Aspartic acid

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Aspartic acid (abbreviated as Asp or D; Asx or B represent either aspartic acid or asparagine) is an α-amino acid with the chemical formula HO₂CCH(NH₂)CH₂CO₂H. The carboxylate anion of aspartic acid is known as aspartate. The L-isomer of aspartate is one of the 20 proteinogenic amino acids, i.e., the building blocks of proteins. Its codons are GAU and GAC.

Aspartic acid is, together with glutamic acid, classified as an acidic amino acid with a pKa of 4.0. Aspartic acid is pervasive in biosynthesis. As with all amino acids, the location of acid protons depends on the pH of the solution and the crystallization conditions.

Role in biosynthesis of amino acids

Aspartic acid is non-essential in mammals, being produced from oxaloacetate by transamination. In plants and microorganisms, aspartic acid is the precursor to several amino acids, including four that are essential: methionine, threonine, isoleucine, and lysine. The conversion of aspartic acid to these other amino acids begins with reduction of aspartic acid to its "semialdehyde," HO₂CCH(NH₂)CH₂CHO. Asparagine is derived from aspartic acid via transamidation:

HO₂CCH(NH₂)CH₂CO₂H + GC(O)NH₂ HO₂CCH(NH₂)CH₂CONH₂ + GC(O)OH

(where GC(O)NH₂ and GC(O)OH are glutamine and glutamic acid, respectively)

Other biochemical roles

Aspartic acid is also 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 (dehydrogenated) derivative of malic acid. Aspartic acid donates one nitrogen atom in the biosynthesis of inositol, the precursor to the purine bases.

Neurotransmitter

Aspartate (the conjugate base of aspartic acid) stimulates NMDA receptors, though not as strongly as the amino acid neurotransmitter glutamate does. It serves as an excitatory neurotransmitter in the brain and is an excitotoxin .

As a neurotransmitter, aspartic acid may provide resistance to fatigue, and, thus, leads to endurance, although the evidence to support this idea is not strong.

Sources

Dietary Sources

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

Animal sources: luncheon meats, sausage meat, wild game
Vegetable sources: sprouting seeds, oat flakes, avocado, asparagus.

Chemical Synthesis

Racemic aspartic acid can be synthesized from diethyl sodium phthalimidomalonate, (C₆H₄(CO)₂NC(CO₂Et)₂).