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Arginine

life science Maturity 11-13 Next article →

Arginine is a vital amino acid used by living things to build proteins.

Arginine-spin
Arginine-spin

It is a white, water-soluble solid. Chemically, its formula is (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. This molecule features a guanidino group attached to a standard amino acid framework. In the body, arginine is often found as a cation. This happens because at physiological pH, the carboxylic acid is deprotonated. Meanwhile, the amino and guanidino groups are protonated. Only the L-arginine enantiomer is found in nature. This specific form is encoded by several different codons, such as CGU and AGA.

To understand how the body makes arginine, we must look at the urea cycle. This is a complex series of steps involving specific enzymes. First, epithelial cells in the small intestine produce a molecule called citrulline. This citrulline is created primarily from glutamine and glutamate. The citrulline is then secreted into the bloodstream. The blood carries it to the proximal tubule cells of the kidney.

Arginine biosynthesis pathway
Arginine biosynthesis pathway

Inside these kidney cells, the citrulline is converted into arginine. The kidney then returns the arginine to the blood. This process uses two ATP equivalents for every molecule of argininosuccinate synthesized. This makes the biosynthesis of arginine an energetically costly task for the body.

Arginine is classified as a semiessential or conditionally essential amino acid. This classification depends on a person's health and developmental stage. For example, preterm infants cannot synthesize arginine internally. For these babies, the amino acid is nutritionally essential. Most healthy adults can synthesize arginine from glutamine via citrulline. However, extra dietary arginine is needed during physiological stress. This includes recovery from injuries, burns, or sepsis. It is also necessary if the small intestine or kidneys are not working well. If these organs fail, arginine becomes essential to the diet.

Different animals have different requirements for this molecule. Arginine is an essential amino acid for all birds. This is because birds do not possess a urea cycle. It is also essential for certain carnivores like cats, dogs, and ferrets. These animals have a very efficient protein catabolism process. This process produces large amounts of ammonia after a meal. The body must process this ammonia through the urea cycle. Without enough arginine, ammonia toxicity can become lethal. Fortunately, meat usually contains enough arginine to prevent this.

Betaine Arginine
Betaine Arginine

The history of arginine began in 1886. A German chemist named Ernst Schulze first isolated it. He found it in the seedlings of the yellow lupin plant. Schulze worked with his assistant, Ernst Steiger, on this discovery. He named the substance after the Greek word "arguros," which means silver. He chose this name because the arginine nitrate crystals looked silver-white. In 1897, Schulze and Ernst Winterstein determined its chemical structure. Later, in 1899, they synthesized it from ornithine and cyanamide. However, doubts about the structure remained until Sørensen provided a synthesis in 1910.

Arginine performs many critical roles in biological systems. It is a precursor for the synthesis of nitric oxide. Nitric oxide is a signaling molecule that helps regulate blood pressure. Arginine also helps with cell division and wound healing. It is necessary for the proper function of T-cells in the immune system. In proteins, arginine's side chain is amphipathic. This means it has both water-loving and water-fearing parts. The guanidinium group is highly polar and stays on the outside of proteins. This allows it to participate in hydrogen bonding and salt bridges.

Arginine-spin
Arginine-spin

Arginine can also undergo citrullination, which is important for fetal development.

Scientific research continues to explore the many effects of arginine. Some clinical trials have looked at its impact on growth hormone. For instance, intravenous arginine can stimulate growth hormone secretion. A meta-analysis also showed that L-arginine may reduce blood pressure. It showed an estimated reduction of 5.4 mmHg for systolic pressure. It also showed a reduction of 2.7 mmHg for diastolic pressure. Researchers are also studying its connection to the brain. Studies of deceased people with schizophrenia show altered arginine metabolism. These studies found different levels of agmatine and glutamate in the brain tissue. Scientists are still working to understand the full biological basis of these connections.

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File:Arginine-spin.gif
Arginine-spin.gif
File:Arginine biosynthesis pathway.png
Arginine biosynthesis pathway.png
File:Betaine Arginine.png
Betaine Arginine.png
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