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chemistry

Homocysteine

Homocysteine is a chemistry 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 Homocysteine rather than just read about it. In short: Homocysteine (symbol Hcy) is a non-proteinogenic α-amino acid. It is a homologue of the amino acid cysteine, differing by an additional methylene bridge (−CH2−).

Homocysteine — main illustration
Homocysteine — illustration

Key takeaways

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

Reference excerpt

Homocysteine (symbol Hcy) is a non-proteinogenic α-amino acid. It is a homologue of the amino acid cysteine, differing by an additional methylene bridge (−CH2−). It is biosynthesized from methionine by the removal of its terminal Cε methyl group. Although the production of homocysteine is a normal part of the metabolism of methionine, an excess of homocysteine can be harmful. There are two primary ways for organisms such as humans to metabolize homocysteine: remethylation and transsulfuration. Remethylation adds a methyl group to the homocysteine molecule, converting homocysteine back into methionine. There are two known remethylation pathways. One pathway requires vitamin B9 (folate) and B12 (cobalamin), which drive the MTR (methionine synthase) and MTRR (methionine synthase reductase) enzymes. The other pathway uses TMG (trimethylglycine) to drive the BHMT (betaine-homocysteine methyltransferase) enzyme. Transsulfuration converts homocysteine to cystathionine. This pathway requires vitamin B6 to drive the CBS (cystathionine beta synthase) enzyme. Cystathionine is the immediate precursor of the amino acid cysteine, which (along with glutamate and glycine), is incorporated into the tripeptide glutathione, a major antioxidant in the human body. Homocysteine is therefore an important metabolic substrate. However, excessive levels of homocysteine can result in hyperhomocysteinemia, which is regarded as an indicator of cardiovascular disease risk. Homocysteine likely contributes to atherogenesis, which can result in ischemic injury. Therefore, hyperhomocysteinemia is a possible risk factor for coronary artery disease. Coronary artery disease occurs when an atherosclerotic plaque blocks blood flow to the coronary arteries, which supply the heart with oxygenated blood. Hyperhomocysteinemia has also been correlated with the occurrence of blood clots, heart attacks, and strokes, although it is unclear whether hyperhomocysteinemia is an independent risk factor for these conditions. Hyperhomocysteinemia has also been associated with early-term spontaneous abortions and with neural tube defects.

Biosynthesis and biochemical roles

Homocysteine is biosynthesized naturally via a multi-step process. First, methionine receives an adenosine group from ATP, a reaction catalyzed by S-adenosyl-methionine synthetase, to give S-adenosyl methionine (SAM). SAM is a widely used source of methyl radicals and a cofactor for radical SAM enzymes. Transfer of the methyl group to an acceptor molecule results in the formation of S-adenosyl-homocysteine. Hydrolysis of this thioether gives L-homocysteine. L-Homocysteine reacts with tetrahydrofolate (THF) to give L-methionine.

Biosynthesis of cysteine Mammals biosynthesize the amino acid cysteine via homocysteine. Cystathionine β-synthase catalyses the condensation of homocysteine and serine to give cystathionine. This reaction uses Pyridoxal phosphate (vitamin B6) as a cofactor. Cystathionine γ-lyase then converts this double amino acid to cysteine, ammonia, and α-ketobutyrate. Bacteria and plants rely on a different pathway to produce cysteine, relying on O-acetylserine.

Methionine salvage Homocysteine can be recycled into methionine. This process uses N5-methyl tetrahydrofolate as the methyl donor and Methylcobalamin (vitamin B12)-related enzymes. More detail on these enzymes can be found in the article for methionine synthase.

Other reactions of biochemical significance Homocysteine can cyclize to give homocysteine thiolactone, a five-membered heterocycle. Because of this "self-looping" reaction, homocysteine-containing peptides tend to cleave themselves by reactions generating oxidative stress. Homocysteine also acts as an allosteric antagonist at Dopamine D2 receptors. It has been proposed that both homocysteine and its thiolactone may have played a significant role in the appearance of life on the early Earth.

Homocysteine levels

Homocysteine levels typically are higher in men than women, and increase with age. Common levels in Western populations are 10 to 12 μmol/L, and levels of 20 μmol/L are found in populations with low B-vitamin intakes or in the elderly (e.g., Rotterdam, Framingham). It is decreased with methyl folate trapping, where it is accompanied by decreased methylmalonic acid, increased folate, and a decrease in formiminoglutamic acid. This is the opposite of MTHFR C677T mutations, which result in an increase in homocysteine.

The ranges above are provided as examples only; test results always should be interpreted using the range provided by the laboratory that produced the result.

Elevated homocysteine

… excerpt ends here. Continue reading the full article.

Illustrations

Homocysteine: Structural formula
Structural formula
Homocysteine: Ball-and-stick model
Ball-and-stick model
Homocysteine illustration
Homocysteine: Zwitterionic forms of (S)-homocysteine (left) and (R)-homocysteine (right)
Zwitterionic forms of (S)-homocysteine (left) and (R)-homocysteine (right)
Homocysteine: Two of homocysteine's main biochemical roles (homocysteine is seen in the left middle of the image). It can be synthesized from methionine and then converted back to methionine via the SAM cycle or used to create cysteine and alpha-ketobutyrate.
Two of homocysteine's main biochemical roles (homocysteine is seen in the left middle of the image). It can be synthesized from methionine and then converted back to methionine via the SAM cycle or used to create cysteine and alpha-ketobutyrate.

Worked examples

Example 1 — a first encounter with Homocysteine

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

In research
Homocysteine appears in chemistry 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 Homocysteine 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
Homocysteine is common in secondary-school and first-year university syllabi. It links to neighbouring topics Alpha-Amino acids, Excitatory amino acids, Non-proteinogenic amino acids, so understanding it makes those chapters shorter.
In everyday life
Look for Homocysteine 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 Homocysteine in 20 minutes

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

Frequently asked questions

What is Homocysteine in simple terms?

Homocysteine (symbol Hcy) is a non-proteinogenic α-amino acid. It is a homologue of the amino acid cysteine, differing by an additional methylene bridge (−CH2−).

Why does Homocysteine matter?

Because it connects several chemistry 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 Homocysteine?

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 Homocysteine.

Tags

  • Alpha-Amino acids
  • Excitatory amino acids
  • Non-proteinogenic amino acids
  • Sulfur amino acids
  • Thiols

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