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Aspartic acid

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Aspartic acid is a vital alpha-amino acid used in the biosynthesis of proteins. It serves as one of the 22 proteinogenic amino acids, which are the fundamental building blocks of proteins. In humans, it is classified as a non-essential amino acid. This means the human body can synthesize it as needed rather than requiring it from the diet. The molecule is characterized by having an amino group and a carboxylic acid. Under physiological conditions, which refer to the typical environment inside a living body, its alpha-amino group is in the protonated form. Meanwhile, its alpha-carboxylic acid group is deprotonated.

Aspartic acid-spin
Aspartic acid-spin

This molecule exists in two distinct forms known as enantiomers. The first form is L-aspartic acid, which is directly incorporated into proteins during their creation. The second form is D-aspartic acid. While L-aspartic acid builds proteins, D-aspartic acid has different roles in mammals. It is not used for protein synthesis, but it is incorporated into certain peptides. It also functions as a neurotransmitter or a neuromodulator. A neurotransmitter is a chemical that helps cells communicate. Specifically, aspartate stimulates NMDA receptors, though it is not as strong as L-glutamate. In fact, the "A" in the name NMDA stands for aspartate.

Aspartic acid has a specific chemical structure that allows it to interact with its environment. It possesses an acidic side chain consisting of CH2COOH. This side chain reacts with other amino acids, enzymes, and proteins within the body. In proteins, this side chain usually exists as the negatively charged aspartate form. Aspartic acid is classified as an acidic amino acid, similar to glutamic acid. It has a pKa of 3.9, which is a measure of its acidity. However, this value can change depending on the local environment in a peptide. In some cases, the pKa can reach as high as 14.

Scientists first discovered aspartic acid in 1827. The discovery was made by Auguste-Arthur Plisson and Étienne-Ossian Henry. They found it through the hydrolysis of asparagine. Asparagine had been previously isolated from asparagus juice in 1806. The original method used by these scientists involved lead hydroxide. Today, researchers use various other acids or bases to perform this process. The one-letter code "D" used for aspartate was assigned arbitrarily. A proposed mnemonic for this code is "asparDic" acid.

In the human body, the biosynthesis of aspartate often occurs through transamination. This process involves the molecule oxaloacetate. An enzyme called an aminotransferase facilitates this reaction. The enzyme transfers an amine group from another molecule, such as glutamine or alanine. This transfer results in the production of aspartate and an alpha-keto acid. In plants and microorganisms, aspartate acts as a precursor for other molecules. It helps create four amino acids that are essential for humans: methionine, threonine, isoleucine, and lysine. The process begins by reducing aspartate to its semialdehyde.

Aspartic acid plays many diverse roles in biochemical systems. It is a metabolite in the urea cycle and participates in gluconeogenesis. It also carries reducing equivalents in the malate-aspartate shuttle. This shuttle uses the interconversion of aspartate and oxaloacetate. Furthermore, aspartate donates a nitrogen atom during the biosynthesis of inosine. Inosine is the precursor to purine bases. Aspartic acid also acts as a hydrogen acceptor in a chain of ATP synthase. In the diet, it can act as an inhibitor of Beta-glucuronidase to regulate bile acids.

Beyond biology, aspartic acid has significant industrial and commercial value. In 2014, the global market for aspartic acid was approximately $117 million annually. The largest market segments include China, Western Europe, and the United States. It is used to create aspartame, which is a low-calorie sweetener. It is also used to manufacture biodegradable polymers known as polyaspartic acid. This is a biodegradable substitute for polyacrylate. Additionally, it is used in the fertilizer industry to help plants with nitrogen uptake and water retention.

Finally, aspartic acid is a key component in the production of superabsorbent polymers. These materials are highly effective at soaking up liquids. Approximately 75% of these polymers are used in disposable diapers. Another 20% are used for feminine hygiene products and adult incontinence products. Because roughly 1 in 20 amino acids in eukaryotic cells is aspartic acid, most dietary proteins contain it. This makes it a common part of the nutrition humans receive from food.

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