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2018 ; 3
(2
): 2452-2462
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Structural Insight of Amyloidogenic Intermediates of Human Insulin
#MMPMID30023834
Dolui S
; Roy A
; Pal U
; Saha A
; Maiti NC
ACS Omega
2018[Feb]; 3
(2
): 2452-2462
PMID30023834
show ga
Engaging Raman spectroscopy as a primary tool, we investigated the early events
of insulin fibrilization and determined the structural content present in
oligomer and protofibrils that are formed as intermediates in the fibril
formation pathway. Insulin oligomer, as obtained upon incubation of zinc-free
insulin at 60 °C, was mostly spherical in shape, with a diameter of 3-5 nm.
Longer incubation produced "necklace"-like beaded protofibrillar assembly
species. These intermediates eventually transformed into 5-8 nm thick fibers with
smooth surface texture. A broad amide I band in the Raman spectrum of insulin
monomer appeared at 1659 cm(-1), with a shoulder band at 1676 cm(-1). This
signature suggested the presence of major helical and extended secondary
structure of the protein backbone. In the oligomeric state, the protein
maintained its helical imprint (?50%) and no substantial increment of the compact
cross-?-sheet structure was observed. A nonamide helix signature band at 940
cm(-1) was present in the oligomeric state, and it was weakened in the fibrillar
structure. The 1-anilino-8-naphthalene-sulfonate binding study strongly suggested
that a collapse in the tertiary structure, not the major secondary structural
realignment, was the dominant factor in the formation of oligomers. In the
fibrillar state, the contents of helical and disordered secondary structures
decreased significantly and the ?-sheet amount increased to ?62%. The narrow
amide I Raman band at 1674 cm(-1) in the fibrillar state connoted the formation
of vibrationally restricted highly organized ?-sheet structure with quaternary
realignment into steric-zipped species.