Letter: Conformation of the cystine linkages in bovine alpha-lactalbumin as revealed by its Raman effect

Tracking the amyloidogenic core of IAPP amyloid fibrils: Insights from micro-Raman spectroscopy

Human islet amyloid polypeptide (hIAPP) is the major protein component of extracellular amyloid deposits, located in the islets of Langerhans, a hallmark of type II diabetes. The underlying mechanisms of IAPP aggregation have not yet been clearly defined, although the highly amyloidogenic sequence of the protein has been extensively studied. Several segments have been highlighted as aggregation-prone regions (APRs), with much attention focused on the central 8-17 and 20-29 stretches. In this work, we employ micro-Raman spectroscopy to identify specific regions that are contributing to or are excluded from the amyloidogenic core of IAPP amyloid fibrils. Our results demonstrate that both the N-terminal region containing a conserved disulfide bond between Cys residues at positions 2 and 7, and the C-terminal region containing the only Tyr residue are excluded from the amyloid core. Finally, by performing detailed aggregation assays and molecular dynamics simulations on a number of IAPP variants, we demonstrate that point mutations within the central APRs contribute to the reduction of the overall amyloidogenic potential of the protein but do not completely abolish the formation of IAPP amyloid fibrils.

Hydrogen sulfide inhibits amyloid formation

Amyloid fibrils are large aggregates of misfolded proteins, which are often associated with various neurodegenerative diseases such as Alzheimer’s, Parkinson’s, Huntington’s, and vascular dementia. The amount of hydrogen sulfide (H₂S) is known to be significantly reduced in the brain tissue of people diagnosed with Alzheimer’s disease relative to that of healthy individuals. These findings prompted us to investigate the effects of H₂S on the formation of amyloids in vitro using a model fibrillogenic protein hen egg white lysozyme (HEWL). HEWL forms typical β-sheet rich fibrils during the course of 70 min at low pH and high temperatures. The addition of H₂S completely inhibits the formation of β-sheet and amyloid fibrils, as revealed by deep UV resonance Raman (DUVRR) spectroscopy and ThT fluorescence. Nonresonance Raman spectroscopy shows that disulfide bonds undergo significant rearrangements in the presence of H₂S. Raman bands corresponding to disulfide (RSSR) vibrational modes in the 550–500 cm^–1 spectral range decrease in intensity and are accompanied by the appearance of a new 490 cm^–1 band assigned to the trisulfide group (RSSSR) based on the comparison with model compounds. The formation of RSSSR was proven further using a reaction with TCEP reduction agent and LC-MS analysis of the products. Intrinsic tryptophan fluorescence study shows a strong denaturation of HEWL containing trisulfide bonds. The presented evidence indicates that H₂S causes the formation of trisulfide bridges, which destabilizes HEWL structure, preventing protein fibrillation. As a result, small spherical aggregates of unordered protein form, which exhibit no cytotoxicity by contrast with HEWL fibrils.

Investigation of the structure of alpha-lactalbumin protein nanotubes using optical spectroscopy

Alpha-lactalbumin (α-la) is one of the major proteins in whey. When partially hydrolysed with Bacillus licheniformis protease, it produces nanotubular structures in the presence of calcium ions by a self-assembly process. This study presents investigation of α-la protein structure during hydrolysis and nanotube formation using optical spectroscopy. Before spectroscopic measurements, nanotubes were examined with microscopy. The observed α-la nanotubes (α-LaNTs) were in the form of regular hollow strands with a diameter of about 20 nm and the average length of 1 μm. Amide and backbone vibration bands of the Raman spectra displayed remarkable conformational changes in α and β domains in the protein structure during nanotube growth. This was confirmed by the Fourier-transform infrared (FTIR) spectroscopy data. Also, FTIR analysis revealed certain bands at calcium (Ca++) binding sites of COO− groups in hydrolysed protein. These sites might be critical in nanotube elongation.

Ultraviolet resonance Raman spectra of insulin and alpha-lactalbumin with 218- and 200-nm laser excitation

Ultraviolet resonance Raman (RR) spectra, with 200- and 218-nm excitation from a H2-shifted quadrupled Nd:YAG laser, are reported for insulin and alpha-lactalbumin in dilute aqueous solution, at pH values known to produce differences in the exposure of the aromatic residues to solvent. At 200 nm, the spectra are dominated by tyrosine bands, whose intensity is lowered somewhat in protein conformations in which tyrosine is exposed to solvent. The expected shift in the relative intensities of the components of the approximately 850-cm-1 tyrosine doublet is difficult to discern because the higher energy component shows much greater resonance enhancement and the lower energy component appears as a weak shoulder. The peptide vibrations, amides I, II, and III, are also enhanced at 200 nm. The infrared active amide II mode is particularly prominent, although it is not observed in Raman spectra with visible excitation. In addition, the amide I band is quite broad in the 200-nm RR spectra, and the peak frequency is lower than that seen in visible excitation Raman spectra and is close to the infrared frequency. It appears that 200-nm excitation produces resonance enhancement of the infrared-active components of both amide I and amide II. Excitation at 218 nm enhances tryptophan modes strongly. The 876-cm-1 band, assigned to a deformation mode of the five-membered ring, shows a measurable upshift upon exposure of tryptophan to solvent, attributable to N-H hydrogen bonding.(ABSTRACT TRUNCATED AT 250 WORDS)

Opsin structure probed by raman spectroscopy of photoreceptor membranes

The first nonresonance Raman spectra of photoreceptor membranes are presented. Information about the membrane protein, opsin, and the membrane phospholipids can be deduced. Opsin appears to contain alpha-helical structure but little beta structure. The tyrosine residues are predominantly hydrogen bonded, and disulfide bonds, if they are present, are not in the normal gauche-gauche configuration.