Structural Biochemistry/Cell Signaling Pathways/ROS

Overview
ROS, which stands for reactive oxygen species, oxidizes DNA, lipids, and proteins. They are molecules that are highly reactive, but small. They modulate activities of oxidized targets when they are controlled tightly. Reactive oxygen species is one of many cell-signaling processes. At first it was once thought to be toxic byproducts of aerobic metabolism, but now it is known that ROS plays an important role in the complicated signaling network of cells in different kinds of organisms. ROS is generated from different kinds of sources like chloroplasts, peroxisomes, cell membrane, germin-like oxalate oxidases, and amine oxidases, etc. ROS can activate autophagy, which is one of their essential roles. They can cause cells to either survive or die. As of the moment, ROS and its involvement with autophagy are not completely understood. Autophagy's redox regulation has a focus on mitochondria's role as one of the sources of ROS as well as clearance of ROS by mitophagy.

Major ROS(reactive oxygen species)
1.	Singlet oxygen (O2) As the first excited electronic state of O2, it is formed during photosynthesis. However, singlet oxygen can cause violent damage on PSI and PSII in the photosynthetic mechanism. Singlet oxygen is more acting as a signal to activate stress-response pathways instead of acting as a toxin like other ROS.

2.	Superoxide radicals (O2•-) Superoxide radicals is moderately reactive ROS, which can protonate or donate electron to iron(3+) result with a iron(2+) product, the product will then lead to reduce hydrogen peroxide(H2O2). Photosynthetically active chloroplasts generates superoxide radicals to active genes in signaling pathways.

3.	Hydrogen peroxide (H2O2) Hydrogen peroxide is also moderately reactive with longer half-time life(1ms) than other ROS (2-4μs). Hydrogen peroxide can oxidize enzyme’s thiol groups and lead to cell death. In low concentration, hydrogen peroxide is a signal molecule that triggers tolerance to biotic/abiotic stress. In high concentration, hydrogen peroxide will lead to programmed cell death.

4.	Hydroxyl radical (OH•) Hydroxyl radical is the most reactive ROS so far. As a product from superoxide radicals and hydrogen peroxide, hydroxyl radicals can be formed at neutral pH with catalyst. Hydroxyl radical can react with protein, nucleic acid, and lipids. Just like hydrogen peroxide, high concentration of hydroxyl radical will lead to programmed cell death.

Autophagy
Autophagy, also known as autophagocytosis, is involved in degradation of cells when they are dysfunctional or unnecessary by lysosomes. Autophagy is a catabolic basic mechanism.

Incompleteness in oxygen one electron reduction forms ROS molecules that are small, highly reactive, and short-lived. Oxygen anions and free radicals are included as a ROS.

Reference
[1]Regulation of autophagy by ROS: physiology and pathology. Scherz-Shouval R, Elazar Z. Trends Biochem Sci. 2011 Jan;36(1):30-8. Epub 2010 Aug 20. Review.

[2]L.-J. Quan, B. Zhang, W.-W. Shi, H.-Y. Li, Hydrogen peroxide in plants: a versatile molecule of the reactive oxygen species network, J. Integrat. Plant Biol. 50 (2008) 2e18.

[3]Gill SS, Tuteja N,. 2010. Reactive oxygen species and antioxidant machinery in abiotic stress tolerance in crop plants. Plant Physiology and Biochemistry. 48. 909–930.

[4]S. Bhattachrjee, Reactive oxygen species and oxidative burst: roles in stress, senescence and signal transduction in plant, Curr. Sci. 89 (2005) 1113e1121.