Homoarginine

Key takeaways

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

Reference excerpt

Homoarginine is an nonproteinogenic alpha-amino acid. It is structurally equivalent to a one-methylene group-higher homolog of arginine and to the guanidino derivative of lysine. L-Homoarginine is the naturally occurring enantiomer. Physiologically, homoarginine increases nitric oxide (NO) supply and betters endothelial functions in the body, with a particular correlation and effect towards cardiovascular outcome and mortality. At physiological pH, homoarginine is cationic: the guanidino group is protonated.

Occurrences Homoarginine is a growth inhibitor of Staphylococcus aureus, Escherichia Coli and Candida albicans, indicating it inhibits particular microbial growth and germination pathways. Homoarginine is assumed to be an antimetabolite of arginine. Many studies have shown that it acts as a competitive inhibitor in most cases, but there are also controversial studies showing that it is also an organ specific, non-competitive inhibitor as well. Studies have also shown that it is toxic when targeting Insecta and Rattus norvegicus. In its inhibition, is also often found in occurrences with the lungs, cervix, testis and is an inhibitor of bone and liver-specific alkaline phosphatase enzymes. This amino acid derivative is also found in occurrence with murine osteosarcoma cell proliferation. Levels of homoarginine have been found to increase during pregnancy, but more studies are underway to confirm this thoroughly.

Production Homoarginine is formed as a derivative from lysine through reactions similar to those of the urea cycle. Just as in the urea cycle, in its synthesis, ornithine is replaced by lysine. Ornithine transcarbamylase is the main enzyme for homoarginine synthesis. The production of homoarginine is based around the activity of this enzyme. Although ornithine transcarbamylase has a higher affinity to ornithine, it ends up catalyzing the transaminidation reaction of lysine as well, which starts homoarginine production. The reason it also catalyzes this reaction with lysine is because of the low substrate selectivity in the reaction. Another pathway for the production of Homoarginine includes glycine amidinotransferase (AGAT). This enzyme normally acts through the transfer of an amidino group from arginine to glycine, resulting in formation of guanidinoacetic acid, which is subsequently methylated by guanidinoacetate methyltransferase (GAMT) to form creatine. However, glycine amidinotransferase (AGAT) sometimes acts by using lysine instead of glycine in the reaction, therefore lysine becomes the acceptor of the amidino group, resulting in the production of homoarginine.

Reactions Homoarginine can increase the availability of nitric oxide, and this is the basis of many of its functions. It can serve as a substrate for NO synthase itself. It can also inhibit arginase, an enzyme that competes with NO synthase for arginine. The resulting increase in the intracellular concentration of arginine leads to increased production of NO from it by NO synthase.

Uses Homoarginine is used clinical studies, often with rats, to explore its effects on cardiovascular health by acting as an inhibitor for organ-specific reactions as well as a stimulator in some cases. A recent study was done on the topic of homoarginine related to heart failure and sudden cardiac death in haemodialysis patients. The study was done on 1255 diabetic haemodialysis patients throughout a median of 4 years of follow-up. Results showed a range of different events such as sudden cardiac death, myocardial infarction, stroke, and even death due to heart failure. The study calculations showed that the risk of sudden cardiac death had a threefold increase in the presence of per unit decrease of homoarginine. This explained the strong association of congestive heart failure and left ventricular hypertrophy with low homoarginine levels. Furthermore, this study presented evidence towards increased risk of stroke with low concentrations of homoarginine. Yet, some cases such as myocardial infarction did not show any significance towards low levels of homoarginine correlation.

References Huynh, Ngan Ngoc; Chin-Dusting, Jaye (2006). "Amino Acids, Arginase and Nitric Oxide in Vascular Health". Clinical and Experimental Pharmacology and Physiology. 33 (1–2): 1–8. doi:10.1111/j.1440-1681.2006.04316.x. PMID 16445692. S2CID 45083834. Schmitz, M; Hagemeister, H; Erbersdobler, HF (1991). "Homoarginine labeling is suitable for determination of protein absorption in miniature pigs". The Journal of Nutrition. 121 (10): 1575–80. doi:10.1093/jn/121.10.1575. PMID 1722509. Lin, C. W.; Fishman, W. H. (1972). "L-Homoarginine: an organ-specific, uncompetitive inhibitor of human liver and bone alkaline phosphohydrolases" (PDF). Journal of Biological Chemistry. 247: 3082–3087. doi:10.1016/S0021-9258(19)45215-0. Ryan, W. L.; Wells, I. C. (1964). "Homocitrulline and Homoarginine Synthesis from Lysine". Science. 144 (3622): 1122–7. Bibcode:1964Sci...144.1122R. doi:10.1126/science.144.3622.1122. PMID 14148430. S2CID 2732208. Drechsler, C.; Meinitzer, A.; Pilz, S.; Krane, V.; Tomaschitz, A.; Ritz, E.; Marz, W.; Wanner, C. (2011). "Homoarginine, heart failure, and sudden cardiac death in haemodialysis patients". European Journal of Heart Failure. 13 (8): 852–9. doi:10.1093/eurjhf/hfr056. PMC 3143829. PMID 21791541.

Illustrations

Worked examples

Example 1 — a first encounter with Homoarginine

Start with the simplest possible case. Write down what Homoarginine 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 Homoarginine 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 Homoarginine 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 Homoarginine

In research: Homoarginine 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 Homoarginine 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: Homoarginine is common in secondary-school and first-year university syllabi. It links to neighbouring topics Alpha-Amino acids, Basic amino acids, Guanidines, so understanding it makes those chapters shorter.
In everyday life: Look for Homoarginine 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.

How to study Homoarginine in 20 minutes

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

Frequently asked questions

What is Homoarginine in simple terms?

Homoarginine is an nonproteinogenic alpha-amino acid. It is structurally equivalent to a one-methylene group-higher homolog of arginine and to the guanidino derivative of lysine.

Why does Homoarginine 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 Homoarginine?

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 Homoarginine.