Aspartic acid


Aspartic acid, is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the protonated –NH form under physiological conditions, while its α-carboxylic acid group is deprotonated −COO− under physiological conditions. Aspartic acid has an acidic side chain which reacts with other amino acids, enzymes and proteins in the body. Under physiological conditions in proteins the side chain usually occurs as the negatively charged aspartate form, −COO−. It is a non-essential amino acid in humans, meaning the body can synthesize it as needed. It is encoded by the codons GAU and GAC.
D-Aspartate is one of two D-amino acids commonly found in mammals.Aspartic acid#cite note-3|
In proteins aspartate sidechains are often hydrogen bonded to form asx turns or asx motifs, which frequently occur at the N-termini of alpha helices.
The L-isomer of Asp is one of the 22 proteinogenic amino acids, i.e., the building blocks of proteins. Aspartic acid, like glutamic acid, is classified as an acidic amino acid, with a pKa of 3.9, however in a peptide this is highly dependent on the local environment, and could be as high as 14. Asp is pervasive in biosynthesis. Because aspartate can be synthesized by the body it is classified as a non-essential amino acid.

Discovery

Aspartic acid was first discovered in 1827 by Auguste-Arthur Plisson and Étienne Ossian Henry by hydrolysis of asparagine, which had been isolated from asparagus juice in 1806. Their original method used lead hydroxide, but various other acids or bases are now more commonly used instead.

Forms and nomenclature

There are two forms or enantiomers of aspartic acid. The name "aspartic acid" can refer to either enantiomer or a mixture of two. Of these two forms, only one, "L-aspartic acid", is directly incorporated into proteins. The biological roles of its counterpart, "D-aspartic acid" are more limited. Where enzymatic synthesis will produce one or the other, most chemical syntheses will produce both forms, "DL-aspartic acid", known as a racemic mixture.

Synthesis

Biosynthesis

In the human body, aspartate is most frequently synthesized through the transamination of oxaloacetate. The biosynthesis of aspartate is facilitated by an aminotransferase enzyme: the transfer of an amine group from another molecule such as alanine or glutamine yields aspartate and an alpha-keto acid.
Aspartate also plays an important role in the urea cycle.

Chemical synthesis

Industrially, aspartate is produced by amination of fumarate catalyzed by L-aspartate ammonia-lyase.
Racemic aspartic acid can be synthesized from diethyl sodium phthalimidomalonate,
2NC.

Metabolism

In plants and microorganisms, aspartate is the precursor to several amino acids, including four that are essential for humans: methionine, threonine, isoleucine, and lysine. The conversion of aspartate to these other amino acids begins with reduction of aspartate to its "semialdehyde", O2CCHCH2CHO. Asparagine is derived from aspartate via transamidation:
NH2 and GC

Participation in the urea cycle

In the urea cycle, aspartate and ammonia donate amino groups leading to the formation of urea.

Other biochemical roles

Aspartate has many other biochemical roles. It is a metabolite in the urea cycle and participates in gluconeogenesis. It carries reducing equivalents in the malate-aspartate shuttle, which utilizes the ready interconversion of aspartate and oxaloacetate, which is the oxidized derivative of malic acid. Aspartate donates one nitrogen atom in the biosynthesis of inosine, the precursor to the purine bases. In addition, aspartic acid acts as a hydrogen acceptor in a chain of ATP synthase. Dietary L-aspartic acid has been shown to act as an inhibitor of Beta-glucuronidase, which serves to regulate enterohepatic circulation of bilirubin and bile acids.

Interactive pathway map

Neurotransmitter

Aspartate stimulates NMDA receptors, though not as strongly as the amino acid neurotransmitter L-glutamate does.

Applications & market

In 2014, the global market for aspartic acid was or about $117 million annually with potential areas of growth accounting for an of $8.78 billion. The three largest market segments include the U.S., Western Europe, and China. Current applications include biodegradable polymers, low calorie sweeteners, scale and corrosion inhibitors, and resins.

Superabsorbent polymers

One area of aspartic acid market growth is biodegradable superabsorbent polymers. The superabsorbent polymers market is anticipated to grow at a compound annual growth rate of 5.5% from 2014 to 2019 to reach a value of $8.78Bn globally. Around 75% of superabsorbent polymers are used in disposable diapers and an additional 20% is used for adult incontinence and feminine hygiene products. Polyaspartic acid, the polymerization product of aspartic acid, is a biodegradable substitute to polyacrylate. The polyaspartate market comprises a small fraction of the total SAP market.

Additional uses

In addition to SAP, aspartic acid has applications in the $19Bn fertilizer industry, where polyaspartate improves water retention and nitrogen uptake; the $1.1Bn concrete floor coatings market, where polyaspartic is a low VOC, low energy alternative to traditional epoxy resins; and lastly the >$5Bn scale and corrosion inhibitors market.

Dietary sources

Aspartic acid is not an essential amino acid, which means that it can be synthesized from central metabolic pathway intermediates in humans. However, aspartic acid is found in: