Glutamate–cysteine ligase

glutamate–cysteine ligase
Identifiers
EC number	6.3.2.2
CAS number	9023-64-7
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / EGO
Search PMC articles PubMed articles NCBI proteins

Glutamate Cysteine Ligase (GCL) (EC 6.3.2.2), previously known as gamma-glutamylcysteine synthetase (GCS), is the first enzyme of the cellular glutathione (GSH) biosynthetic pathway that catalyzes the chemical reaction:

L-glutamate + L-cysteine + ATP $\rightleftharpoons$ gamma-glutamyl cysteine + ADP + P_i

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, Alzheimers 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 glutathione (GSH), involving the ATP-dependent condensation of cysteine and glutamate to form the dipeptide gamma-glutamylcysteine (γ-GC).^[5] This peptide coupling is unique in that it occurs between the amino moiety of the cysteine and the terminal carboxylic acid of the glutamate side chain (hence the name gamma-glutamyl cysteine).^[6] This peptide bond is resistant to cleavage by cellular peptidases and requires a specialized enzyme, gamma-glutamyl transpeptidase (γGT), to metabolize γ-GC and GSH into its constituent amino acids.^[7]

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).^{[13][14][15][16]} Although baseline constitutive expression is required to maintain cell viability, expression of the GCL subunits is also inducible in response to oxidative stress, GSH depletion, and exposure to toxic chemicals, with the Nrf2, AP-1, and NF-κB transcription factors regulating the inducible and constitutive expression of both subunits ^[17][18]

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 K_i 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 K_i for monomeric GCLC.^[20][21]

Structure

Glutamate cysteine ligase is a heterodimeric holoenzyme composed of two protein subunits that are coded by independent genes located on separate chromosomes:

• 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.

As of late 2007, 6 structures have been solved for this class of enzymes, with PDB accession codes 1V4G, 1VA6, 2D32, 2D33, 2GWC, and 2GWD.

References

1. Dalton TP, et al. (2004). "Genetically altered mice to evaluate glutathione homeostasis in health and disease". Free Radic Biol Med. 37 (10): 1511–26. doi:10.1016/j.freeradbiomed.2004.06.040. PMID 15477003.
2. Lu SC (2009). "Regulation of glutathione synthesis". Mol Aspects Med. 30 (1-2): 42–59. doi:10.1016/j.mam.2008.05.005. PMC 2704241. PMID 18601945.
3. Franklin CC, et al. (2009). "Structure, function, and post-translational regulation of the catalytic and modifier subunits of glutamate cysteine ligase". Mol Aspects Med. 30 (1-2): 86–98. doi:10.1016/j.mam.2008.08.009. PMC 2714364. PMID 18812186.
4. Backos DS, et al. (2012). "The role of glutathione in brain tumor drug resistance". Biochem Pharmacol. 83 (8): 1005–12. doi:10.1016/j.bcp.2011.11.016. PMID 22138445.
5. Franklin CC, et al. (2009). "Structure, function, and post-translational regulation of the catalytic and modifier subunits of glutamate cysteine ligase". Mol Aspects Med. 30 (1-2): 86–98. doi:10.1016/j.mam.2008.08.009. PMC 2714364. PMID 18812186.
6. Njålsson R, Norgren S (2005). "Physiological and pathological aspects of GSH metabolism". Acta Paediatr. 94 (2): 132–137. doi:10.1080/08035250410025285. PMID 15981742.
7. Lu SC (2009). "Regulation of glutathione synthesis". Mol Aspects Med. 30 (1-2): 42–59. doi:10.1016/j.mam.2008.05.005. PMC 2704241. PMID 18601945.
8. Backos DS, et al. (2010). "Manipulation of cellular GSH biosynthetic capacity via TAT-mediated protein transduction of wild-type or a dominant-negative mutant of glutamate cysteine ligase alters cell sensitivity to oxidant-induced cytotoxicity". Toxicol Appl Pharmacol. 243 (1): 35–45. doi:10.1016/j.taap.2009.11.010. PMC 2819613. PMID 19914271.
9. Backos DS, et al. (2011). "Posttranslational modification and regulation of glutamate-cysteine ligase by the α,β-unsaturated aldehyde 4-hydroxy-2-nonenal". Free Radic Biol Med. 50 (1): 14–26. doi:10.1016/j.freeradbiomed.2010.10.694. PMC 3014730. PMID 20970495.
10. Backos DS, et al. (2013). "Glycation of glutamate cysteine ligase by 2-deoxy-d-ribose and its potential impact on chemoresistance in glioblastoma". Neurochem Res. 38 (9): 1838–49. doi:10.1007/s11064-013-1090-4. PMID 23743623.
11. Franklin CC, et al. (2009). "Structure, function, and post-translational regulation of the catalytic and modifier subunits of glutamate cysteine ligase". Mol Aspects Med. 30 (1-2): 86–98. doi:10.1016/j.mam.2008.08.009. PMC 2714364. PMID 18812186.
12. Griffith OW, Meister A (1979). "Potent and specific inhibition of glutathione synthesis by buthionine sulfoximine (S-n-butyl homocysteine sulfoximine)". J Biol Chem. 254 (16): 7558–60. PMID 38242.
13. Lu SC (2009). "Regulation of glutathione synthesis". Mol Aspects Med. 30 (1-2): 42–59. doi:10.1016/j.mam.2008.05.005. PMC 2704241. PMID 18601945.
14. Franklin CC, et al. (2009). "Structure, function, and post-translational regulation of the catalytic and modifier subunits of glutamate cysteine ligase". Mol Aspects Med. 30 (1-2): 86–98. doi:10.1016/j.mam.2008.08.009. PMC 2714364. PMID 18812186.
15. Backos DS, et al. (2011). "Posttranslational modification and regulation of glutamate-cysteine ligase by the α,β-unsaturated aldehyde 4-hydroxy-2-nonenal". Free Radic Biol Med. 50 (1): 14–26. doi:10.1016/j.freeradbiomed.2010.10.694. PMC 3014730. PMID 20970495.
16. Backos DS, et al. (2013). "Glycation of glutamate cysteine ligase by 2-deoxy-d-ribose and its potential impact on chemoresistance in glioblastoma". Neurochem Res. 38 (9): 1838–49. doi:10.1007/s11064-013-1090-4. PMID 23743623.
17. Lu SC (2009). "Regulation of glutathione synthesis". Mol Aspects Med. 30 (1-2): 42–59. doi:10.1016/j.mam.2008.05.005. PMC 2704241. PMID 18601945.
18. Franklin CC, et al. (2009). "Structure, function, and post-translational regulation of the catalytic and modifier subunits of glutamate cysteine ligase". Mol Aspects Med. 30 (1-2): 86–98. doi:10.1016/j.mam.2008.08.009. PMC 2714364. PMID 18812186.
19. Backos DS, et al. (2013). "Glycation of glutamate cysteine ligase by 2-deoxy-d-ribose and its potential impact on chemoresistance in glioblastoma". Neurochem Res. 38 (9): 1838–49. doi:10.1007/s11064-013-1090-4. PMID 23743623.
20. McConnachie LA, Mohar I, et al. (2007). "Glutamate cysteine ligase modifier subunit deficiency and gender as determinants of acetaminophen-induced hepatotoxicity in mice". Toxicological Sciences. 99 (2): 628–636. doi:10.1093/toxsci/kfm165. PMID 17584759.
21. Backos DS, et al. (2013). "Glycation of glutamate cysteine ligase by 2-deoxy-d-ribose and its potential impact on chemoresistance in glioblastoma". Neurochem Res. 38 (9): 1838–49. doi:10.1007/s11064-013-1090-4. PMID 23743623.

• 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. doi:10.1111/j.1432-1033.1987.tb13644.x. 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.