Question

Maintenance of cellular redox homeostasis relies on a fresh supply of high energy electrons. What is the ultimate source of those electrons? Explain.

Answer 1

Redox homeostasis in a cell is maintained by the generation and elimination of the reactive oxygen or nitrogen species (ROS/RNS). This process happens due to the gain and loss of high energy electrons. for eg.- In ROS, species like superoxide (O₂⁻), hydroxyl radical (HO^·) and hydrogen peroxide (H₂O₂) are formed due to high energy electrons.

Formation of reactive oxygen and nitrogen species (ROS/RNS):

Exogenous sources - There are several exogenous factors that contribute to their generation:

1. Irradiation- eg- UV irradiation, X-ray or gamma-rays etc.
2. Atmospheric particles eg- dust or pollutants.
3. Biochemicals - There are several components in the junk food that contribute to ROS formation.

Endogenous sources -

1. Electron leakage- In Mitochondria, where O₂⁻ is generated by electron leakage from complex I and III of the electron-transport chain (ETC).
2. Microsomes and peroxisomes
3. membrane-associated NAD(P)H oxidase, cytochrome c oxidase, and xanthine oxidase.

Antioxidant removal by enzymatic and non-enzymatic means-

Enzymatic- There are several enzymes that play a role in enzymatic removal of the ROS/RNS species. for eg.-The catalysis of the dismutation of O₂⁻ to H₂O₂, is done by the enzyme superoxide dismutase (SOD). While, the conversion of H₂O₂ to H₂O + O₂ is carried out by the action of catalase.

Non-enzymatic- Antioxidants like glutathione (GSH) and thioredoxin play a role in this by carrying out thiol–disulfide exchange reactions.

Ultimately, the electrons generated in this process are passed onto the O₂ through the electron-transport chain (ETC) in order to generate energy in the form of ATP.