited molecules which can inflict reactive damage, although they do not cause sunburn (erythema).

Antioxidant

antioxidantsanti-oxidantantioxidative
As such, they are an important antioxidant defense in nearly all cells exposed to oxygen.
A paradox in metabolism is that, while the vast majority of complex life on Earth requires oxygen for its existence, oxygen is a highly reactive molecule that damages living organisms by producing reactive oxygen species.

Electron transport chain

respiratory chainelectron transportmitochondrial respiratory chain
The process of ATP production in the mitochondria, called oxidative phosphorylation, involves the transport of protons (hydrogen ions) across the inner mitochondrial membrane by means of the electron transport chain.
A small percentage of electrons do not complete the whole series and instead directly leak to oxygen, resulting in the formation of the free-radical superoxide, a highly reactive molecule that contributes to oxidative stress and has been implicated in a number of diseases and aging.

Ultraviolet

UVultraviolet lightultraviolet radiation
However, during times of environmental stress (e.g., UV or heat exposure), ROS levels can increase dramatically.
However, sunscreen chemicals cannot dissipate the energy of the excited state as efficiently as melanin and therefore, if sunscreen ingredients penetrate into the lower layers of the skin, the amount of reactive oxygen species may be increased.

Oxygen

OO 2 molecular oxygen
In a biological context, ROS are formed as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling and homeostasis.
Reactive oxygen species, such as superoxide ion and hydrogen peroxide, are reactive by-products of oxygen use in organisms.

Superoxide dismutase

SODorgoteinSOD1
Superoxide dismutases (SOD) are a class of enzymes that catalyze the dismutation of superoxide into oxygen and hydrogen peroxide.
In higher plants, superoxide dismutase enzymes (SODs) act as antioxidants and protect cellular components from being oxidized by reactive oxygen species (ROS).

Carotenoid

carotenoidscarotinoidketo-carotenoid
Various substances such as carotenoids, tocopherols and plastoquinones contained in chloroplasts quench singlet oxygen and protect against its toxic effects.
Both light and oxygen produce damaging species during photosynthesis, with the most damaging being reactive oxygen species (ROS).

Plastoquinone

plastoquinolplastiquinone
Various substances such as carotenoids, tocopherols and plastoquinones contained in chloroplasts quench singlet oxygen and protect against its toxic effects.
Plastoquinol, the reduced form, also functions as an antioxidant by reducing reactive oxygen species, some produced from the photosynthetic reactions, that could harm the cell membrane.

Singlet oxygen

singlet 1 O 2 dioxygen molecule
Examples include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.
In mammalian biology, singlet oxygen is one of the reactive oxygen species, which is linked to oxidation of LDL cholesterol and resultant cardiovascular effects.

Radiolysis

pulse radiolysisradiolyticRadiolyse
Ionizing radiation can generate damaging intermediates through the interaction with water, a process termed radiolysis.

Lipid peroxidation

lipid oxidationperoxidationlipid peroxides
The most notable initiators in living cells are reactive oxygen species (ROS), such as OH· and HOO·, which combines with a hydrogen atom to make water and a fatty acid radical.

Vitamin C

ascorbic acidascorbateC
To cope with this natural source of ROS, the steroidogenic tissues, ovary and testsis, have a large concentration of antioxidants such as vitamin C (ascorbate) and β-carotene and anti-oxidant enzymes.
Ascorbic acid is a common enzymatic cofactor in mammals used in the synthesis of collagen, as well as a powerful reducing agent capable of rapidly scavenging a number of reactive oxygen species (ROS).

Coenzyme Q – cytochrome c reductase

Complex IIIUbiquinol Cytochrome c ReductaseCoenzyme Q - cytochrome c reductase
In normal conditions, the oxygen is reduced to produce water; however, in about 0.1–2% of electrons passing through the chain (this number derives from studies in isolated mitochondria, though the exact rate in live organisms is yet to be fully agreed upon), oxygen is instead prematurely and incompletely reduced to give the superoxide radical, most well documented for Complex I and Complex III.
The relevance of this otherwise minor side reaction is that superoxide and other reactive oxygen species are highly toxic and are thought to play a role in several pathologies, as well as aging (the free radical theory of aging).

Respiratory complex I

Complex INADH dehydrogenase (ubiquinone)mitochondrial complex I
In normal conditions, the oxygen is reduced to produce water; however, in about 0.1–2% of electrons passing through the chain (this number derives from studies in isolated mitochondria, though the exact rate in live organisms is yet to be fully agreed upon), oxygen is instead prematurely and incompletely reduced to give the superoxide radical, most well documented for Complex I and Complex III.
Recent investigations suggest that complex I is a potent source of reactive oxygen species.

Jasmonate

JasmonatesJA-Ile
Levels of jasmonate play a key role in the decision between cell acclimation or cell death in response to elevated levels of this reactive oxygen species.
JA can also induce mitochondrial death by inducing the accumulation of reactive oxygen species (ROSs).

Platelet

plateletsplatelet aggregationplatelet count
In particular, platelets involved in wound repair and blood homeostasis release ROS to recruit additional platelets to sites of injury.
Platelets are also the largest source of soluble CD40L which induces production of reactive oxygen species (ROS) and upregulate expression of adhesion molecules, such as E-selectin, ICAM-1 and VCAM-1, in neutrophils, activates macrophages and activates cytotoxic response in T and B lymphocytes.

Free-radical theory of aging

free radical theory of agingfree-radical theoryfree radical theory
According to the free radical theory of aging, oxidative damage initiated by reactive oxygen species is a major contributor to the functional decline that is characteristic of aging.
Strictly speaking, the free radical theory is only concerned with free radicals such as superoxide ( O 2 − ), but it has since been expanded to encompass oxidative damage from other reactive oxygen species such as hydrogen peroxide (H 2 O 2 ), or peroxynitrite (OONO − ).