Existence of different structural intermediates and aggregates on the folding pathway of ovalbumin

Structural modifications of ovalbumin in presence of different concentration of guanidine hydrochloride (Gdn HCl) and glucose were investigated by using intrinsic fluorescence, Fourier transform infra-red spectroscopy, circular dichroism and 8-anilino-1-naphthalene-sulphonic acid, to confirm that partially folded intermediates of ovalbumin lead to aggregation. The two partially folded intermediates of ovalbumin were observed one at 1 M Gdn HCl and another in the presence of 20 mM glucose at 3 M Gdn HCl. Both intermediates exist as compact states with altered intrinsic fluorescence, prominent β-sheet secondary structure and enhanced ANS binding. Ovalbumin in the presence of glucose required more concentration of Gdn HCl (3 M) to exist as an intermediate state than control (1 M). Such alpha-helix/beta-sheet transition of proteins is a crucial step in amyloidogenic diseases and represents an internal rearrangement of local contacts in an already folded protein. Further, incubation for 24 h resulted in the formation of aggregates as detected by thioflavin T-assay. On further increasing the concentration of glucose to 50 mM and incubation time for various days resulted in the formation of molten globule state of ovalbumin at 6th day. Later on, at 10th day advanced glycated end products were observed.