S-nitrosylation is the formation of S -nitrosothiol (SNO) by covalent addition of a nitric oxide (NO) moiety (formally as NO + ) to thiol group of cysteine (Cys) residue(s) in the proteins. In other words, S-nitrosyaltion of a protein is posttranslational modification in which attachment of a nitric oxide group on the sulfur atom of one or more cysteine residues takes place. Proteins acquire NO groups from a pool of intracellular S-nitroglutathione (GSNO), and other small molecular mass S-nitrosothiols or from the S-nitrosylated peptides/proteins.

S -nitrosylation thus mediates significant biological effects of NO molecule. S-nitrosylation is thought to regulate a wide variety of biological responses, from activation of Ras, inhibition of JNK, ornithine decarboxylase, inhibition/activation of caspases, regulation of inflammation by iNOS through NFkB, and its suggested role in membrane trafficking and ER stress in vascular smooth muscle.

The importance of S-nitrosylation in physiology raises the possibility that dysregulated, S-nitrosylation contributes to pathophysiology. Indeed it has been reported that S-nitrosation/denitrosation can serve as a regulatory process in signal-transduction pathways.Recent data suggest that excessive NO production and protein S-nitrosylation may contribute to disease. It is even postulated that the some of the cancer chemotherapies may execute their functions via S-nitrosylation of specific cytotoxic effectors (e.g. SNO-GAPDH).

Since proteins from signal transduction pathways, metabolic pathways, cell growth and maintenance pathways, transport, cytosketeton and much more have found to be S-nitrosylated, it is likely that S-nitrosylation regulates diverse cellular processes. Reversal of S-nitrosyaltion can occur via attack by thiols such as glutathione and reduced thiredoxin to yield mixed disulfides, which could serve as intermediates in subsequent S-nitrosylation reactions.

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