Aspartic acid (Asp, D) - Structure, pKa, pI, and Codons

Side chain

The R group of aspartic acid is -CH₂COOH. Carboxymethyl; a carboxylic acid. This is what distinguishes it from the other 19 amino acids, since all of them share the same backbone: an alpha carbon bonded to an amino group, a carboxyl group, and a hydrogen.

Ionization and isoelectric point

Aspartic acid has 3 ionizable groups: alpha-COOH (pKa 1.88), side chain COOH (pKa 3.65), alpha-NH3+ (pKa 9.60). The isoelectric point is the pH at which the molecule carries no net charge, found by averaging the two pKa values that bracket the neutral species.

pI = (1.88 + 3.65) / 2 = 2.77

Note that this uses the side chain pKa rather than both alpha group values, because the side chain ionizes within the range that brackets the neutral form.

These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.

Stereochemistry

The alpha carbon of L-aspartic acid is (S).

Genetic code

2 codons encode aspartic acid: GAU, GAC.

MCAT notes

• The lowest pI of the 20 (2.77). Deprotonated and negatively charged at pH 7.
• A member of the serine protease catalytic triad (Ser-His-Asp).
• Transaminates to oxaloacetate; central to the urea cycle, where it donates the second nitrogen, and to the malate-aspartate shuttle.

D and E are the two acids, and they sit next to each other in the alphabet.

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