Is polyproline II helix the killer conformation? a raman optical activity study of the amyloidogenic prefibrillar intermediate of human lysozyme

The amyloidogenic prefibrillar partially denatured intermediate of human lysozyme, prepared by heating the native protein to 57°C at pH 2.0, was studied using Raman optical activity (ROA). A positive band in the room temperature ROA spectrum of the native protein at ∼1345 cm−1, assigned to a hydrated form of α-helix, is not present in that of the prefibrillar intermediate, where a new strong positive band at ∼1318 cm−1 appears instead that is assigned to the poly(l-proline) II (PPII)-helical conformation. A sharp negative band at ∼1241 cm−1 in the native protein, assigned to β-strand, shows little change in the ROA spectrum of the prefibrillar intermediate. The disappearance of a positive ROA band at ∼1551 cm−1 assigned to vibrations of tryptophan side-chains indicates that major conformational changes have occurred among the five tryptophan residues present in human lysozyme, four of which are located in the α-domain. The various ROA data suggest that a substantial loss of tertiary structure has occurred in the prefibrillar intermediate and that this is located more in the α-domain than in the β-domain. There is no evidence for any increase in β-structure. The ROA spectrum of hen lysozyme, which does not form amyloid fibrils so readily, remains much more native-like on heating to 57°C at pH 2.0. The thermal behaviour of the alanine-rich α-helical peptide AK21 in aqueous solution was found to be similar to that of human lysozyme. Hydrated α-helix therefore appears to readily undergo a conformational change to PPII structure on heating, which may be a key step in the conversion of α-helix into β-sheet in the formation of amyloid fibrils in human lysozyme. Since it is extended, flexible, lacks intrachain hydrogen bonds and is fully hydrated in aqueous solution, PPII helix has the appropriate characteristics to be implicated as a critical conformational element in many conformational diseases. Disorder of the PPII type may be a sine qua non for the formation of regular fibrils; whereas the more dynamic disorder of the random coil may lead only to amorphous aggregates.