Glutamate–cysteine ligase
Glutamate–cysteine ligase | |||||||||
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Identifiers | |||||||||
ExPASy | NiceZyme view | ||||||||
KEGG | KEGG entry | ||||||||
MetaCyc | metabolic pathway | ||||||||
PRIAM | profile | ||||||||
PDB structures | RCSB PDB PDBe PDBsum | ||||||||
Gene Ontology | AmiGO / QuickGO | ||||||||
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Glutamate–cysteine ligase (GCL) EC 6.3.2.2), previously known as γ-glutamylcysteine synthetase (GCS), is the first enzyme of the cellular glutathione (GSH) biosynthetic pathway that catalyzes the chemical reaction:
L-glutamate + L-cysteine + ATP γ-glutamyl cysteine + ADP + Pi
GSH, and by extension GCL, is critical to cell survival. Nearly every eukaryotic cell, from plants to yeast to humans, expresses a form of the GCL protein for the purpose of synthesizing GSH. To further highlight the critical nature of this enzyme, genetic knockdown of GCL results in embryonic lethality.[1] Furthermore, dysregulation of GCL enzymatic function and activity is known to be involved in the vast majority of human diseases, such as diabetes, Parkinson's disease, Alzheimer's disease, COPD, HIV/AIDS, and cancer.[2][3] This typically involves impaired function leading to decreased GSH biosynthesis, reduced cellular antioxidant capacity, and the induction of oxidative stress. However, in cancer, GCL expression and activity is enhanced, which serves to both support the high level of cell proliferation and confer resistance to many chemotherapeutic agents.[4]
Function
Glutamate cysteine ligase (GCL) catalyzes the first and rate-limiting step in the production of the cellular antioxidant
GCL enzymatic activity generally dictates cellular GSH levels and GSH biosynthetic capacity. GCL enzymatic activity is influenced by numerous factors, including cellular expression of the GCL subunit proteins, access to substrates (cysteine is typically limiting in the production of γ-GC), the degree of negative feedback inhibition by GSH, and functionally relevant post-translational modifications to specific sites on the GCL subunits.[8][9][10] Given its status as the rate-limiting enzyme in GSH biosynthesis, changes in GCL activity directly equate to changes in cellular GSH biosynthetic capacity.[11] Therefore, therapeutic strategies to alter GSH production have focused on this enzyme.[12]
Regulation
In keeping with its critical importance in maintaining life, GCL is subject to a multi-level regulation of its expression, function, and activity. GCL expression is regulated at the transcriptional (transcription of the GCLC and GCLM DNA to make mRNA), posttranscriptional (the stability of the mRNA over time), translational (processing of the mRNA into protein), and posttranslational levels (involving modifications to the existing proteins).
In terms of enzyme functional regulation, GSH itself acts as a feedback inhibitor of GCL activity. Under normal physiologic substrate concentrations, the GCLC monomer alone may synthesize gamma-glutamylcysteine; however, the normal physiologic levels of GSH (estimated at around 5 mM) far exceeds the GSH Ki for GCLC,[19] suggesting that only the GCL holoenzyme is functional under baseline conditions. However, during oxidative stress or toxic insults that can result in the depletion of cellular GSH or its oxidation to glutathione disulfide (GSSG), the function of any monomeric GCLC in the cell is likely to become quite important. In support of this hypothesis, mice lacking expression of the GCLM subunit due to genetic knockdown exhibit low levels of tissue GSH (~10–20% of the normal level), which is roughly the level of the GSH Ki for monomeric GCLC.[20][21]
Structure
Animal glutamate–cysteine ligase
Animal glutamate cysteine ligase (GCL) is a
- Glutamate cysteine ligase catalytic subunit (GCLC, ~73 kDa) possesses all of substrate and cofactor binding sites and is responsible for all of the catalysis.
- Glutamate cysteine ligase modifier subunit (GCLM, ~31 kDa) has no enzymatic activity on its own but increases the catalytic efficiency of GCLC when complexed in the holoenzyme.
In the majority of cells and tissues, the expression of GCLM protein is lower than GCLC and GCLM is therefore limiting in the formation of the holoenzyme complex. Thus, the sum total of cellular GCL activity is equal to the activity of the holoenzyme + the activity of the remaining monomeric GCLC. composed of a catalytic and a modulatory subunit. The catalytic subunit is necessary and sufficient for all GCL enzymatic activity, whereas the modulatory subunit increases the catalytic efficiency of the enzyme. Mice lacking the catalytic subunit (i.e., lacking all de novo GSH synthesis) die before birth.[22] Mice lacking the modulatory subunit demonstrate no obvious phenotype, but exhibit marked decrease in GSH and increased sensitivity to toxic insults.[23][24][25]
Plant glutamate cysteine ligase
The plant glutamate cysteine ligase is a redox-sensitive
As of late 2007, six
References
- PMID 15477003.
- PMID 18601945.
- PMID 18812186.
- PMID 22138445.
- PMID 18812186.
- PMID 15981742.
- PMID 18601945.
- PMID 19914271.
- PMID 20970495.
- S2CID 7738579.
- PMID 18812186.
- PMID 38242.
- PMID 18601945.
- PMID 18812186.
- PMID 20970495.
- S2CID 7738579.
- PMID 18601945.
- PMID 18812186.
- S2CID 7738579.
- PMID 17584759.
- S2CID 7738579.
- PMID 11118286.
- PMID 12384496.
- PMID 17084875.
- PMID 17584759.
- PMID 16766527.
- PMID 17766407.
- PMID 15610346.
- PMID 23691990.
- PMID 18088327.
- MacKinnon, Charlotte M.; Carter, Philip E.; Smyth, S. Jane; Dunbar, Bryan; Fothergill, John E. (1987). "Molecular cloning of cDNA for human complement component C1s. The complete amino acid sequence". European Journal of Biochemistry. 169 (3): 547–553. PMID 3500856.
- Snoke, JE; Yanari, S; Bloch, K (1953). "Synthesis of glutathione from gamma-glutamylcysteine". The Journal of Biological Chemistry. 201 (2): 573–586. PMID 13061393.
- Mandeles, S; Block, K (1955). "Enzymatic synthesis of gamma-glutamylcysteine". The Journal of Biological Chemistry. 214 (2): 639–646. PMID 14381401.