Aspartic acid (symbolAsp orD;[4] the ionic form is known asaspartate), is an α-amino acid that is used in the biosynthesis of proteins.[5] TheL-isomer of aspartic acid is one of the 22proteinogenic amino acids, i.e., the building blocks ofproteins.D-aspartic acid is one of twoD-amino acids commonly found in mammals.[6][7] Apart from a few rare exceptions,D-aspartic acid is not used for protein synthesis but is incorporated into somepeptides and plays a role as aneurotransmitter/neuromodulator.[6]
Like all other amino acids, aspartic acid contains an amino group and a carboxylic acid. Its α-amino group is in the protonated –NH+ 3 form under physiological conditions, while its α-carboxylic acid group is deprotonated −COO− under physiological conditions. Aspartic acid has an acidic side chain (CH2COOH) which reacts with other amino acids, enzymes and proteins in the body.[5] Under physiological conditions (pH 7.4) in proteins the side chain usually occurs as the negatively charged aspartate form, −COO−.[5] It is a non-essential amino acid in humans, meaning the body cansynthesize it as needed. It isencoded by thecodons GAU and GAC.
In proteins aspartate sidechains are often hydrogen bonded to formasx turns orasx motifs, which frequently occur at the N-termini ofalpha helices.
Aspartic acid, likeglutamic acid, is classified as an acidic amino acid, with apKa of 3.9; however, in a peptide this is highly dependent on the local environment, and could be as high as 14.
The one-letter code D for aspartate was assigned arbitrarily,[8] with the proposed mnemonic asparDic acid.[9]
There are two forms orenantiomers of aspartic acid. The name "aspartic acid" can refer to either enantiomer or a mixture of two.[13] 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 aracemic mixture.[citation needed]
In the human body, aspartate is most frequently synthesized through thetransamination ofoxaloacetate. The biosynthesis of aspartate is facilitated by anaminotransferase enzyme: the transfer of anamine group from another molecule such as alanine or glutamine yields aspartate and an alpha-keto acid.[5]
In plants andmicroorganisms, aspartate is the precursor to several amino acids, including four that are essential for humans:methionine,threonine,isoleucine, andlysine. The conversion of aspartate to these other amino acids begins with reduction of aspartate to itssemialdehyde, O2CCH(NH2)CH2CHO.[16]Asparagine is derived from aspartate via transamidation:
Aspartate has many other biochemical roles. It is ametabolite in theurea cycle[17] and participates ingluconeogenesis. It carries reducing equivalents in themalate-aspartate shuttle, which utilizes the ready interconversion of aspartate andoxaloacetate, which is the oxidized (dehydrogenated) derivative ofmalic acid. Aspartate donates one nitrogen atom in the biosynthesis ofinosine, the precursor to thepurine 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 ofBeta-glucuronidase, which serves to regulateenterohepatic circulation ofbilirubin and bile acids.[18]
Aspartate (theconjugate base of aspartic acid) stimulatesNMDA receptors, though not as strongly as the amino acid neurotransmitterL-glutamate does.[19] Aspartate is the "A" in NMDA (N-methyl-D-aspartate receptor).
In 2014, the global market for aspartic acid was 39.3 thousandshort tons (35.7 thousandtonnes)[20] or about $117 million annually.[21] The three largest market segments include the U.S., Western Europe, and China. Current applications include biodegradable polymers (polyaspartic acid), low calorie sweeteners (aspartame), scale and corrosion inhibitors, and resins.[citation needed]
Aspartic acid is not anessential amino acid, which means that it can be synthesized from central metabolic pathway intermediates in humans, and does not need to be present in the diet. Ineukaryotic cells, roughly 1 in 20 amino acids incorporated into a protein is an aspartic acid,[26] and accordingly almost any source of dietary protein will include aspartic acid. Additionally, aspartic acid is found in:
^abcdVoet, Donald; Voet, Judith G.; Pratt, Charlotte W. (2016-02-29).Fundamentals of Biochemistry: Life at the Molecular Level. John Wiley & Sons.ISBN9781118918401.OCLC910538334.
^Berzelius JJ, Öngren OG (1839).Traité de chimie (in French). Vol. 3. Brussels: A. Wahlen et Cie. p. 81. Retrieved25 August 2015.
^Plimmer R (1912) [1908]. Plimmer R, Hopkins F (eds.).The chemical composition of the proteins. Monographs on Biochemistry. Vol. Part I. Analysis (2nd ed.). London: Longmans, Green and Co. p. 112. RetrievedJanuary 18, 2010.
^"Nomenclature and symbolism for amino acids and peptides (IUPAC-IUB Recommendations 1983)".Pure and Applied Chemistry.56 (5):595–624. 1984.doi:10.1351/pac198456050595..
^Chen PE, Geballe MT, Stansfeld PJ, Johnston AR, Yuan H, Jacob AL, Snyder JP, Traynelis SF, Wyllie DJ (May 2005). "Structural features of the glutamate binding site in recombinant NR1/NR2A N-methyl-D-aspartate receptors determined by site-directed mutagenesis and molecular modeling".Molecular Pharmacology.67 (5):1470–84.doi:10.1124/mol.104.008185.PMID15703381.S2CID13505187.
