A single Asp isomer substitution in an αA-crystallin-derived peptide induces a large change in peptide properties

The eye lens is mainly composed of crystallins, which undergo modifications such as oxidation, deamidation and isomerization with aging. Asp58, Asp76, Asp84, and Asp151 residues of αA-crystallin are site-specifically isomerized to L-iso, D-, and D-iso isomers in aged-related cataract lenses. In addition, an αA66-80 peptide, corresponding to the 66-80 (⁶⁶SDRDKFVIFLDVKHF⁸⁰) fragment of human αA-crystallin, is detected in aged lens. This peptide induces protein aggregation and causes loss of the chaperone function of α-crystallin. The αA66-80 peptide contains Asp76, but it is not known whether isomerization of Asp76 in αA66-80 specifically induces protein aggregation or affects α-crystallin function. Using Fmoc-based solid-phase synthesis, here we synthesized four αA66-80 peptides, each containing L-, L-iso, D-, or D-isoAsp at position 76, and compared their structures and properties. Normal αA66-80 peptide containing the L-Asp76 isomer increased the EDTA-induced aggregation of ADH protein, DTT-induced aggregation of insulin, and heat-induced aggregation of β_L-crystallin. αA66-80 peptide containing D- or D-isoAsp76 had similar or no effects on the aggregation of these proteins. By contrast, αA66-80 peptide containing L-isoAsp76 inhibited the aggregation of all three proteins, indicating that it has chaperone activity. With regard to secondary structure, αA66-80 peptide containing the L-, D-, or D-isoAsp76 isomer had random-coil structure, whereas αA66-80 peptide containing L-isoAsp76 had β-sheet like structure. A Thioflavin T (ThT) assay indicated that only the L-isoAsp-containing αA66-80 peptide has β-sheet structure and generates amyloid fibrils. Collectively, these observations indicate that isomerization of Aps76 to the Lβ isomer endows β-sheet structure and chaperone function on this peptide.