Protein S-glutathiolation: redox-sensitive regulation of protein function

Abstract

Reversible protein S-glutathiolation has emerged as an important mechanism of post-translational modification. Under basal conditions several proteins remain adducted to glutathione, and physiological glutathiolation of proteins has been shown to regulate protein function. Enzymes that promote glutathiolation (e.g., glutathione-S-transferase-P) or those that remove glutathione from proteins (e.g., glutaredoxin) have been identified. Modification by glutathione has been shown to affect protein catalysis, ligand binding, oligomerization and protein-protein interactions. Conditions associated with oxidative or nitrosative stress, such as ischemia-reperfusion, hypertension and tachycardia increase protein glutathiolation via changes in the glutathione redox status (GSH/GSSG) or through the formation of sulfenic acid (SOH) or nitrosated (SNO) cysteine intermediates. These "activated" thiols promote reversible S-glutathiolation of key proteins involved in cell signaling, energy production, ion transport, and cell death. Hence, S-glutathiolation is ideally suited for integrating and mounting fine-tuned responses to changes in the redox state. S-glutathiolation also provides a temporary glutathione "cap" to protect protein thiols from irreversible oxidation and it could be an important mechanism of protein "encryption" to maintain proteins in a functionally silent state until they are needed during conditions of stress. Current evidence suggests that the glutathiolation-deglutathiolation cycle integrates and interacts with other post-translational mechanisms to regulate signal transduction, metabolism, inflammation, and apoptosis. This article is part of a Special Section entitled "Post-translational Modification."

Figure 1. Mechanisms of protein S-glutathiolation

Nitric oxide and reactive oxygen species activate protein or glutathionyl thiols resulting in S-nitrosocysteine (GSNO or PSNO) or sulfenylated (GSOH or PSOH) intermediates. In addition, the cellular abundance of GSSG is regulated by oxidative stress. These “activated” intermediate species react readily to form protein-glutathione adducts. Protein glutathiolation can regulate cellular function by modulating the activity of key metabolic and signaling enzymes. The glutathione adduct also forms a molecular “cap” that may protect protein thiols from advanced protein oxidation.

Figure 2. Regulation of the S-glutathiolation cycle

Steps in the glutathiolation cycle include: 1) Thiol activation: nitric oxide synthases (NOS), cellular oxidases, or the electron transport chain (ETC) generate reactive nitrogen or oxygen species that promote the formation of nitrosated or sulfenylated proteins; 2) Glutathiolation: GSH can react non-enzymatically with the activated thiols, or the glutathiolation reaction can be accelerated in the presence of GSH-loaded GSTP; 3) Thiol recovery: GSH can reduce glutathiolated proteins non-enzymatically, or the reaction can be catalyzed via glutaredoxin (GRX), sulfiredoxin (SRX), protein disulfide isomerase (PDI), or other proteins with reactive cysteines such as FXYD protein.