science

Transsulfuration pathway

Transsulfuration pathway is a science 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 Transsulfuration pathway rather than just read about it. In short: The transsulfuration pathway is a metabolic pathway involving the interconversion of cysteine and homocysteine through the intermediate cystathionine. Two transsulfurylation pathways are known: the forward and the reverse.

Key takeaways

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

Reference excerpt

The transsulfuration pathway is a metabolic pathway involving the interconversion of cysteine and homocysteine through the intermediate cystathionine. Two transsulfurylation pathways are known: the forward and the reverse. The forward pathway is present in several bacteria, such as Escherichia coli and Bacillus subtilis, and involves the transfer of the thiol group from cysteine to homocysteine (methionine precursor with the S-methyl group), thanks to the γ-replacement of the acetyl or succinyl group of a homoserine with cysteine via its thiol group to form cystathionine (catalysed by cystathionine γ-synthase, which is encoded by metB in E. coli and metI in B. subtilis). Cystathionine is then cleaved by means of the β-elimination of the homocysteine portion of the molecule leaving behind an unstable imino acid, which is attacked by water to form pyruvate and ammonia (catalysed by the metC-encoded cystathionine β-lyase). The production of homocysteine through transsulfuration allows the conversion of this intermediate to methionine, through a methylation reaction carried out by methionine synthase. The reverse pathway is present in several organisms, including humans, and involves the transfer of the thiol group from homocysteine to cysteine via a similar mechanism. In Klebsiella pneumoniae the cystathionine β-synthase is encoded by mtcB, while the γ-lyase is encoded by mtcC. Humans are auxotrophic for methionine, therefore it's considered to be an essential amino acid. However humans are autotrophic for cysteine due to the reverse trans-sulfurylation pathway, meaning that cysteine is not considered to be an essential amino acid. Mutations in the CBS pathway can cause a condition known as homocystinuria, an outcome of higher homocysteine levels (hyperhomocysteinemia).

Role of pyridoxal phosphate

All four transsulfuration enzymes require vitamin B6 in its active form (pyridoxal phosphate or PLP). Three of these enzymes (cystathionine γ-synthase excluded) are part of the Cys/Met metabolism PLP-dependent enzyme family (type I PLP enzymes). There are five different structurally related types of PLP enzymes. Members of this family belong to the type I and are:

in the transsulfurylation route for methionine biosynthesis: Cystathionine γ-synthase (metB) which joins an activated homoserine ester (acetyl or succinyl) with cysteine to form cystathionine Cystathionine β-lyase (metC) which splits cystathionine into homocysteine and a deaminated alanine (pyruvate and ammonia) in the direct sulfurylation pathway for methionine biosynthesis: O-acetyl homoserine sulfhydrylase (metY) which adds a thiol group to an activated homoserine ester O-succinylhomoserine sulfhydrylase (metZ) which adds a thiol group to an activated homoserine ester in the reverse transsulfurylation pathway for cysteine biosynthesis: Cystathionine γ-lyase (no common gene name) which joins an activated serine ester (acetyl or succinyl) with homocysteine to form cystathionine Not Cystathionine β-synthase which is a PLP enzyme type II cysteine biosynthesis from serine: O-acetyl serine sulfhydrylase (cysK or cysM) which adds a thiol group to an activated serine ester methionine degradation: Methionine gamma-lyase (mdeA) which breaks down methionine at the thioether and amine bounds Note: MetC, metB, metZ are closely related and have fuzzy boundaries so fall under the same NCBI orthologue cluster (COG0626).

Direct sulfurization The direct sulfurylation pathways for the synthesis of cysteine or homocysteine proceeds via the replacement of the acetyl/succinyl group with free sulfide (via the cysK or cysM -encoded cysteine synthase. and the metZ or metY -encoded homocysteine synthase,

References

Illustrations

Worked examples

Example 1 — a first encounter with Transsulfuration pathway

Start with the simplest possible case. Write down what Transsulfuration pathway claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Transsulfuration pathway 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 Transsulfuration pathway 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 Transsulfuration pathway

In research: Transsulfuration pathway appears in science 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 Transsulfuration pathway 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: Transsulfuration pathway is common in secondary-school and first-year university syllabi. It links to neighbouring topics Metabolic pathways, Nitrogen cycle, Sulfur metabolism, so understanding it makes those chapters shorter.
In everyday life: Look for Transsulfuration pathway 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 Transsulfuration pathway in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Transsulfuration pathway 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 Transsulfuration pathway out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Transsulfuration pathway in simple terms?

Why does Transsulfuration pathway matter?

Because it connects several science 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 Transsulfuration pathway?

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 Transsulfuration pathway.