engineering

Glutamate–cysteine ligase

Glutamate–cysteine ligase is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Glutamate–cysteine ligase rather than just read about it. In short: 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 ⇌ {\displaystyle \rightleftharpoons } γ-glutamyl cysteine + ADP + Pi GSH, and by extension GCL, is critical to cell survival. Nearly every eukaryotic cell, from plants to ye…

Key takeaways

Glutamate–cysteine ligase belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Glutamate–cysteine ligase to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Glutamate–cysteine ligase from memory before moving on to harder problems.

Reference excerpt

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 ⇌ {\displaystyle \rightleftharpoons } 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 knockout of GCL results in embryonic lethality. 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. 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.

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). 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). 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. 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. Given its status as the rate-limiting enzyme in GSH biosynthesis, changes in GCL activity directly equate to changes in cellular GSH biosynthetic capacity. Therefore, therapeutic strategies to alter GSH production have focused on this enzyme.

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). 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 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, 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.

Structure

Animal glutamate–cysteine ligase Animal glutamate cysteine ligase (GCL) is a heterodimeric enzyme 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. 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. Mice lacking the modulatory subunit demonstrate no obvious phenotype, but exhibit marked decrease in GSH and increased sensitivity to toxic insults.

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Worked examples

Example 1 — a first encounter with Glutamate–cysteine ligase

Start with the simplest possible case. Write down what Glutamate–cysteine ligase claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Glutamate–cysteine ligase before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Glutamate–cysteine ligase ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Glutamate–cysteine ligase

In research: Glutamate–cysteine ligase appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Glutamate–cysteine ligase in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Glutamate–cysteine ligase is common in secondary-school and first-year university syllabi. It links to neighbouring topics EC 6.3.2, Enzymes of known structure, so understanding it makes those chapters shorter.
In everyday life: Look for Glutamate–cysteine ligase outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Glutamate–cysteine ligase in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Glutamate–cysteine ligase means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Glutamate–cysteine ligase out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Glutamate–cysteine ligase in simple terms?

Why does Glutamate–cysteine ligase matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Glutamate–cysteine ligase?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Glutamate–cysteine ligase.