Search for New Aggregable Fragments of Human Insulin

Enhanced chromium and nitrogen removal by constructing a biofilm reaction system based on denitrifying bacteria preferential colonization theory

Understanding the developmental characteristics of microbial communities in biofilms is crucial for designing targeted functional microbial enhancements for the remediation of complex contamination scenarios. The strong prioritization effect of microorganisms confers the ability to colonize strains that arrive first dominantly. In this study, the auto-aggregating denitrifying bacterial Pseudomonas stutzeri strain YC-34, which has both nitrogen and chromium removal characteristics, was used as a biological material to form a stable biofilm system based on the principle of dominant colonization and biofortification. The effect of the biofilm system on nitrogen and chromium removal was characterized by measuring the changes in the quality of influent and effluent water. The pattern of biofilm changes was analyzed by measuring biofilm content and thickness and characterizing extracellular polymer substances (EPS). Further analysis of the biofilm microbiota characteristics and potential functions revealed the mechanism of strain YC-34 biofortified biofilm. The results revealed that the biofilm system formed could achieve 90.56% nitrate-nitrogen removal with an average initial nitrate-nitrogen concentration of 51.9 mg/L and 40% chromium removal with an average initial hexavalent chromium Cr(VI) concentration of 7.12 mg/L. The biofilm properties of the system were comparatively analyzed during the biofilm formation period, the fluctuation period of Cr(VI)-stressed water quality, and the stabilization period of Cr(VI)-stressed water quality. The biofilm system may be able to increase the structure of hydrogen bonds, the type of protein secondary structure, and the abundance of amino acid-like components in the EPS, which may confer biofilm tolerance to Cr(VI) stress and allow the system to maintain a stable biofilm structure. Furthermore, microbial characterization indicated an increase in microbial diversity in the face of chromium stress, with an increase in the abundance of nitrogen removal-associated functional microbiota and an increasing trend in the abundance of nitrogen transfer pathways. These results demonstrate that the biofilm system is stable in nitrogen and chromium removal. This bioaugmentation method may provide a new way for the remediation of heavy metal-polluted water bodies and also provides theoretical and application parameters for the popularization and application of biofilm systems.

Structural, kinetic, and thermodynamic aspects of insulin aggregation

Given the significance of protein aggregation in proteinopathies and the development of therapeutic protein pharmaceuticals, revamped interest in assessing and modelling the aggregation kinetics has been observed. Quantitative analysis of aggregation includes data of gradual monomeric depletion followed by the formation of subvisible particles. Kinetic and thermodynamic studies are essential to gain key insights into the aggregation process. Despite being the medical marvel in the world of diabetes, insulin suffers from the challenge of aggregation. Physicochemical stresses are experienced by insulin during industrial formulation, storage, delivery, and transport, considerably impacting product quality, efficacy, and effectiveness. The present review briefly describes the pathways, mathematical kinetic models, and thermodynamics of protein misfolding and aggregation. With a specific focus on insulin, further discussions include the structural heterogeneity and modifications of the intermediates incurred during insulin fibrillation. Finally, different model equations to fit the kinetic data of insulin fibrillation are discussed. We believe that this review will shed light on the conditions that induce structural changes in insulin during the lag phase of fibrillation and will motivate scientists to devise strategies to block the initialization of the aggregation cascade. Subsequent abrogation of insulin fibrillation during bioprocessing will ensure stable and globally accessible insulin for efficient management of diabetes.

Chemical Modification of the Amino Groups of Human Insulin: Investigating Structural Properties and Amorphous Aggregation of Acetylated Species

The efficacy of human recombinant insulin can be affected by its aggregation. Effects of acetylation were observed on insulin structure, stability, and aggregation at 37 and 50 °C and pH of 5.0 and 7.4 with the use of spectroscopy, circular dichroism (CD), dynamic light scattering (DLS), and atomic force microscopy (AFM). Raman and FTIR results were indicative of structural changes in AC-INS, and CD analyses showed a slight increase in β-sheet content in AC-INS. Melting temperature (T_m) measurements indicated an overall more stable structure and spectroscopic assessment showed a more compact one. Formation of amorphous aggregates was followed over time and kinetics parameters showed a longer nucleation phase (higher t* amount) and lower aggregates amount (lower A_lim) for acetylated insulin (AC-INS) compared to native (N-INS) in all tested conditions. The results of amyloid-specific probes approved the formation of amorphous aggregates. Size particle and microscopic analysis suggested that AC-INS was less prone to form aggregates, which were smaller if formed. In conclusion, this study has demonstrated that controlled acetylation of insulin may lead to its higher stability and lower propensity toward amorphous aggregation and has provided insight into the result of this type of post-translational protein modification.

Non-Aggregating Amylin Fragments as an Inhibitors of the Aggregation Process of Susceptible to Aggregation Fragments 18-22, 23-27, and 33-37 of Hormone

Amylin aggregation is one of the factors in the development of diabetes mellitus, which is classified as a civilization disease. The aim of this research was to find whether non-aggregating fragments 1-7, 8-12, 13-17 and 28-32 of amylin would inhibit the aggregation of the amyloidogenic cores 18-22, 23-27, 33-37 of hormone. In the study of the inhibitory potential of non-aggregating amylin fragments, a set of independent methods were used to study aggregation properties (spectroscopic and fluorescence studies with the use of indicators, microscopic studies, circular dichroism studies) and the method of prediction of aggregation properties. The performed research allowed to select the cyclic fragment (1-7) H-KCNTATC-OH with disulfide bond as an inhibitor capable of inhibiting the aggregation of all amyloidogenic cores 18-22, 23-27, 33-37 of the hormone. Additionally, it was found that this peptide inhibits insulin hot spot aggregation, which may indicate its universal utility in inhibiting the process of aggregation of hormones regulating carbohydrate metabolism directly related to the development of diabetes. Research on the possibility of the extensive use of the cyclic fragment (1-7) of H-KCNTATC-OH as a peptide inhibitor of the polypeptide/protein aggregation process is ongoing.

FKBP22 from the psychrophilic bacterium Shewanella sp. SIB1 selectively binds to the reduced state of insulin to prevent its aggregation

FKBP22 of a psychrophilic bacterium, Shewanella sp. SIB1 (SIB1 FKBP22), is a member of peptidyl-prolyl cis-trans isomerase (PPIase) and consists of N- and C-domains responsible for chaperone-like and PPIase catalytic activities, respectively. The chaperone-like activity of SIB1 FKBP22 was previously evidenced by its ability to prevent dithiothreitol (DTT)-induced insulin aggregation. Nevertheless, the mechanism by which this protein inhibits the aggregation remains unclear. To address this, the binding affinity of SIB1 FKBP22 to the native or reduced states of insulin was examined using surface plasmon resonance (SPR). The native and reduced states refer to insulin in the absence or DTT presence, respectively. The SPR sensorgram showed that SIB1 FKBP22 binds specifically to the reduced state of insulin, with a KD value of 37.31 ± 3.20 μM. This binding was facilitated by the N-domain, as indicated by the comparable KD values of the N-domain and SIB1 FKBP22. Meanwhile, the reduced state of insulin was found to have no affinity towards the C-domain. The KD value of SIB1 FKBP22 was slightly decreased by NaCl but was not severely affected by FK506, a specific FKBP inhibitor. Similarly, the prevention of DTT-induced aggregation by SIB1 FKBP22 was also modulated by the N-domain and was not affected by FK506. Further, the reduced and native states of insulin had no effect on the catalytic efficiency (kcat/KM) of SIB1 FKBP22 towards a peptide substrate. Nevertheless, the reduced state of insulin slightly reduced the catalytic efficiency towards refolding RNase T1, at up to 1.5-fold lower than in the absence of insulin. These results suggested that the binding event was mainly facilitated by hydrophobic interaction and was independent from its PPIase activity. Altogether, a possible mechanism by which SIB1 FKBP22 prevents DTT-induced insulin aggregation was proposed.

N-Methylated Analogs of hIAPP Fragments 18-22, 23-27, 33-37 Inhibit Aggregation of the Amyloidogenic Core of the Hormone

Amylin (hIAPP) aggregation leads to the formation of insoluble deposits and is one of the factors in the development of type II diabetes. The aim of this research was to find N-methylated analogs of the aggregating amylin fragments 18-22, 23-27, and 33-37, which would not themselves be susceptible to aggregation and would inhibit the aggregation of the amyloidogenic cores of the hormone. None of the analogs of fragment 18-22 containing one or two N-methylated amino acid residues showed any tendency to aggregate. Only the peptide H-F(N-Me)GA(N-Me) IL-OH (6) derived from the 23-27 hIAPP hot spot did not form fibrous structures. All analogs of the 33-37 amylin fragment were characterized by the ability to form aggregates, despite the presence of N-methylated amino acids in their structures. N-Methylated peptides 1-5 demonstrated inhibitory properties against the aggregation of fragment 18-22. Aggregation of the amyloidogenic core of 23-27 was significantly inhibited by N-methylated peptides 1-3 derived from the (18-22) H-HSSNN-OH fragment and by the H-F(N-Me)GA(N-Me)IL-OH (6) fragment derived from the 23-27 amylin hot spot. Fragment (33-37) H-GSNTY-NH₂ was found to be inhibited in the presence of N-methylated peptides 1-3 derived from the 18-22 fragment and by the double methylated peptide H-F(N-Me)GA(N-Me)IL-OH (6). Research on the possibility of using N-methylated analogs of amyloidogenic amylin cores as inhibitors of hormone aggregation is ongoing, with a focus on finding the minimum concentration of N-methylated peptides capable of inhibiting the aggregation of hIAPP hot spots.

Human Serum Albumin Binds Native Insulin and Aggregable Insulin Fragments and Inhibits Their Aggregation

The purpose of this study was to investigate whether Human Serum Albumin (HSA) can bind native human insulin and its A13–A19 and B12–B17 fragments, which are responsible for the aggregation of the whole hormone. To label the hormone and both hot spots, so that their binding positions within the HSA could be identified, 4-(1-pyrenyl)butyric acid was used as a fluorophore. Triazine coupling reagent was used to attach the 4-(1-pyrenyl)butyric acid to the N-terminus of the peptides. When attached to the peptides, the fluorophore showed extended fluorescence lifetimes in the excited state in the presence of HSA, compared to the samples in buffer solution. We also analyzed the interactions of unlabeled native insulin and its hot spots with HSA, using circular dichroism (CD), the microscale thermophoresis technique (MST), and three independent methods recommended for aggregating peptides. The CD spectra indicated increased amounts of the α-helical secondary structure in all analyzed samples after incubation. Moreover, for each of the two unlabeled hot spots, it was possible to determine the dissociation constant in the presence of HSA, as 14.4 µM (A13–A19) and 246 nM (B12–B17). Congo Red, Thioflavin T, and microscopy assays revealed significant differences between typical amyloids formed by the native hormone or its hot-spots and the secondary structures formed by the complexes of HSA with insulin and A13–A19 and B12–B17 fragments. All results show that the tested peptide-probe conjugates and their unlabeled analogues interact with HSA, which inhibits their aggregation.

Insulin Hot-Spot Analogs Formed with N-Methylated Amino Acid Residues Inhibit Aggregation of Native Hormone

In this study, N-methylated analogs of hot-spots of insulin were designed and synthesized, in the expectation that they would inhibit the aggregation of both insulin hot-spots and the entire hormone. Synthesis of insulin “amyloidogenic” analogs containing N-methylated amino acid residues was performed by microwave-assisted solid phase according to the Fmoc/tert-Bu strategy. As a coupling reagent 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium toluene-4-sulfonate (DMT/NMM/TosO^-) was used. Three independent methods were applied in aggregation studies of the complexes of insulin with its N-methylated peptides. Additionally, circular dichroism (CD) measurements were used to confirm that aggregation processes did not occur in the presence of the N-methylated analogs of hot-spot insulin fragments, and that insulin retains its native conformation. Of the seven N-methylated analogs of the A- and B-chain hot-spots of insulin, six inhibited insulin aggregation (peptides 1 and 3–7). All tested peptides were found to have a lower ability to inhibit the aggregation of insulin hot-spots compared to the capability to inhibit native hormone aggregation.