Aspartic acid

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 HOOCCH(NH2)CH2COOH. The carboxylate anion, salt, or ester 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. Aspartate is pervasive in biosynthesis. As with all amino acids, the presence of acid protons depends on the residue's local chemical environment and the pH of the solution.

Discovery
Aspartic acid was first discovered in 1827 by Plisson, synthesized by boiling asparagine (which had been isolated from asparagus juice in 1806) with a base.

Forms and nomenclature
The term "aspartic acid" refers to either of two forms or a mixture of two. Of these two forms, only one, " L -aspartic acid", is directly incorporated into amino acids. 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.

Role in biosynthesis of amino acids
Aspartate is non-essential in mammals, being produced from oxaloacetate by transamination. It can also be made in the Urea Cycle from Ornithine and Citrulline. 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," O2CCH(NH2)CH2CHO. Asparagine is derived from aspartate via transamidation:
 * -O2CCH(NH2)CH2CO2- +  GC(O)NH3+   O2CCH(NH2)CH2CONH3+  +  GC(O)O

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

Other biochemical roles
Aspartate 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. Aspartate donates one nitrogen atom in the biosynthesis of inosine, 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.

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, young sugarcane, and molasses from sugar beets.
 * Dietary supplements, either as aspartic acid itself or salts (such as magnesium aspartate)
 * The sweetener aspartame (NutraSweet, Equal, Canderel, etc.)

Chemical synthesis
Racemic aspartic acid can be synthesized from diethyl sodium phthalimidomalonate, (C6H4(CO)2NC(CO2Et)2). The major disadvantage of the above technique is that equimolar amounts of each enantiomer are made, the body only utilises L-amino acids. Using biotechnology it is now possible to use immobilised enzymes to create just one type of enantiomer owing to their stereospecificity. Aspartic acid is made synthetically using ammonium fumarate and aspartase from E.coli, E.coli usually breaks down the aspartic acid as a nitrogen source but using excess amounts of ammonium fumarate a reversal of the enzyme's job is possible, and so aspartic acid is made to very high yields, 98.7 mM from 1 M.