Aspartic acid

Aspartic acid is an alpha-amino acid with two carboxyl groups, represented in a neutral structural form as $\mathrm{HOOC}-\mathrm{CH}(\mathrm{N}{\mathrm{H}}_2)-\mathrm{C}{\mathrm{H}}_2-\mathrm{COOH}$. Its molecular formula is ${\mathrm{C}}_4{\mathrm{H}}_7\mathrm{N}{\mathrm{O}}_4$. One carboxyl group belongs to the amino-acid backbone and the other terminates the side chain.^[ref]

The protein-associated L configuration and the sequence symbols Asp and D are separate naming conventions. A one-letter D in a protein sequence denotes the aspartic-acid residue; it is not a declaration of D stereochemistry.^[ref]

Loss or redistribution of protons changes the ionic form. The name aspartate commonly denotes the anionic form. For example, a form with two carboxylates carrying $-1$ each and an ammonium group carrying $+1$ has net charge $-1-1+1=-1$. This charge count must be kept distinct from the L/D configuration and from atom connectivity.

The side-chain carboxyl chemistry also matters in aspartic proteases, where appropriately positioned acidic groups participate in peptide bond hydrolysis. Presence of an aspartic-acid residue alone does not establish catalytic activity: its geometry and chemical environment matter.^[ref]

Aspartate denotes an anionic form of aspartic acid. A common biochemical example is the L-configured monoanion, with two carboxylate groups and one positively charged amino group. Its formula is ${\mathrm{C}}_4{\mathrm{H}}_6\mathrm{N}{\mathrm{O}}_4^{-}$ and its net charge is $-1$.^[ref]

The charge count isHaving both positive and negative groups therefore does not imply a zero total charge. Compared with neutral aspartic acid, formation of this conjugate base removes a proton; it does not change the four-carbon connectivity or, by itself, reverse the L/D stereochemistry.

An illustrative acid–base comparison keeps these questions separate. For one protonation step in a dilute solution, the Henderson-Hasselbalch equation gives $[\mathrm{base}]/[\mathrm{acid}]\simeq 10^{\mathrm{pH}-\mathrm{p}{K}_a}$. One pH unit above that step's $\mathrm{p}{K}_a$ gives a ratio of about ten. This is a symbolic illustration, not a numerical assignment of the molecule's several dissociation constants. Specifying ionic form, pH and stereochemistry is more informative than treating every use of the name aspartate as one context-free structure.