The pathogenesis of type 2 diabetes (T2D) is highly related to the abnormal self-assembly of the human islet amyloid polypeptide (hIAPP) into amyloid aggregates. To inhibit hIAPP aggregation is considered a promising therapeutic strategy for T2D treatment. Melatonin (Mel) was reported to effectively impede the accumulation of hIAPP aggregates and dissolve preformed fibrils. However, the underlying mechanism at the atomic level remains elusive. Here, we performed replica-exchange molecular dynamics (REMD) simulations to investigate the inhibitory effect of Mel on hIAPP oligomerization by using hIAPP20–29 octamer as templates. The conformational ensemble shows that Mel molecules can significantly prevent the β-sheet and backbone hydrogen bond formation of hIAPP20–29 octamer and remodel hIAPP oligomers and transform them into less compact conformations with more disordered contents. The interaction analysis shows that the binding behavior of Mel is dominated by hydrogen bonding with a peptide backbone and strengthened by aromatic stacking and CH–π interactions with peptide sidechains. The strong hIAPP–Mel interaction disrupts the hIAPP20–29 association, which is supposed to inhibit amyloid aggregation and cytotoxicity. We also performed conventional MD simulations to investigate the influence and binding affinity of Mel on the preformed hIAPP1–37 fibrillar octamer. Mel was found to preferentially bind to the amyloidogenic region hIAPP20–29, whereas it has a slight influence on the structural stability of the preformed fibrils. Our findings illustrate a possible pathway by which Mel alleviates diabetes symptoms from the perspective of Mel inhibiting amyloid deposits. This work reveals the inhibitory mechanism of Mel against hIAPP20–29 oligomerization, which provides useful clues for the development of efficient anti-amyloid agents.

Enzymes from thermophilic microorganisms usually show high thermostability, which is of great potential in industrial application; to understand the structural logic of these enzymes is helpful for the construction of robust biocatalysts. In this study, based on the crystal structure of an N-demethylase�TrSOX�with outstanding thermostability from Thermomicrobium roseum, substitutions were introduced on the aggregation interface and rigid spots to reduce the aggregation ratio and the rigidity. Four substitutions on the aggregation interface�V162S, M308S, F170S, and V306S�considerably reduced the thermostability and slightly enhanced the catalytic efficiency. In addition, the thermostable framework was considerably disrupted in several multiple P → G substitutions in several local motifs (P129G/P134G, P237G/P259G, and P259G/P276G). These structural fluctuations were in good accordance with whole-structure or partial root-mean-square deviation, radius of gyration H-bonds, and solvent-accessible surface area values in molecular dynamics simulation. Furthermore, these key spots were introduced into an unstable homolog from Bacillus sp., resulting in a dramatical increase in the half-life at 60 °C from <10 to 1440 min. These results could help understand the natural stable framework of thermophilic enzymes, which could be references for the construction of robust enzymes in industrial applications.

Human islet amyloid polypeptide (hIAPP) is the major component of the amyloid deposited in the pancreas of patients with type 2 diabetes mellitus. Its aggregation and consequent production of intermediates are believed to be responsible for its cytotoxicity and pathological processes. Recently, graphene quantum dots (GQDs) are proved to effectively inhibit a range of amyloid deposits. This work focuses on the influence of the charged GQDs on hIAPP inhibition. Microsecond all-atom molecular dynamics simulations in explicit water were performed to study the influence of charged GQDs on the structural stability of hIAPP fibril. GQDs were found to be able to destabilize the hIAPP fibril and reduce the β-sheet content. The stability of the hydrophobic core was greatly disturbed, and the hydrogen bond formation at protofibril interfaces was also hindered. The negatively and positively charged GQDs have different binding sites, dynamics, and interactions at hIAPP fibril, which is dominated by electrostatic interaction and assisted by π−π stacking, salt bridge, and hydrogen bonding interactions. The π−π stacking between GQDs and hIAPP may be influenced by the electrostatic interaction in a facilitative or competitive manner. In addition, the negatively charged GQD is suggested to be a better candidate of amyloid inhibition than the positively charged one in disruptive effect, binding modes, and binding intensity. These findings may provide useful perspectives for the design of nanomedicine for amyloid inhibition and are helpful to the development of diagnosis and screening nanotechnology for neurodegenerative diseases.