Aspartic acid is an alpha-amino acid with two carboxyl groups, represented in a neutral structural form as . Its molecular formula is . 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 each and an ammonium group carrying has net charge . 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]
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