Aβ Interacts with Both the Iron Center and the Porphyrin Ring of Heme: Mechanism of Heme’s Action on Aβ Aggregation and Disaggregation

Curcumin hybrid molecules for the treatment of Alzheimer's disease: Structure and pharmacological activities

Alzheimer's disease (AD) is the most common neurodegenerative disease among the elderly. Contemporary treatments can only relieve symptoms but fail to delay disease progression. Curcumin is a naturally derived compound that has demonstrated significant therapeutic effects in AD treatment. Recently, molecular hybridization has been utilized to combine the pharmacophoric groups present in curcumin with those of other AD drugs, resulting in a series of novel compounds that enhance the therapeutic efficacy through multiple mechanisms. In this review, we firstly provide a concise summary of various pathogenetic hypotheses of AD and the mechanism of action of curcumin in AD, as well as the concept of molecular hybridization. Subsequently, we focus on the recent development of hybrid molecules derived from curcumin, summarizing their structures and pharmacological activities, including cholinesterase inhibitory activity, Aβ aggregation inhibitory activity, antioxidant activity, and other activities. The structure-activity relationships were further discussed.

Amyloid Disassembly: What Can We Learn from Chaperones?

Protein aggregation and subsequent accumulation of insoluble amyloid fibrils with cross-β structure is an intrinsic characteristic of amyloid diseases, i.e., amyloidoses. Amyloid formation involves a series of on-pathway and off-pathway protein aggregation events, leading to mature insoluble fibrils that eventually accumulate in multiple tissues. In this cascade of events, soluble oligomeric species are formed, which are among the most cytotoxic molecular entities along the amyloid cascade. The direct or indirect action of these amyloid soluble oligomers and amyloid protofibrils and fibrils in several tissues and organs lead to cell death in some cases and organ disfunction in general. There are dozens of different proteins and peptides causing multiple amyloid pathologies, chief among them Alzheimer's, Parkinson's, Huntington's, and several other neurodegenerative diseases. Amyloid fibril disassembly is among the disease-modifying therapeutic strategies being pursued to overcome amyloid pathologies. The clearance of preformed amyloids and consequently the arresting of the progression of organ deterioration may increase patient survival and quality of life. In this review, we compiled from the literature many examples of chemical and biochemical agents able to disaggregate preformed amyloids, which have been classified as molecular chaperones, chemical chaperones, and pharmacological chaperones. We focused on their mode of action, chemical structure, interactions with the fibrillar structures, morphology and toxicity of the disaggregation products, and the potential use of disaggregation agents as a treatment option in amyloidosis.

Structure and mechanism behind the inhibitory effect of water soluble metalloporphyrins on Aβ1-42 aggregation

Although the exact molecular mechanism of the pathogenesis of Alzheimer’s disease (AD) is still unclear, compounds that can inhibit the aggregation of amyloid-β peptide (Aβ1-42) or scavenge the highly toxic...

Hydroxy-Porphyrin as an Effective, Endogenous Molecular Clamp during Early Stages of Amyloid Fibrillization

Amyloid fibril formation of proteins is of great concern in neurodegenerative disease and can be detrimental to the storage and stability of biologics. Recent evidence suggests that insulin fibril formation reduces the efficacy of type II diabetes management and may lead to several complications. To develop anti-amyloidogenic compounds of endogenous origin, we have utilized the hydrogen bond anchoring, π stacking ability of porphyrin, and investigated its role on the inhibition of insulin amyloid formation. We report that hydroxylation and metal removal from the heme moiety yields an excellent inhibitor of insulin fibril formation. Thioflavin T, tyrosine fluorescence, Circular Dichorism (CD) spectroscopy, Field emission scanning electron microscopy (FESEM) and molecular dynamics (MD) simulation studies suggest that hematoporphyrin (HP) having hydrogen bonding ability on both sides is a superior inhibitor compared to hemin and protoporphyrin (PP). Experiments with hen egg white lysozyme (HEWL) amyloid fibril formation also validated the efficacy of endogenous porphyrin based small molecules. Our results will help to decipher a general therapeutic strategy to counter amyloidogenesis.

In vitro coordination of Fe-protoheme with amyloid β is non-specific and exhibits multiple equilibria

In addition to copper and zinc, heme is thought to play a role in Alzheimer's disease and its metabolism is strongly affected during the course of this disease. Amyloid β, the peptide associated with Alzheimer's disease, was shown to bind heme in vitro with potential catalytic activity linked to oxidative stress. To date, there is no direct determination of the structure of this complex. In this work, we studied the binding mode of heme to amyloid β in different conditions of pH and redox state by using isotopically labelled peptide in combination with advanced magnetic and vibrational spectroscopic methods. Our results show that the interaction between heme and amyloid β leads to a variety of species in equilibrium. The formation of these species seems to depend on many factors suggesting that the binding site is neither very strong nor highly specific. In addition, our data do not support the currently accepted model where a water molecule is bound to the ferric heme as sixth ligand. They also exclude structural models mimicking a peroxidatic site in the amyloid β-Fe-protoheme complexes.

Insights Into the Mechanism of Tyrosine Nitration in Preventing β-Amyloid Aggregation in Alzheimer's Disease

Nitration of tyrosine at the tenth residue (Tyr10) in amyloid-β (Aβ) has been reported to reduce its aggregation and neurotoxicity in our previous studies. However, the exact mechanism remains unclear. Here, we used Aβ_1-42 peptide with differently modified forms at Tyr10 to investigate the molecular mechanism to fill this gap. By using immunofluorescent assay, we confirmed that nitrated Aβ was found in the cortex of 10-month-old female triple transgenic mice of Alzheimer's disease (AD). And then, we used the surface-enhanced Raman scattering (SERS) method and circular dichroism (CD) to demonstrate that the modification and mutation of Tyr10 in Aβ have little impact on conformational changes. Then, with the aids of fluorescence assays of thioflavin T and 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid, transmission electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS), we found that adding a large group to the phenolic ring of Tyr10 of Aβ could not inhibit Aβ fibrilization and aggregation. Nitration of Aβ reduces its aggregation mainly because it could induce the deprotonation of the phenolic hydroxyl group of Tyr10 of Aβ at physiological pH. We proposed that the negatively charged Tyr10 caused by nitration at physiological pH could interact with the salt bridge between Glu11 and His6 or His13 and block the kink around Tyr10, thereby preventing Aβ fibrilization and aggregation. These findings provide us new insights into the relationship between Tyr10 nitration and Aβ aggregation, which would help to further understand that keeping the balance of nitric oxide in vivo is important for preventing AD.

Discovery and Biological Evaluation of New Selective Acetylcholinesterase Inhibitors with Anti-Aβ Aggregation Activity through Molecular Docking-Based Virtual Screening

Discovery of novel multifunctional inhibitors targeting acetylcholinesterase (AChE) has becoming a hot spot in anti-Alzheimer's disease (AD) drug development. In the present study, four potent small molecule inhibitors (A01, A02, A03 and A04) of AChE with new chemical scaffold were identified. Inhibitor A03 displayed the most potent inhibition activity on AChE at enzymatic level with IC₅₀ value of 180 nM, and high selectivity towards AChE over butyrylcholinesterase (BChE) by more than 100-fold. The binding modes of compounds A01-A04 were carefully analyzed by molecular docking and molecular dynamics (MD) simulation to provide informative clues for further structure modification. Finally, the anti-amyloid beta (Aβ) aggregation and neuroprotective activity were also well investigated. Our findings highlighted the therapeutic promise of AChE inhibitors A01-A04 for AD treatment.

Metal Ions in Alzheimer's Disease: A Key Role or Not?

Despite tremendous research efforts in universities and pharmaceutical companies, effective drugs are still lacking for the treatment of Alzheimer's disease (AD). The biochemical mechanisms of this devastating neurodegenerative disease have not yet been clearly understood. Besides a small percentage of cases with early onset disease having a genetic origin (<5%, familial AD), most cases develop in the elderly as a sporadic form due to multiple and complex parameters of aging. Consequently, AD is spreading in all countries with a long life expectancy. AD is characterized by deposition of senile plaques made of β-amyloid proteins (Aβ) and by hyperphosphorylation of tau proteins, which have been considered as the main drug targets up to now. However, antibodies targeting amyloid aggregates, as well as enzyme inhibitors aiming to modify the amyloid precursor protein processing, have failed to improve cognition in clinical trials. Thus, to set up effective drugs, it is urgent to enlarge the panel of drug targets. Evidence of the link between AD and redox metal dysregulation has also been supported by post-mortem analyses of amyloid plaques, which revealed accumulation of copper, iron, and zinc by 5.7, 2.8, and 3.1 times, respectively, the levels observed in normal brains. Copper-amyloid complexes, in the presence of endogenous reductants, are able to catalyze the reduction of dioxygen and to produce reduced, reactive oxygen species (ROS), leading to neuron death. The possibility of using metal chelators to regenerate normal trafficking of metal ions has been considered as a promising strategy in order to reduce the redox stress lethal for neurons. However, most attempts to use metal chelators as therapeutic agents have been limited to existing molecules available from the shelves. Very few chelators have resulted from a rational design aiming to create drugs with a safety profile and able to cross the blood-brain barrier after an oral administration. In the human body, metals are handled by a sophisticated protein network to strictly control their transport and reactivity. Abnormal concentrations of certain metals may lead to pathological events due to misaccumulation and irregular reactivity. Consequently, therapeutic attempts to restore metal homeostasis should carefully take into account the coordination chemistry specificities of the concerned redox-active metal ions. This Account is focused on the role of the main biologically redox-active transition metals, iron and copper. For iron, the recent debate on the possible role of magnetite in AD pathogenesis is presented. The section devoted to copper is focused on the design of specific copper chelators as drug candidates able to regulate copper homeostasis and to reduce the oxidative damage responsible for the neuron death observed in AD brains. A short survey on non-redox-active metal ions is also included at the beginning, such as aluminum and its controversial role in AD and zinc which is a key metal ion in the brain.

Discovery of New Selective Butyrylcholinesterase (BChE) Inhibitors with Anti-Aβ Aggregation Activity: Structure-Based Virtual Screening, Hit Optimization and Biological Evaluation

In this study, a series of selective butyrylcholinesterase (BChE) inhibitors was designed and synthesized from the structural optimization of hit 1, a 4-((3,4-dihydroisoquinolin-2(1H)-yl)methyl)benzoic acid derivative identified by virtual screening our compound library. The in vitro enzyme assay results showed that compounds 9 ((4-((3,4-dihydroisoquinolin-2(1H)-yl)methyl)phenyl)(pyrrolidin-1-yl)methanone) and 23 (N-(2-bromophenyl)-4-((3,4-dihydroisoquinolin-2(1H)-yl)methyl)benzamide) displayed improved BChE inhibitory activity and good selectivity towards BChE versus AChE. Their binding modes were probed by molecular docking and further validated by molecular dynamics simulation. Kinetic analysis together with molecular modeling studies suggested that these derivatives could target both the catalytic active site (CAS) and peripheral anionic site (PAS) of BChE. In addition, the selected compounds 9 and 23 displayed anti-Aβ_1–42 aggregation activity in a dose-dependent manner, and they did not show obvious cytotoxicity towards SH-SY5Y neuroblastoma cells. Also, both compounds showed significantly protective activity against Aβ_1-42-induced toxicity in a SH-SY5Y cell model. The present results provided a new valuable chemical template for the development of selective BChE inhibitors.

Heme prevents highly amyloidogenic human calcitonin (hCT) aggregation: A potential new strategy for the clinical reuse of hCT

Irreversible aggregation can extremely limit the bioavailability and therapeutic activity of peptide-based drugs. Thus, peptide fibrillation is an excellent challenge for biotechnological drug development. Human calcitonin (hCT) is such a peptide hormone known for its hypocalcaemic effect but has limited pharmaceutical potential due to a high tendency to aggregate. hCT is therefore not widely used preparation in clinical practice. Nonetheless, hCT seems to be still an ideal target for clinical therapy when fibrillation is effectively inhibited, because the alternatives of hCT can stimulate undesirable immune responses in patients and cause side effects. Interestingly, heme is an essential component for many livings and has been shown a strong inhibitory effect on some amyloidogenic peptides aggregation. Here we demonstrate that it may be a most suitable, safe, biocompatible small molecule inhibitor on hCT aggregation, and thereby improving its activity when guiding the drug peptide in clinical therapeutics. In this work, we found that heme was able to reversibly bind with hCT to form a heme-hCT complex with a moderate binding constant (9.17 × 10⁶ M^-1) and significantly suppress the aggregation of hCT probably accomplished by heme binding to it, blocking the β-sheet structure assembly which is essential in hCT fibril aggregation. Meanwhile, the heme-hCT complexes showed enhanced bioactivity compared to hCT itself after a 24 h incubation time in reducing blood calcium levels in mice. This study may develop a new strategy to reuse the wild-type hCT in clinical therapeutics.

Binding Modes of Phthalocyanines to Amyloid β Peptide and Their Effects on Amyloid Fibril Formation

The inherent tendency of proteins to convert from their native states into amyloid aggregates is associated with a range of human disorders, including Alzheimer's and Parkinson's diseases. In that sense, the use of small molecules as probes for the structural and toxic mechanism related to amyloid aggregation has become an active area of research. Compared with other compounds, the structural and molecular basis behind the inhibitory interaction of phthalocyanine tetrasulfonate (PcTS) with proteins such as αS and tau has been well established, contributing to a better understanding of the amyloid aggregation process in these proteins. We present here the structural characterization of the binding of PcTS and its Cu(II) and Zn(II)-loaded forms to the amyloid β-peptide (Aβ) and the impact of these interactions on the peptide amyloid fibril assembly. Elucidation of the PcTS binding modes to Aβ₄₀ revealed the involvement of specific aromatic and hydrophobic interactions in the formation of the Aβ₄₀-PcTS complex, ascribed to a binding mode in which the planarity and hydrophobicity of the aromatic ring system in the phthalocyanine act as main structural determinants for the interaction. Our results demonstrated that formation of the Aβ₄₀-PcTS complex does not interfere with the progression of the peptide toward the formation of amyloid fibrils. On the other hand, conjugation of Zn(II) but not Cu(II) at the center of the PcTS macrocyclic ring modified substantially the binding profile of this phthalocyanine to Aβ₄₀ and became crucial to reverse the effects of metal-free PcTS on the fibril assembly of the peptide. Overall, our results provide a firm basis to understand the structural rules directing phthalocyanine-protein interactions and their implications on the amyloid fibril assembly of the target proteins; in particular, our results contradict the hypothesis that PcTS might have similar mechanisms of action in slowing the formation of a variety of pathological aggregates.

Free Heme and Amyloid-β: A Fatal Liaison in Alzheimer's Disease

While the etiology of Alzheimer's disease (AD) is still unknown, an increased formation of amyloid-β (Aβ) peptide and oxidative processes are major pathological mechanism of the disease. The interaction of Aβ with free heme leads to the formation of peroxidase-active Aβ-heme complexes. However, enzyme-kinetic data and systematic mutational studies are still missing. These aspects were addressed in this study to evaluate the role of Aβ-heme complexes in AD. The enzyme-kinetic measurements showed peroxidase-specific pH- and H2O2-dependencies. In addition, the enzymatic activity of Aβ-heme complexes constantly increased at higher peptide excess. Moreover, the role of the Aβ sequence for the named enzymatic activity was tested, depicting human-specific R5, Y10, and H13 as essential amino acids. Also by studying Y10 as an endogenous peroxidase substrate for Aβ-heme complexes, ratio-specific effects were observed, showing an optimal dityrosine formation at an about 40-fold peptide excess. As dityrosine formation promotes Aβ fibrillation while free heme disturbs protein aggregation, we also investigated the effect of Aβ-heme complex-derived peroxidase activity on the formation of Aβ fibrils. The fluorescence measurements showed a different fibrillation behavior at strong peroxidase activity, leading also to altered fibril morphologies. The latter was detected by electron microscopy. As illustrated by selected in vivo measurements on a mouse model of AD, the disease is also characterized by Aβ-derived microvessel destructions and hemolytic processes. Thus, thrombo-hemorrhagic events are discussed as a source for free heme in brain tissue. In summary, we suggest the formation and enzymatic activity of Aβ-heme complexes as pathological key features of AD.

Photoactive chlorin e6 is a multifunctional modulator of amyloid-β aggregation and toxicity via specific interactions with its histidine residues

Photoactive chlorin e6 selectively damage the histidine residues of amyloid-β and reduce its aggregation and toxicity even in the presence of Cu ions.

The self-assembly of Aβ to β-sheet-rich neurotoxic oligomers is a main pathological event leading to Alzheimer's disease (AD). Selective targeting of Aβ oligomers without affecting other functional proteins is therefore an attractive approach to prevent the disease and its progression. In this study, we report that photodynamic treatment of Aβ in the presence of catalytic amounts of chlorin e6 can selectively damage Aβ and inhibit its aggregation and toxicity. Chlorin e6 also reversed the amyloid aggregation process in the dark by binding its soluble and low molecular weight oligomers, as shown by thioflavin T (ThT) fluorescence and photoinduced cross-linking of unmodified protein (PICUP) methods. Using HSQC NMR spectroscopy, ThT assays, amino acid analysis, SDS/PAGE, and EPR spectroscopy, we show that catalytic amounts of photoexcited chlorin e6 selectively damage the Aβ histidine residues H6, H13, and H14, and induce Aβ cross-linking by generating singlet oxygen. In contrast, photoexcited chlorin e6 was unable to cross-link ubiquitin and α-synuclein, demonstrating its high selectivity for Aβ. By binding to the Aβ histidine residues, catalytic amounts of chlorin e6 can also inhibit the Cu²⁺-induced aggregation and toxicity in darkness, while at stoichiometric amounts it acts as a chelator to reduce the amount of free Cu²⁺. This study demonstrates the great potential of chlorin e6 as a multifunctional agent for treatment of AD, and shows that the three N-terminal Aβ histidine residues are a suitable target for Aβ-specific drugs.

Hemin and bile pigments are the secondary structure regulators of intrinsically disordered antimicrobial peptides

The interaction of protoporphyrin compounds of human origin with the major bee venom component melittin (26 a.a., Z +6) and its hybrid derivative (CM15, 15 a.a., Z +6) were studied by a combination of various spectroscopic methods. Throughout a two-state, concentration-dependent process, hemin and its metabolites (biliverdin, bilirubin, bilirubin ditaurate) increase the parallel β-sheet content of the natively unfolded melittin, suggesting the oligomerization of the peptide chains. In contrast, α-helix promoting effect was observed with the also disordered but more cationic CM15. According to fluorescence quenching experiments, the sole Trp residue of melittin is the key player during the binding, in the vicinity of which the first pigment molecule is accommodated presumably making indole-porphyrin π-π stacking interaction. As circular dichroism titration data suggest, cooperative association of additional ligands subsequently occurs, resulting in multimeric complexes with an apparent dissociation constant ranged from 20 to 65 μM. Spectroscopic measurements conducted with the bilirubin catabolite urobilin and stercobilin refer to the requirement of intact dipyrrinone moieties for inducing secondary structure transformations. The binding topography of porphyrin rings on a model parallel β-sheet motif was evaluated by absorption spectroscopy and computational modeling showing a slipped-cofacial binding mode responsible for the red shift and hypochromism of the Soret band. Our results may aid to recognize porphyrin-responsive binding motifs of biologically relevant, intrinsically disordered peptides and proteins, where transient conformations play a vital role in their functions.

TMPyP Inhibits Amyloid-β Aggregation and Alleviates Amyloid-Induced Cytotoxicity

The aggregation or misfolding of amyloid-β (Aβ) is a major pathological hallmark of Alzheimer's disease (AD). The regulation of Aβ aggregation is thought to be an effective strategy for AD treatment. The capability of a water-soluble porphyrin, 5,10,15,20-tetrakis(N-methyl-4-pyridyl)porphyrin (TMPyP), to inhibit Aβ aggregation and to lower Aβ-induced toxicity was demonstrated. As evidenced by surface plasmon resonance and circular dichroism, TMPyP can not only disrupt Aβ aggregation but also disassemble the preformed Aβ aggregates. The atomic force microscopy imaging proves that TMPyP inhibits the formation of both oligomers and fibrils. Molecular dynamic simulations provide an insight into the interaction between TMPyP and Aβ at the molecular level. The half-maximal inhibitory concentrations of TMPyP acting on the oligomers and fibrils were determined to be 0.6 and 0.43 μM, respectively. As a member of porphyrin family, TMPyP is of rather low cytotoxicity, and the cytotoxicity of the Aβ aggregates was also relieved upon coincubation with TMPyP. The excellent performance of TMPyP thus makes it a potential drug candidate for AD therapy.

N-Methyl Mesoporphyrin IX as an Effective Probe for Monitoring Alzheimer's Disease β-Amyloid Aggregation in Living Cells

Formation of amyloid fibrils by amyloid-β peptide (Aβ) is an important step in Alzheimer's disease (AD) progression. Screening and designing of new molecules which can monitor the amyloidosis process especially in cells are diagnostically and therapeutically important. Utilizing Thioflavin T (ThT), the commonly used amyloid dye, is the most standardized way to monitor amyloid. However, with the green fluorescence emission and small Stokes shift, the fluorescence of ThT can overlap with that arising from other intrinsic fluorescent components in the cells, making it not suitable for detection of protein aggregates in vivo. Therefore, it is urgent for developing amyloid probes with large Stokes shifts and red-shifted fluorescence emission to detect Aβ aggregates in cells. In this report, we found that N-methyl mesoporphyrin IX (NMM), a widely used G-quadruplex DNA specific fluorescent binder, can be an efficient probe for monitoring Aβ fibrillation in living cells. NMM is nonfluorescent in aqueous solution or monomeric Aβ environments. However, through stacking with the Aβ assemblies, NMM emits strong fluorescence. Furthermore, the large Stokes shift and stable photoluminescence make it an ideal probe for detecting Aβ aggregates in highly fluorescent environments and cell culture. Our results provide a new sight to design and screen new reagents for monitoring the diseases associated with protein conformational disorders.

Nitration of Tyrosine Residue Y10 of Aβ1-42 Significantly Inhibits Its Aggregation and Cytotoxicity

Amyloid-β plaques and oxidative stress are the major hallmarks of Alzheimer's disease. Our previous study found that the heme-Aβ complex enhanced the catalytic effect of free heme on protein tyrosine nitration in the presence of hydrogen peroxide (H₂O₂) and nitrite (NO₂^-). Y10 in Aβ could be the first target to be nitrated. We also found that nitration of Aβ_1-40 significantly decreased its aggregation. However, a contrary report showed that nitration of Aβ_1-42 by peroxynitrite enhanced its aggregation. To rule out the interference of peroxynitrite caused Aβ oxidation, we used synthetic Y10 nitrated Aβ_1-42 to study the influence of Y10 nitration on Aβ_1-42's aggregation and cytotoxicity in this study. We confirmed that Aβ_1-42 could be nitrated in the presence of H₂O₂, NO₂^-, and heme by dot blotting. CD spectroscopy showed an increase of β-sheet structure of Aβ_1-42 and its mutants. The thioflavin T (ThT) flourescence assay revealed that both nitration and chlorination significantly inhibited Aβ_1-42 fibril formation. TEM and AFM observations of Aβ peptide aggregates further confirmed that Y10 modification inhibited Aβ_1-42 fibril formation. The cytotoxicity study of native and modified Aβ peptides on SH-SY5Y cells revealed that nitration of Aβ_1-42 remarkably decreased the neurotoxicity of Aβ_1-42. On the basis of these results, we hypothesized that nitration of Y10 may block the π-π stacking interactions of Aβ_1-42 so that it inhibit its aggregation and neurotoxicity. More importantly, considerable evidence suggested that the levels of nitrite plus nitrate significantly decreased in the brain of AD patients. Thus, we believe that these findings would be helpful for further understanding the function of Aβ in AD.

Is there correlation between Aβ-heme peroxidase activity and the peptide aggregation state? A literature review combined with hypothesis

Alzheimer's disease (AD) is an age-related neurodegenerative disorder characterized by aggregation of amyloid-β (Aβ) peptide, formation of neurofibrillary tangles, synaptic loss and neuronal cell death, and is manifested clinically by progressive cognitive dysfunction and memory loss. Disease pathogenesis is mainly linked to the formation of Aβ insoluble or soluble oligomeric assemblies. Binding of heme to Aβ has been suggested as the origin of the heme deficiency, peroxidase activity, as well as some oxidative stress-mediated AD pathologies, and then differential affinity of heme for human and rodent Aβ peptide has been proposed to account for the susceptibility of humans to AD. This review highlights whether there is any dependency of peroxidase activity of heme-bound Aβ on the Aβ aggregation state or not, with focusing on emerging role of heme in neurodegeneration. Here, several lines of evidence supporting existing contradictory conjectures are discussed.

Effect of protein structure and/or conformation on the dityrosine cross-linking induced by haem-hydrogen peroxide

BACKGROUND

Haem, an essential cofactor in aerobic organisms, can cause oxidative stress and impose toxic effects on tissues and organs. It can induce aggregation of proteins via dityrosine cross-linking and cause neurodegenerative diseases. Although dityrosine cross-linking in many proteins induced by haem has been reported, not all the proteins have the same effect or the efficiency of cross-linking varies, while the reason has not been clarified.

METHODS

The correlation of protein structure/conformation with its aggregation tendency via dityrosine induced by hematin (oxidized form of haem) in the presence of hydrogen peroxide (H2O2) was studied through reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), fluorescence and circular dichroism (CD) measurements, and the mechanism was investigated by performing UV-Vis absorbance, Raman spectroscopy and low-temperature electron spin resonance (ESR) experiments.

RESULTS

It was found that proteins in unstructured state are more readily to be cross-linked via dityrosine formation by hematin-H2O2. The unstructured protein without steric effect can coordinate with hematin to form six-coordinated protein-hematin complex, in which the produced tyrosyl radicals by H2O2 are with high tendency to dimerize to form dityrosine.

CONCLUSIONS

Our results demonstrate that protein structure/conformation can affect its coordination state with haem, and the tendency of reaction of two tyrosyl radicals, further influencing the yield and efficiency of dityrosine cross-linking in the presence of H2O2.

GENERAL SIGNIFICANCE

This research can help to deepen our understanding of the protein aggregation and inactivation mechanisms in varied sophisticated conditions, and especially give us the new insight into the toxic effects under haem stress.

Key roles of Arg(5), Tyr(10) and his residues in Aβ-heme peroxidase: relevance to Alzheimer's disease

Recent reports show that heme binds to amyloid β-peptide (Aβ) in the brain of Alzheimer's disease (AD) patients and forms Aβ-heme complexes, thus leading a pathological feature of AD. However, the important biological relevance to AD etiology, resulting from human Aβ-heme peroxidase formation, was not well characterized. In this study, we used wild-type and mutated human Aβ1-16 peptides and investigated their Aβ-heme peroxidase activities. Our results indicated that both histidine residues (His(13), His(14)) in Aβ1-16 and free histidine enhanced the peroxidase activity of heme, hence His residues were essential in peroxidase activity of Aβ-heme complexes. Moreover, Arg(5) was found to be the key residue in making the Aβ1-16-heme complex as a peroxidase. Under oxidative and nitrative stress conditions, the Aβ1-16-heme complexes caused oxidation and nitration of the Aβ Tyr(10) residue through promoting peroxidase-like reactions. Therefore, these residues (Arg(5), Tyr(10) and His) were pivotal in human Aβ-heme peroxidase activity. However, three of these residues (Arg(5), Tyr(10) and His(13)) identified in this study are all absent in rodents, where rodent Aβ-heme complex lacks peroxidase activity and it does not show AD, implicating the novel significance of these residues as well as human Aβ-heme peroxidase in the pathology of AD.

New insights into Alzheimer's disease amyloid inhibition: nanosized metallo-supramolecular complexes suppress aβ-induced biosynthesis of heme and iron uptake in PC12 cells

Nanosized metallo-supramolecular compounds, [Ni2 L3 ](4+) and [Fe2 L3 ](4+) , can not only strongly inhibit Aβ aggregation but also reduce the peroxidase activity of Aβ-heme. Further studies demonstrate that through blocking the heme-binding site, these two compounds can suppress Aβ-induced biosynthesis of heme and iron uptake in PC12 cells. This work provides new insights into molecular mechanisms of Aβ inhibitors on Aβ-mediated neurotoxicity.

The iron regulatory capability of the major protein participants in prevalent neurodegenerative disorders

As with most bioavailable transition metals, iron is essential for many metabolic processes required by the cell but when left unregulated is implicated as a potent source of reactive oxygen species. It is uncertain whether the brain’s evident vulnerability to reactive species-induced oxidative stress is caused by a reduced capability in cellular response or an increased metabolic activity. Either way, dys-regulated iron levels appear to be involved in oxidative stress provoked neurodegeneration. As in peripheral iron management, cells within the central nervous system tightly regulate iron homeostasis via responsive expression of select proteins required for iron flux, transport and storage. Recently proteins directly implicated in the most prevalent neurodegenerative diseases, such as amyloid-β precursor protein, tau, α-synuclein, prion protein and huntingtin, have been connected to neuronal iron homeostatic control. This suggests that disrupted expression, processing, or location of these proteins may result in a failure of their cellular iron homeostatic roles and augment the common underlying susceptibility to neuronal oxidative damage that is triggered in neurodegenerative disease.

Electrochemical detection of β-amyloid peptides on electrode covered with N-terminus-specific antibody based on electrocatalytic O2 reduction by Aβ(1-16)-heme-modified gold nanoparticles

β-Amyloid (Aβ) peptides are believed to be important for the diagnosis and prognosis of Alzheimer's disease (AD) serving as reliable molecular biomarkers. In this work, we reported a simple and sensitive electrochemical strategy for the detection of total Aβ peptides using gold nanoparticles modified with Aβ(1-16)-heme (denoted as Aβ(1-16)-heme-AuNPs). Monoclonal antibody (mAb) specific to the common N-terminus of Aβ was immobilized onto gold electrode for the capture of Aβ(1-16)-heme-AuNPs. The anchored Aβ(1-16)-heme-AuNPs showed strong electrocatalytic O2 reduction. Pre-incubation of the mAb-covered electrode with native Aβ decreased the amount of Aβ(1-16)-heme-AuNPs immobilized onto the electrode, resulting in the decrease of the reduction current of O2 to H2O2. The competitive assay is sensitive and selective to Aβ peptides. The voltammetric responses were found to be proportional to the concentrations of Aβ ranging from 0.02 to 1.50nM, and a detection limit of 10 pM was achieved. To demonstrate the viability of the method for the analysis of Aβ in real sample, artificial cerebrospinal fluid (aCSF) containing Aβ(1-40), Aβ(1-42) and Aβ(1-16) was tested. We believe that the method would offer a useful means for quantifying Aβ in a biological matrix, and be valuable in the design of new types of electrochemical biosensors for the detection of peptides and proteins.