Structural changes induced by the deamidation and isomerization of asparagine revealed by the crystal structure of Ustilago sphaerogena ribonuclease U2B.

Under physiological conditions, the deamidation and isomerization of asparagine to isoaspartate (isoAsp) proceeds nonenzymatically via succinimide. Although a large number of proteins have been reported to contain isoAsp, information concerning the three-dimensional structure of proteins containing isoaspartate is still limited. We have crystallized isoAsp containing Ustilago sphaerogena ribonuclease U2B, and determined the crystal structure at 1.32 Å resolution. The structure revealed that the formation of isoAsp32 induces a single turn unfolding of the α-helix from Asp29 to Asp34, and the region from Asp29 to Arg35 forms a U-shaped loop structure. The electron density map shows that isoAsp32 retained the L-configuration at the C(α) atom. IsoAsp32 is in gauche conformation about a C(α)--C(β) bond, and the polypeptide chain bends by ∼90° at isoAsp32. IsoAsp32 protrudes from the surface of the protein, and the abnormal β-peptide bond in the main-chain and α-carboxylate in the side-chain is fully exposed. The structure suggests that the deamidation of the Asn and the isoAsp formation in proteins could confer immunogenicity.