Mechanism of heme degradation by heme oxygenase

Electrosynthesis of Verdoheme and Biliverdin Derivatives Following Enzymatic Pathways

Artificial syntheses of biologically active molecules have been fruitful in many bioinspired catalysis applications. Specifically, verdoheme and biliverdin, bearing polypyrrole frameworks, have inspired catalyst designs to address energy and environmental challenges. Despite remarkable progress in benchtop synthesis of verdoheme and biliverdin derivatives, all reported syntheses, starting from metalloporphyrins or inaccessible biliverdin precursors, require multiple steps to achieve the final desired products. Additionally, such synthetic procedures use multiple reactants/redox agents and involve multistep purification/extraction processes that often lower the yield. However, in a single step using atmospheric oxygen, heme oxygenases selectively generate verdoheme or biliverdin from heme. Motivated by such enzymatic pathways, we report a single-step electrosynthesis of verdoheme or biliverdin derivatives from their corresponding meso-aryl-substituted metalloporphyrin precursors. Our electrosynthetic methods have produced a copper-coordinating verdoheme analog in >80% yield at an applied potential of 0.65 V vs ferrocene/ferrocenium in air-exposed acetonitrile solution with a suitable electrolyte. These electrosynthetic routes reached a maximum product yield within 8 h of electrolysis at room temperature. The major products of verdoheme and biliverdin derivatives were isolated, purified, and characterized using electrospray mass spectrometry, absorption spectroscopy, cyclic voltammetry, and nuclear magnetic resonance spectroscopy techniques. Furthermore, X-ray crystallographic data were collected for select cobalt (Co)- and Cu-chelating verdoheme and metal-free biliverdin products. Electrosynthesis routes for the selective modification at the macrocycle ring in a single step are not known yet, and therefore, we believe that this report would advance the scopes of electrosynthesis strategies.

Intricacies of Mass Transport during Electrocatalysis: A Journey through Iron Porphyrin-Catalyzed Oxygen Reduction

Electrochemical steps are increasingly attractive for green chemistry. Understanding reactions at the electrode-solution interface, governed by kinetics and mass transport, is crucial. Traditional insights into these mechanisms are limited, but our study bridges this gap through an integrated approach combining voltammetry, electrochemical impedance spectroscopy, and electrospray ionization mass spectrometry. This technique offers real-time monitoring of the chemical processes at the electrode-solution interface, tracking changes in intermediates and products during reactions. Applied to the electrochemical reduction of oxygen catalyzed by the iron(II) tetraphenyl porphyrin complex, it successfully reveals various reaction intermediates and degradation pathways under different kinetic regimes. Our findings illuminate complex electrocatalytic processes and propose new ways for studying reactions in alternating current and voltage-pulse electrosynthesis. This advancement enhances our capacity to optimize electrochemical reactions for more sustainable chemical processes.

Radiolysis of myoglobin concentrated gels by protons: specific changes in secondary structure and production of carbon monoxide

While particle therapy has been used for decades for cancer treatment, there is still a lack of information on the molecular mechanisms of biomolecules radiolysis by accelerated ions. Here, we examine the effects of accelerated protons on highly concentrated native myoglobin, by means of Fourier transform infrared and UV-Visible spectroscopies. Upon irradiation, the secondary structure of the protein is drastically modified, from mostly alpha helices conformation to mostly beta elements at highest fluence. These changes are accompanied by significant production of carbon monoxide, which was shown to come from heme degradation under irradiation. The radiolytic yields of formation of denatured protein, carbon monoxide, and of heme degradation were determined, and found very close to each other: G+denatured Mb ≈ G+CO ≈ G-heme = 1.6 × 10-8 ± 0.1 × 10-8 mol/J = 0.16 ± 0.01 species/100 eV. The denaturation of the protein to a beta structure and the production of carbon monoxide under ion irradiation are phenomena that may play an important role in the biological effects of ionizing radiation.

Heme metabolism in nonerythroid cells

Heme is an iron-containing prosthetic group necessary for the function of several proteins termed "hemoproteins." Erythrocytes contain most of the body's heme in the form of hemoglobin and contain high concentrations of free heme. In nonerythroid cells, where cytosolic heme concentrations are 2 to 3 orders of magnitude lower, heme plays an essential and often overlooked role in a variety of cellular processes. Indeed, hemoproteins are found in almost every subcellular compartment and are integral in cellular operations such as oxidative phosphorylation, amino acid metabolism, xenobiotic metabolism, and transcriptional regulation. Growing evidence reveals the participation of heme in dynamic processes such as circadian rhythms, NO signaling, and the modulation of enzyme activity. This dynamic view of heme biology uncovers exciting possibilities as to how hemoproteins may participate in a range of physiologic systems. Here, we discuss how heme is regulated at the level of its synthesis, availability, redox state, transport, and degradation and highlight the implications for cellular function and whole organism physiology.

Improved methods for the detection of heme and protoporphyrin IX adducts and quantification of heme B from cytochrome P450 containing systems

Heme B is a critical prosthetic group for the function of numerous proteins including the cytochrome P450 (CYP) family of enzymes. CYP enzymes are involved in the metabolism of endogenous and xenobiotic molecules that are of central interest in drug development. Formation of reactive metabolites by CYPs can lead to heme modification and destruction of the enzyme. The structure of the adducted heme can provide key information on the mechanism of inactivation, which is of great interest during preclinical drug discovery. Historically, techniques to extract the modified heme or protoporphyrin IX species involved harsh extraction conditions and esterification of propionate groups to aid chromatography. We have developed a simplified extraction method and LC/MS chromatography system that does not require derivatization to quantify heme B and identify modified heme B species from multiple CYP-containing matrices. The method uses mass defect filter triggered data dependent MS2 scans to rapidly identify heme and protoporphyrin IX adducts. These methods may also be useful for the analysis of other heme variants and hemoproteins.

Changes in heme oxygenase level during development affect the adult life of Drosophila melanogaster

Heme oxygenase (HO) has been shown to control various cellular processes in both mammals and Drosophila melanogaster. Here, we investigated how changes in HO levels in neurons and glial cells during development affect adult flies, by using the TARGET Drosophila system to manipulate the expression of the ho gene. The obtained data showed differences in adult survival, maximum lifespan, climbing, locomotor activity, and sleep, which depended on the level of HO (after ho up-regulation or downregulation), the timing of expression (chronic or at specific developmental stages), cell types (neurons or glia), sex (males or females), and age of flies. In addition to ho, the effects of changing the mRNA level of the Drosophila CNC factor gene (NRF2 homolog in mammals and master regulator of HO), were also examined to compare with those observed after changing ho expression. We showed that HO levels in neurons and glia must be maintained at an appropriate physiological level during development to ensure the well-being of adults. We also found that the downregulation of ho in either neurons or glia in the brain is compensated by ho expressed in the retina.

Searching for molecular hypoxia sensors among oxygen-dependent enzymes

The ability to sense and respond to changes in cellular oxygen levels is critical for aerobic organisms and requires a molecular oxygen sensor. The prototypical sensor is the oxygen-dependent enzyme PHD: hypoxia inhibits its ability to hydroxylate the transcription factor HIF, causing HIF to accumulate and trigger the classic HIF-dependent hypoxia response. A small handful of other oxygen sensors are known, all of which are oxygen-dependent enzymes. However, hundreds of oxygen-dependent enzymes exist among aerobic organisms, raising the possibility that additional sensors remain to be discovered. This review summarizes known and potential hypoxia sensors among human O2-dependent enzymes and highlights their possible roles in hypoxia-related adaptation and diseases.

Molecular Taphonomy of Heme: Chemical Degradation of Hemin under Presumed Fossilization Conditions

The metalloporphyrin heme acts as the oxygen-complexing prosthetic group of hemoglobin in blood. Heme has been noted to survive for many millions of years in fossils. Here, we investigate its stability and degradation under various conditions expected to occur during fossilization. Oxidative, reductive, aerobic, and anaerobic conditions were studied at neutral and alkaline pH values. Elevated temperatures were applied to accelerate degradation. High-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) identified four main degradation products. The vinyl residues are oxidized to formyl and further to carboxylate groups. In the presence of air or H₂O₂, cleavage of the tetrapyrrole ring occurs, and hematinic acid is formed. The highest stability of heme was observed under anaerobic reductive conditions (half-life 9.5 days), while the lowest stability was found in the presence of H₂O₂ (half-life 1 min). We confirmed that the iron cation plays a crucial role in degradation, since protoporphyrin IX, lacking iron, remained significantly more stable. Under anaerobic, reductive conditions, the above-mentioned degradation products were not observed, suggesting a different degradation pathway. To our knowledge, this is the first molecular taphonomy study on heme, which will be useful for understanding its fate during fossilization.

Self-assembly of a fluorescent virus-like particle for imaging in tissues with high autofluorescence

Virus-like particles (VLPs) are engineered nanoparticles that mimic the properties of viruses-like high tolerance to heat and proteases-but lack a viral genome, making them non-infectious. They are easily modified chemically and genetically, making them useful in drug delivery, enhancing vaccine efficacy, gene delivery, and cancer immunotherapy. One such VLP is Qβ, which has an affinity towards an RNA hairpin structure found in its viral RNA that drives the self-assembly of the capsid. It is possible to usurp the native way infectious Qβ self-assembles to encapsidate its RNA to place enzymes inside the VLP's lumen as a protease-resistant cage. Further, using RNA templates that mimic the natural self-assembly of the native capsid, fluorescent proteins (FPs) have been placed inside VLPs in a "one pot" expression system. Autofluorescence in tissues can lead to misinterpretation of results and unreliable science, so we created a single-pot expression system that uses the fluorescent protein smURFP, which avoids autofluorescence and has spectral properties compatible with standard commercial filter sets on confocal microscopes. In this work, we were able to simplify the existing "one-pot" expression system while creating high-yielding fluorescent VLP nanoparticles that could easily be imaged inside lung epithelial tissue.

The dual role of heme oxygenase in regulating apoptosis in the nervous system of Drosophila melanogaster

Accumulating evidence from mammalian studies suggests the dual-faced character of heme oxygenase (HO) in oxidative stress-dependent neurodegeneration. The present study aimed to investigate both neuroprotective and neurotoxic effects of heme oxygenase after the ho gene chronic overexpression or silencing in neurons of Drosophila melanogaster. Our results showed early deaths and behavioral defects after pan-neuronal ho overexpression, while survival and climbing in a strain with pan-neuronal ho silencing were similar over time with its parental controls. We also found that HO can be pro-apoptotic or anti-apoptotic under different conditions. In young (7-day-old) flies, both the cell death activator gene (hid) expression and the initiator caspase Dronc activity increased in heads of flies when ho expression was changed. In addition, various expression levels of ho produced cell-specific degeneration. Dopaminergic (DA) neurons and retina photoreceptors are particularly vulnerable to changes in ho expression. In older (30-day-old) flies, we did not detect any further increase in hid expression or enhanced degeneration, however, we still observed high activity of the initiator caspase. In addition, we used curcumin to further show the involvement of neuronal HO in the regulation of apoptosis. Under normal conditions, curcumin induced both the expression of ho and hid, which was reversed after exposure to high-temperature stress and when supplemented in flies with ho silencing. These results indicate that neuronal HO regulates apoptosis and this process depends on ho expression level, age of flies, and cell type.

Crystal structure and reaction mechanism of a bacterial Mg-dechelatase homolog from the Chloroflexi Anaerolineae

Chlorophyll degradation plays a myriad of physiological roles in photosynthetic organisms, including acclimation to light environment and nutrient remobilization during senescence. Mg extraction from chlorophyll a is the first and committed step of the chlorophyll degradation pathway. This reaction is catalyzed by the Mg-dechelatase enzyme encoded by Stay-Green (SGR). The reaction mechanism of SGR protein remains elusive since metal ion extraction from organic molecules is not a common enzymatic reaction. Additionally, experimentally derived structural information about SGR or its homologs has not yet been reported. In this study, the crystal structure of the SGR homolog from Anaerolineae bacterium was determined using the molecular replacement method at 1.85 Å resolution. Our previous study showed that three residues-H32, D34, and D62 are essential for the catalytic activity of the enzyme. Biochemical analysis involving mutants of D34 residue further strengthened its importance in the functioning of the dechelatase. Docking simulation also revealed the interaction between the D34 side chain and central Mg ion of chlorophyll a. Structural analysis showed the arrangement of D34/H32/D62 in the form of a catalytic triad that is generally found in hydrolases. The probable reaction mechanism suggests that deprotonated D34 side chain coordinates and destabilizes Mg, resulting in Mg extraction. Besides, H32 possibly acts as a general base catalyst and D62 facilitates H32 to be a better proton acceptor. Taken together, the reaction mechanism of SGR partially mirrors the one observed in hydrolases.

Secondary Brain Injury by Oxidative Stress After Cerebral Hemorrhage: Recent Advances

Intracerebral hemorrhage (ICH) is a clinical syndrome in which blood accumulates in the brain parenchyma because of a nontraumatic rupture of a blood vessel. Because of its high morbidity and mortality rate and the lack of effective therapy, the treatment of ICH has become a hot research topic. Meanwhile, Oxidative stress is one of the main causes of secondary brain injury(SBI) after ICH. Therefore, there is a need for an in-depth study of oxidative stress after ICH. This review will discuss the pathway and effects of oxidative stress after ICH and its relationship with inflammation and autophagy, as well as the current antioxidant therapy for ICH with a view to deriving better therapeutic tools or targets for ICH.

Detection of glioblastoma multiforme using quantitative molecular magnetic resonance imaging based on 5-aminolevulinic acid: in vitro and in vivo studies

OBJECTIVES

We demonstrated a novel metabolic method based on sequential administration of 5-aminolevulinic acid (ALA) and iron supplement, and ferric ammonium citrate (FAC), for glioblastoma multiforme (GBM) detection using R₂' and quantitative susceptibility mapping (QSM).

MATERIALS AND METHODS

Intra-cellular iron accumulation in glioblastoma cells treated with ALA and/or FAC was measured. Cell phantoms containing glioblastoma cells and Wistar rats bearing C₆ glioblastoma were imaged using a 3 T MRI scanner after sequential administration of ALA and FAC. The relaxivity and QSM analysis were performed on the images.

RESULTS

The intra-cellular iron deposition was significantly higher in the glioma cells with sequential treatment of ALA and FAC for 6 h compared to those treated with the controls. The relaxivity and magnetic susceptibility values of the glioblastoma cells and rat brain tumors treated with ALA + FAC (115 ± 5 s-1 for R₂', and 0.1 ± 0.02 ppm for magnetic susceptibility) were significantly higher than those treated with the controls (55 ± 18 (FAC), 45 ± 15 (ALA) s-1 for R₂', p < 0.05, and 0.03 ± 0.03 (FAC), 0.02 ± 0.02 (ALA) ppm for magnetic susceptibility, p < 0.05).

DISCUSSION

Sequential administration of ALA and iron supplements increases the iron deposition in glioblastoma cells, enabling clinical 3 T MRI to detect GBM using R₂' or QSM.

10-Dehydrogingerdione Attenuates Tramadol-Induced Nephrotoxicity by Modulating Renal Oxidative Stress, Inflammation and Apoptosis in Experimental Rats: Role of HO-1 Activation and TLR4/NF-κB/ERK Inhibition

Tramadol represents a synthetic opioid analgesic especially for mild to severe pain. Its dose must be commonly monitored according to pain status and to alleviate the appearance of any adverse effects such as renal cellular damage during its excretion. Present work aimed mainly to study the effects of tramadol intake on renal tissues and 10-dehydrogingerdione (10-DHGD) potential as a protective agent. Tramadol administration induced an increase in serum levels of urea, creatinine, uric acid, the renal immune expression of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), and caspase-3 which turned out to be decreased by 10-DHGD intake. Our results also recorded a significant increase in renal malondialdehyde (MDA), toll-like receptor 4 (TLR4), and extracellular signal-regulated protein kinase-1 (ERK1) along with glutathione (GSH), superoxide dismutase (SOD), and heme oxygenase-1 (HO-1) decrease due to tramadol intake, which were counteracted by 10-DHGD administration as illustrated and supported by the histopathological findings. Our conclusion refers to renoprotective potential of 10-DHGD against tramadol adverse effects.

Heme oxygenase-2 (HO-2) binds and buffers labile ferric heme in human embryonic kidney cells

Heme oxygenases (HOs) detoxify heme by oxidatively degrading it into carbon monoxide, iron, and biliverdin, which is reduced to bilirubin and excreted. Humans express two isoforms of HO: the inducible HO-1, which is upregulated in response to excess heme and other stressors, and the constitutive HO-2. Much is known about the regulation and physiological function of HO-1, whereas comparatively little is known about the role of HO-2 in regulating heme homeostasis. The biochemical necessity for expressing constitutive HO-2 is dependent on whether heme is sufficiently abundant and accessible as a substrate under conditions in which HO-1 is not induced. By measuring labile heme, total heme, and bilirubin in human embryonic kidney HEK293 cells with silenced or overexpressed HO-2, as well as various HO-2 mutant alleles, we found that endogenous heme is too limiting a substrate to observe HO-2-dependent heme degradation. Rather, we discovered a novel role for HO-2 in the binding and buffering of heme. Taken together, in the absence of excess heme, we propose that HO-2 regulates heme homeostasis by acting as a heme buffering factor that controls heme bioavailability. When heme is in excess, HO-1 is induced, and both HO-2 and HO-1 can provide protection from heme toxicity via enzymatic degradation. Our results explain why catalytically inactive mutants of HO-2 are cytoprotective against oxidative stress. Moreover, the change in bioavailable heme due to HO-2 overexpression, which selectively binds ferric over ferrous heme, is consistent with labile heme being oxidized, thereby providing new insights into heme trafficking and signaling.

Iron homeostasis and organismal aging

Iron is indispensable for normal body functions across species because of its critical roles in red blood cell function and many essential proteins and enzymes required for numerous physiological processes. Regulation of iron homeostasis is an intricate process involving multiple modulators at the systemic, cellular, and molecular levels. Interestingly, emerging evidence has demonstrated that many modulators of iron homeostasis contribute to organismal aging and longevity. On the other hand, the age-related dysregulation of iron homeostasis is often associated with multiple age-related pathologies including bone resorption and neurodegenerative diseases such as Alzheimer's disease. Thus, a thorough understanding on the interconnections between systemic and cellular iron balance and organismal aging may help decipher the etiologies of multiple age-related diseases, which could ultimately lead to developing therapeutic strategies to delay aging and treat various age-related diseases. Here we present the current understanding on the mechanisms of iron homeostasis. We also discuss the impacts of aging on iron homeostatic processes and how dysregulated iron metabolism may affect aging and organismal longevity.

Novel mutual prodrug of 5-fluorouracil and heme oxygenase-1 inhibitor (5-FU/HO-1 hybrid): design and preliminary in vitro evaluation

In this work, the first mutual prodrug of 5-fluorouracil and heme oxygenase1 inhibitor (5-FU/HO-1 hybrid) has been designed, synthesised, and evaluated for its in vitro chemical and enzymatic hydrolysis stability. Predicted in silico physicochemical properties of the newly synthesised hybrid (3) demonstrated a drug-like profile with suitable Absorption, Distribution, Metabolism, and Excretion (ADME) properties and low toxic liabilities. Preliminary cytotoxicity evaluation towards human prostate (DU145) and lung (A549) cancer cell lines demonstrated that 3 exerted a similar effect on cell viability to that produced by the reference drug 5-FU. Among the two tested cancer cell lines, the A549 cells were more susceptible for 3. Of note, hybrid 3 also had a significantly lower cytotoxic effect on healthy human lung epithelial cells (BEAS-2B) than 5-FU. Altogether our results served as an initial proof-of-concept to develop 5-FU/HO-1 mutual prodrugs as potential novel anticancer agents.

Significance of Heme and Heme Degradation in the Pathogenesis of Acute Lung and Inflammatory Disorders

The heme molecule serves as an essential prosthetic group for oxygen transport and storage proteins, as well for cellular metabolic enzyme activities, including those involved in mitochondrial respiration, xenobiotic metabolism, and antioxidant responses. Dysfunction in both heme synthesis and degradation pathways can promote human disease. Heme is a pro-oxidant via iron catalysis that can induce cytotoxicity and injury to the vascular endothelium. Additionally, heme can modulate inflammatory and immune system functions. Thus, the synthesis, utilization and turnover of heme are by necessity tightly regulated. The microsomal heme oxygenase (HO) system degrades heme to carbon monoxide (CO), iron, and biliverdin-IXα, that latter which is converted to bilirubin-IXα by biliverdin reductase. Heme degradation by heme oxygenase-1 (HO-1) is linked to cytoprotection via heme removal, as well as by activity-dependent end-product generation (i.e., bile pigments and CO), and other potential mechanisms. Therapeutic strategies targeting the heme/HO-1 pathway, including therapeutic modulation of heme levels, elevation (or inhibition) of HO-1 protein and activity, and application of CO donor compounds or gas show potential in inflammatory conditions including sepsis and pulmonary diseases.

Neuroprotection of Heme Oxygenase-2 in Mice AfterIntracerebral Hemorrhage

There are few effective preventive or therapeutic strategies to mitigate the effects of catastrophic intracerebral hemorrhage (ICH) in humans. Heme oxygenase is the rate-limiting enzyme in heme metabolism; heme oxygenase-2 (HO-2) is a constitutively expressed heme oxygenase. We explored the involvement of HO-2 in a collagenase-induced mouse model of ICH in C57BL/6 wild-type and HO-2 knockout mice. We assessed oxidative stress injury, blood-brain barrier permeability, neuronal damage, late-stage angiogenesis, and hematoma clearance using immunofluorescence, Western blot, MRI, and special staining methods. Our results show that HO-2 reduces brain injury volume and brain edema, alleviates cytotoxic injury, affects vascular function in the early stage of ICH, and improves hematoma absorbance and angiogenesis in the late stage of ICH in this model. Thus, we found that HO-2 has a protective effect on brain injury after ICH.

Carbon Monoxide: from Poison to Clinical Trials

Every cell has a highly sophisticated system for regulating heme levels, which is particularly important with regard to turnover. Heme degradation generates CO and while CO has long been viewed as a metabolic waste product, and at higher concentrations cellularly lethal, we now know that CO is an indispensable gasotransmitter that participates in fundamental physiological processes necessary for survival. Irrefutable preclinical data have resulted in concerted efforts to develop CO as a safe and effective therapeutic agent, but against this notion lies dogma that CO is a poison, especially to the brain. The emergence of this debate is discussed here highlighting the neuroprotective properties of CO through its role on the central circadian clock and ongoing strategies being developed for CO administration for clinical use.

Removal of hard COD from acidic eucalyptus kraft pulp bleach plant effluent streams using oxidoreductases

The bleach plant of a pulp and paper (P&P) mill presents a major source of wastewater containing toxic organic matter characterized as chemical oxygen demand (COD). Due to their high oxidizing power, oxidoreductases hold promise to be a key solution for the removal of dissolved organic material. Here, four oxidoreductases from different enzyme families were selected to treat bleach plant effluents. Haloperoxidase treatment of the final effluent resulted in the highest levels of decolorization (71%) and reduction of aromatic compounds (36%). Using single compound analysis, 27 low molecular weight compounds were found to be persistent throughout the wastewater treatment process and, therefore, classified as hard COD. The tested enzymes efficiently removed several of the identified COD compounds. Hence, this study suggests that the application of oxidoreductases will serve as an environmental-friendly solution for reducing waste from P&P production.

Novel Heme Oxygenase-1 (HO-1) Inducers Based on Dimethyl Fumarate Structure

Novel heme oxygenase-1 (HO-1) inducers based on dimethyl fumarate (DMF) structure are reported in this paper. These compounds are obtained by modification of the DMF backbone. Particularly, maintaining the α, β-unsaturated dicarbonyl function as the central chain crucial for HO-1 induction, different substituted or unsubstituted phenyl rings are introduced by means of an ester or amide linkage. Symmetric and asymmetric derivatives are synthesized. All compounds are tested on a human hepatic stellate cell line LX-2 to assay their capacity for modifying HO-1 expression. Compounds 1b, 1l and 1m stand out for their potency as HO-1 inducers, being 2–3 fold more active than DMF, and for their ability to reverse reactive oxygen species (ROS) production mediated using palmitic acid (PA). These properties, coupled with a low toxicity toward LX-2 cell lines, make these compounds potentially useful for treatment of diseases in which HO-1 overexpression may counteract inflammation, such as hepatic fibrosis. Docking studies show a correlation between predicted binding free energy and experimental HO-1 expression data. These preliminary results may support the development of new approaches in the management of liver fibrosis.

Recent Advances in the Understanding of the Reaction Chemistries of the Heme Catabolizing Enzymes HO and BVR Based on High Resolution Protein Structures

In mammals, catabolism of the heme group is indispensable for life. Heme is first cleaved by the enzyme Heme Oxygenase (HO) to the linear tetrapyrrole Biliverdin IXα (BV), and BV is then converted into bilirubin by Biliverdin Reductase (BVR). HO utilizes three Oxygen molecules (O2) and seven electrons supplied by NADPH-cytochrome P450 oxidoreductase (CPR) to open the heme ring and BVR reduces BV through the use of NAD(P)H. Structural studies of HOs, including substrate-bound, reaction intermediate-bound, and several specific inhibitor-bound forms, reveal details explaining substrate binding to HO and mechanisms underlying-specific HO reaction progression. Cryo-trapped structures and a time-resolved spectroscopic study examining photolysis of the bond between the distal ligand and heme iron demonstrate how CO, produced during the HO reaction, dissociates from the reaction site with a corresponding conformational change in HO. The complex structure containing HO and CPR provides details of how electrons are transferred to the heme-HO complex. Although the tertiary structure of BVR and its complex with NAD+ was determined more than 10 years ago, the catalytic residues and the reaction mechanism of BVR remain unknown. A recent crystallographic study examining cyanobacterial BVR in complex with NADP+ and substrate BV provided some clarification regarding these issues. Two BV molecules are bound to BVR in a stacked manner, and one BV may assist in the reductive catalysis of the other BV. In this review, recent advances illustrated by biochemical, spectroscopic, and crystallographic studies detailing the chemistry underlying the molecular mechanism of HO and BVR reactions are presented.

Heme Oxygenases: Cellular Multifunctional and Protective Molecules against UV-Induced Oxidative Stress

Ultraviolet (UV) irradiation can be considered as a double-edged sword: not only is it a crucial environmental factor that can cause skin-related disorders but it can also be used for phototherapy of skin diseases. Inducible heme oxygenase-1 (HO-1) in response to a variety of stimuli, including UV exposure, is vital to maintain cell homeostasis. Heme oxygenase-2 (HO-2), another member of the heme oxygenase family, is constitutively expressed. In this review, we discuss how heme oxygenase (HO), a vital rate-limiting enzyme, participates in heme catabolism and cytoprotection. Phylogenetic analysis showed that there may exist a functional differentiation between HO-1 and HO-2 during evolution. Furthermore, depending on functions in immunomodulation and antioxidation, HO-1 participates in disease progression, especially in pathogenesis of skin diseases, such as vitiligo and psoriasis. To further investigate the particular role of HO-1 in diseases, we summarized the profile of the HO enzyme system and its related signaling pathways, such as Nrf2 and endoplasmic reticulum crucial signaling, both known to regulate HO-1 expression. Furthermore, we report on a C-terminal truncation of HO-1, which is generally considered as a signal molecule. Also, a newly identified alternative splice isoform of HO-1 not only provides us a novel perspective on comprehensive HO-1 alternative splicing but also offers us a basis to clarify the relationship between HO-1 transcripts and oxidative diseases. To conclude, the HO system is not only involved in heme catabolism but also involved in biological processes related to the pathogenesis of certain diseases, even though the mechanism of disease progression still remains sketchy. Further understanding the role of the HO system and its relationship to UV is helpful for revealing the HO-related signaling networks and the pathogenesis of many diseases.

Theoretical study of first row transitional metals effects on stabilization of verdoheme analogues

Heme catabolism is an important physiological process that converts heme to biliverdin in the presence of heme oxygenase which has an essential role in destroying unwanted heme. Verdohemes, the green iron (II) complexes of the 5-oxaporphyrin macrocycle are produced by oxidative destruction of heme. The main goal of this study is clarification of the central metal effect on stabilization of metal 5-oxaporphyrin molecules. To investigate the role of central metal on geometric and electronic properties of five coordinated verdoheme analogues, the first row transitional metals, including Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, as the central metal of five-coordinated metal 5-oxaporphyrins were systematically calculated without any symmetry constraint by using the B3LYP as DFT method and the 6-31G basis set in gas and solvent phases. According to the results, the stabilization energy of metal 5-oxaporphyrins increases with atomic mass in the solvent phase more than in the gas phase. By reviewing the properties such as the computed frontier orbital energy, HOMO and LUMO gap energy [Formula: see text], hardness [Formula: see text], chemical potential [Formula: see text], softness (s) and electrophilicity [Formula: see text], the pharmaceutical use of this compound can be discussed.

Synthesis of Hydroxyisooxophlorins by Oxidative Degradation of meso-Hydroxyporphyrins

Hydroxyisooxophlorin is an oxidatively degraded porphyrin that contains one carbonyl carbon, one hydroxymethylene group, and two sp²-methine groups at its meso positions. Here we report that oxidation of a free-base meso-hydroxyporphyrin with [bis(trifluoroacetoxy)iodo]benzene in the presence of H₂O afforded two regioisomers of hydroxyisooxophlorins, 10-hydroxy-10-iso-5-oxophlorin and 15-hydroxy-15-iso-5-oxophlorin, as major and minor products, respectively. We also examined the reaction mechanism, acid-mediated isomerization, metal complexation behavior, and physical properties of these molecules.

5-Aminolevulinic acid with ferrous iron improves early renal damage and hepatic steatosis in high fat diet-induced obese mice

5-Aminolevulinic acid, a natural amino acid, activates mitochondrial respiration and induces heme oxygenase-1 expression. Obesity and type 2 diabetes mellitus are associated with age-related mitochondrial respiration defect, oxidative stress and inflammation. The aim of this study is to investigate the effects of 5-aminolevulinic acid with sodium ferrous citrate on early renal damage and hepatic steatosis. 7-Month-old C57BL/6 mice were fed with a standard diet or high fat diet for 9 weeks, which were orally administered 300 mg/kg 5-aminolevulinic acid combined with 47 mg/kg sodium ferrous citrate (5-aminolevulinic acid/sodium ferrous citrate) or vehicle for the last 5 weeks. We observed that 5-aminolevulinic acid/sodium ferrous citrate significantly decreased body weight, fat weight, hepatic lipid deposits and improved levels of blood glucose and oral glucose tolerance test. In addition, 5-aminolevulinic acid/sodium ferrous citrate suppressed increased glomerular tuft area in high fat diet-fed mice, which was associated with increased heme oxygenase-1 protein expression. Our findings demonstrate additional evidence that 5-aminolevulinic acid/sodium ferrous citrate could improve glucose and lipid metabolism in diabetic mice. 5-Aminolevulinic acid/sodium ferrous citrate has potential application in obesity or type 2 diabetes mellitus-associated disease such as diabetic nephropathy and nonalcoholic fatty liver disease.

Density functional theory studies on the conversion of hydroxyheme to iron-verdoheme in the presence of dioxygen

Detailed insight into the second step of heme degradation by heme oxygenase, oxophlorin to verdoheme and biliverdin, is presented. Density functional theory methods are reported for the conversion of oxophlorin to verdoheme. Since it is currently unclear whether dioxygen binding to iron oxophlorin is followed by a reduction or not, in this work we have focused on the difference in reactivity between [(Im)(O₂˙)Fe^III(PO˙)] (PO˙ is the oxophlorin dianion radical) and [(Im)(O₂˙)Fe^III(PO)]^- (PO is the oxophlorin trianion). Thus, we have shown that in [(Im)(O₂˙)Fe^III(PO˙)] and [(Im)(O₂˙)Fe^III(PO)]^-, the mechanisms are stepwise with an initial C-O bond activation to form a ring-structure where the oxophlorin is distorted from planarity. This is followed by homolytic dioxygen bond breaking that directly leads to iron-oxo verdoheme products. The [(Im)(O₂˙)Fe^III(PO˙)] mechanism proceeds via two-state-reactivity patterns on the adjacent doublet and quartet spin state surfaces, whereas the [(Im)(O₂˙)Fe^III(PO)]^- route shows single-state-reactivity on a triplet spin state surface. In both, the rate determining step is the C-O bond activation, with substantially lower barriers on the [(Im)(O₂˙)Fe^III(PO˙)] surface of 12.15 kcal mol^-1 in the gas phase compared to 22.55 kcal mol^-1 for the intermediate-spin of [(Im)(O₂˙)Fe^III(PO)]^-. The complete active space self-consistent-field wave functions with second-order multi-reference perturbation theory were also studied. Finally, the effects of the solvent and the medium on the reaction barriers were tested and shown to be considerable.

Interaction between Mitochondrial Reactive Oxygen Species, Heme Oxygenase, and Nitric Oxide Synthase Stimulates Phagocytosis in Macrophages

Background

Macrophages are cells of the innate immune system that populate every organ. They are required not only for defense against invading pathogens and tissue repair but also for maintenance of tissue homeostasis and iron homeostasis.

Aim

The aim of this study is to understand whether heme oxygenase (HO) and nitric oxide synthase (NOS) contribute to the regulation of nicotinamide adenine dinucleotide phosphate oxidase (NOX) activity and phagocytosis, two key components of macrophage function.

Methods

This study was carried out using resting J774A.1 macrophages treated with hemin or vehicle. Activity of NOS, HO, or NOX was inhibited using specific inhibitors. Reactive oxygen species (ROS) formation was determined by Amplex^® red assay, and phagocytosis was measured using fluorescein isothiocyanate-labeled bacteria. In addition, we analyzed the fate of the intracellular heme by using electron spin resonance.

Results

We show that both enzymes NOS and HO are essential for phagocytic activity of macrophages. NOS does not directly affect phagocytosis, but stimulates NOX activity via nitric oxide-triggered ROS production of mitochondria. Treatment of macrophages with hemin results in intracellular accumulation of ferrous heme and an inhibition of phagocytosis. In contrast to NOS, HO products, including carbon monoxide, neither clearly affect NOX activity nor clearly affect phagocytosis, but phagocytosis is accelerated by HO-mediated degradation of heme.

Conclusion

Both enzymes contribute to the bactericidal activity of macrophages independently, by controlling different pathways.

Atherothrombosis and Oxidative Stress: Mechanisms and Management in Elderly

SIGNIFICANCE

The incidence of cardiovascular events (CVEs) increases with age, representing the main cause of death in an elderly population. Aging is associated with overproduction of reactive oxygen species (ROS), which may affect clotting and platelet activation, and impair endothelial function, thus predisposing elderly patients to thrombotic complications. Recent Advances: There is increasing evidence to suggest that aging is associated with an imbalance between oxidative stress and antioxidant status. Thus, upregulation of ROS-producing enzymes such as nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and myeloperoxidase, along with downregulation of antioxidant enzymes, such as superoxide dismutase and glutathione peroxidase, occurs during aging. This imbalance may predispose to thrombosis by enhancing platelet and clotting activation and eliciting endothelial dysfunction. Recently, gut-derived products, such as trimethylamine N-oxide (TMAO) and lipopolysaccharide, are emerging as novel atherosclerotic risk factors, and gut microbiota composition has been shown to change by aging, and may concur with the increased cardiovascular risk in the elderly.

CRITICAL ISSUES

Antioxidant treatment is ineffective in patients at risk or with cardiovascular disease. Further, anti-thrombotic treatment seems to work less in the elderly population.

FUTURE DIRECTIONS

Interventional trials with antioxidants targeting enzymes implicated in aging-related atherothrombosis are warranted to explore whether modulation of redox status is effective in lowering CVEs in the elderly. Antioxid. Redox Signal. 27, 1083-1124.

Synthesis and structures of photoactive rhenium carbonyl complexes derived from 2-(pyridin-2-yl)-1,3-benzothiazole, 2-(quinolin-2-yl)-1,3-benzothiazole and 1,10-phenanthroline

Carbon monoxide (CO) has recently been identified as a gaseous signaling molecule that exerts various salutary effects in mammalian pathophysiology. Photoactive metal carbonyl complexes (photoCORMs) are ideal exogenous candidates for more controllable and site-specific CO delivery compared to gaseous CO. Along this line, our group has been engaged for the past few years in developing group-7-based photoCORMs towards the efficient eradication of various malignant cells. Moreover, several such complexes can be tracked within cancerous cells by virtue of their luminescence. The inherent luminecscent nature of some photoCORMs and the change in emission wavelength upon CO release also provide a covenient means to track the entry of the prodrug and, in some cases, both the entry and CO release from the prodrug. In continuation of the research circumscribing the development of trackable photoCORMs and also to graft such molecules covalently to conventional delivery vehicles, we report herein the synthesis and structures of three rhenium carbonyl complexes, namely, fac-tricarbonyl[2-(pyridin-2-yl)-1,3-benzothiazole-κ²N,N'](4-vinylpyridine-κN)rhenium(I) trifluoromethanesulfonate, [Re(C₇H₇N)(C₁₂H₈N₂S)(CO)₃](CF₃SO₃), (1), fac-tricarbonyl[2-(quinolin-2-yl)-1,3-benzothiazole-κ²N,N'](4-vinylpyridine-κN)rhenium(I) trifluoromethanesulfonate, [Re(C₇H₇N)(C₁₆H₁₀N₂S)(CO)₃](CF₃SO₃), (2), and fac-tricarbonyl[1,10-phenanthroline-κ²N,N'](4-vinylpyridine-κN)rhenium(I) trifluoromethanesulfonate, [Re(C₇H₇N)(C₁₂H₈N₂)(CO)₃](CF₃SO₃), (3). In all three complexes, the Re^I center resides in a distorted octahedral coordination environment. These complexes exhibit CO release upon exposure to low-power UV light. The apparent CO release rates of the complexes have been measured to assess their comparative CO-donating capacity. The three complexes are highly luminescent and this in turn provides a convenient way to track the entry of the prodrug molecules within biological targets.

Eradication of HT-29 colorectal adenocarcinoma cells by controlled photorelease of CO from a CO-releasing polymer (photoCORP-1) triggered by visible light through an optical fiber-based device

The gaseous signaling molecule carbon monoxide (CO) has recently been recognized for its wide range of physiological activity as well as its antineoplastic properties. However, site-specific delivery of this noxious gas presents a major challenge in hospital settings. In this work, a visible light-sensitive CO-releasing molecule (photoCORM) derived from manganese(I) and 2-(quinolyl)benzothiazole (qbt) namely, [Mn(CO)₃(qbt)(4-vpy)](CF₃SO₃) (1), has been co-polymerized within a gas-permeable HEMA/EGDMA hydrogel. The resulting photoactive CO-releasing polymer (photoCORP-1) incorporates 1 such that neither the carbonyl complex nor its photoproduct(s) exits the polymer at any time. The material can be triggered to photorelease CO remotely by low-power broadband visible light (<1mWcm^-2) with the aid of fiber optics technology. The CO photorelease rates of photoCORP-1 (determined by spectrophotometry) can be modulated by both the concentration of 1 in the hydrogel and the intensity of the light. A CO-delivery device has been assembled to deliver CO to a suspension of human colorectal adenocarcinoma cells (HT-29) under the control of visible light and the extent of CO-induced apoptotic death of the cancer cells has been determined via Annexin V/Propidium iodide stain and flow cytometry. This photoactive CO-releasing polymer could find use in delivering controlled doses of CO to cellular targets such as malignant tissues in remote parts of the body.

The iron chaperone poly(rC)-binding protein 2 forms a metabolon with the heme oxygenase 1/cytochrome P450 reductase complex for heme catabolism and iron transfer

Mammals incorporate a major proportion of absorbed iron as heme, which is catabolized by the heme oxygenase 1 (HO1)-NADPH-cytochrome P450 reductase (CPR) complex into biliverdin, carbon monoxide, and ferrous iron. Moreover, intestinal iron is incorporated as ferrous iron, which is transported via the iron importer, divalent metal transporter 1 (DMT1). Recently, we demonstrated that the iron chaperone poly(rC)-binding protein 2 (PCBP2) can directly receive ferrous iron from DMT1 or transfer iron to the iron exporter, ferroportin 1. To promote intracellular iron flux, an iron chaperone may be essential for receiving iron generated by heme catabolism, but this hypothesis is untested so far. Herein, we demonstrate that HO1 binds to PCBP2, but not to other PCBP family members, namely PCBP1, PCBP3, or PCBP4. Interestingly, HO1 formed a complex with either CPR or PCBP2, and it was demonstrated that PCBP2 competes with CPR for HO1 binding. Using PCBP2-deletion mutants, we demonstrated that the PCBP2 K homology 3 domain is important for the HO1/PCBP2 interaction. In heme-loaded cells, heme prompted HO1-CPR complex formation and decreased the HO1/PCBP2 interaction. Furthermore, in vitro reconstitution experiments with purified recombinant proteins indicated that HO1 could bind to PCBP2 in the presence of heme, whereas loading of PCBP2 with ferrous iron caused PCBP2 to lose its affinity for HO1. These results indicate that ferrous iron released from heme can be bound by PCBP2 and suggest a model for an integrated heme catabolism and iron transport metabolon.

Copper porphyrin catalyzed esterification of C(sp3)–H via a cross-dehydrogenative coupling reaction

Copper porphyrin was an efficient catalyst for the direct esterification of C(sp³)–H bonds using DTBP as an oxidant.

Heme Oxygenases in Cardiovascular Health and Disease

Heme oxygenases are composed of two isozymes, Hmox1 and Hmox2, that catalyze the degradation of heme to carbon monoxide (CO), ferrous iron, and biliverdin, the latter of which is subsequently converted to bilirubin. While initially considered to be waste products, CO and biliverdin/bilirubin have been shown over the last 20 years to modulate key cellular processes, such as inflammation, cell proliferation, and apoptosis, as well as antioxidant defense. This shift in paradigm has led to the importance of heme oxygenases and their products in cell physiology now being well accepted. The identification of the two human cases thus far of heme oxygenase deficiency and the generation of mice deficient in Hmox1 or Hmox2 have reiterated a role for these enzymes in both normal cell function and disease pathogenesis, especially in the context of cardiovascular disease. This review covers the current knowledge on the function of both Hmox1 and Hmox2 at both a cellular and tissue level in the cardiovascular system. Initially, the roles of heme oxygenases in vascular health and the regulation of processes central to vascular diseases are outlined, followed by an evaluation of the role(s) of Hmox1 and Hmox2 in various diseases such as atherosclerosis, intimal hyperplasia, myocardial infarction, and angiogenesis. Finally, the therapeutic potential of heme oxygenases and their products are examined in a cardiovascular disease context, with a focus on how the knowledge we have gained on these enzymes may be capitalized in future clinical studies.

Heme Oxygenase-1 in Kidney Health and Disease

SIGNIFICANCE

Acute kidney injury (AKI) and chronic kidney disease (CKD) represent a considerable burden in healthcare. The heme oxygenase (HO) system plays an important role in regulating oxidative stress and is protective in a variety of human and animal models of kidney disease. Preclinical studies of the HO system have led to the development of several clinical trials targeting the enzyme or its products.

RECENT ADVANCES

Connection of HO, ferritin, and other proteins involved in iron regulation has provided important insight into mechanisms of damage in AKI. Also, HO-1 expression is important in the pathogenesis of hypertension, diabetic kidney disease, and progression to end-stage renal disease.

CRITICAL ISSUES

Despite intriguing discoveries, no drugs targeting the HO system have been translated to the clinic. Meanwhile, treatments for AKI and CKD are urgently needed. Many factors have likely contributed to challenges in clinical translation, including variation in animal models, difficulties in obtaining human tissue, and complexity of the disease processes being studied.

FUTURE DIRECTIONS

The HO system represents a promising avenue of investigation that may lead to targeted therapeutics. Tissue-specific gene modulation, widening the scope of animal studies, and continued clinical research will provide valuable insight into the role HO plays in kidney homeostasis and disease. Antioxid. Redox Signal. 25, 165-183.

Targeting heme oxygenase-1 and carbon monoxide for therapeutic modulation of inflammation

The heme oxygenase-1 (HO-1) enzyme system remains an attractive therapeutic target for the treatment of inflammatory conditions. HO-1, a cellular stress protein, serves a vital metabolic function as the rate-limiting step in the degradation of heme to generate carbon monoxide (CO), iron, and biliverdin-IXα (BV), the latter which is converted to bilirubin-IXα (BR). HO-1 may function as a pleiotropic regulator of inflammatory signaling programs through the generation of its biologically active end products, namely CO, BV and BR. CO, when applied exogenously, can affect apoptotic, proliferative, and inflammatory cellular programs. Specifically, CO can modulate the production of proinflammatory or anti-inflammatory cytokines and mediators. HO-1 and CO may also have immunomodulatory effects with respect to regulating the functions of antigen-presenting cells, dendritic cells, and regulatory T cells. Therapeutic strategies to modulate HO-1 in disease include the application of natural-inducing compounds and gene therapy approaches for the targeted genetic overexpression or knockdown of HO-1. Several compounds have been used therapeutically to inhibit HO activity, including competitive inhibitors of the metalloporphyrin series or noncompetitive isoform-selective derivatives of imidazole-dioxolanes. The end products of HO activity, CO, BV and BR may be used therapeutically as pharmacologic treatments. CO may be applied by inhalation or through the use of CO-releasing molecules. This review will discuss HO-1 as a therapeutic target in diseases involving inflammation, including lung and vascular injury, sepsis, ischemia-reperfusion injury, and transplant rejection.

Spectroscopic studies reveal that the heme regulatory motifs of heme oxygenase-2 are dynamically disordered and exhibit redox-dependent interaction with heme

Heme oxygenase (HO) catalyzes a key step in heme homeostasis: the O2- and NADPH-cytochrome P450 reductase-dependent conversion of heme to biliverdin, Fe, and CO through a process in which the heme participates both as a prosthetic group and as a substrate. Mammals contain two isoforms of this enzyme, HO2 and HO1, which share the same α-helical fold forming the catalytic core and heme binding site, as well as a membrane spanning helix at their C-termini. However, unlike HO1, HO2 has an additional 30-residue N-terminus as well as two cysteine-proline sequences near the C-terminus that reside in heme regulatory motifs (HRMs). While the role of the additional N-terminal residues of HO2 is not yet understood, the HRMs have been proposed to reversibly form a thiol/disulfide redox switch that modulates the affinity of HO2 for ferric heme as a function of cellular redox poise. To further define the roles of the N- and C-terminal regions unique to HO2, we used multiple spectroscopic techniques to characterize these regions of the human HO2. Nuclear magnetic resonance spectroscopic experiments with HO2 demonstrate that, when the HRMs are in the oxidized state (HO2(O)), both the extra N-terminal and the C-terminal HRM-containing regions are disordered. However, protein NMR experiments illustrate that, under reducing conditions, the C-terminal region gains some structure as the Cys residues in the HRMs undergo reduction (HO2(R)) and, in experiments employing a diamagnetic protoporphyrin, suggest a redox-dependent interaction between the core and the HRM domains. Further, electron nuclear double resonance and X-ray absorption spectroscopic studies demonstrate that, upon reduction of the HRMs to the sulfhydryl form, a cysteine residue from the HRM region ligates to a ferric heme. Taken together with EPR measurements, which show the appearance of a new low-spin heme signal in reduced HO2, it appears that a cysteine residue(s) in the HRMs directly interacts with a second bound heme.

A globin domain in a neuronal transmembrane receptor of Caenorhabditis elegans and Ascaris suum: molecular modeling and functional properties

We report the structural and biochemical characterization of GLB-33, a putative neuropeptide receptor that is exclusively expressed in the nervous system of the nematode Caenorhabditis elegans. This unique chimeric protein is composed of a 7-transmembrane domain (7TM), GLB-33 7TM, typical of a G-protein-coupled receptor, and of a globin domain (GD), GLB-33 GD. Comprehensive sequence similarity searches in the genome of the parasitic nematode, Ascaris suum, revealed a chimeric protein that is similar to a Phe-Met-Arg-Phe-amide neuropeptide receptor. The three-dimensional structures of the separate domains of both species and of the full-length proteins were modeled. The 7TM domains of both proteins appeared very similar, but the globin domain of the A. suum receptor surprisingly seemed to lack several helices, suggesting a novel truncated globin fold. The globin domain of C. elegans GLB-33, however, was very similar to a genuine myoglobin-type molecule. Spectroscopic analysis of the recombinant GLB-33 GD showed that the heme is pentacoordinate when ferrous and in the hydroxide-ligated form when ferric, even at neutral pH. Flash-photolysis experiments showed overall fast biphasic CO rebinding kinetics. In its ferrous deoxy form, GLB-33 GD is capable of reversibly binding O2 with a very high affinity and of reducing nitrite to nitric oxide faster than other globins. Collectively, these properties suggest that the globin domain of GLB-33 may serve as a highly sensitive oxygen sensor and/or as a nitrite reductase. Both properties are potentially able to modulate the neuropeptide sensitivity of the neuronal transmembrane receptor.

4-Aminoquinoline-hybridization en route towards the development of rationally designed antimalarial agents

Recent developments in 4-aminoquinoline-hybridization, as an attractive strategy for averting and delaying the drug resistance along with improvement in efficacy of new antimalarials, are described.

Pitfalls in bromination reactions of zinc porphyrins: two-sided ring opening of the porphyrin macrocycle

Reaction of [Zn(II)(TTP)] (1) (TTP = dianion of 5,10,15,20-meso-tetrakis(p-tolyl)porphyrin) with 16 equiv of N-bromosuccinamide (NBS) in methanol at reflux led to the unexpected two-sidedd open-ring brominated product [Zn(II)(C26H20N2O2Br5)2] (2). Similar observations have been made with other meso-substituted zinc porphyrins as well [Zn(II)(por)] {por = dianion of 5,10,15,20-meso-tetrakis(aryl)porphyrin; aryl = phenyl (TPP), p-(t)Bu-phenyl (TBPP), m-Cl-phenyl (TClPP)}. The respective products [Zn(II)(C24H16N2O2Br5)2] (3), [Zn(II)(C32H32N2O2Br5)2] (4), and [Zn(II)(C24H14N2O2Cl2Br5)2] (5) have been isolated in good to moderate yields and characterized by elemental analysis and UV-vis, (1)H NMR, and mass spectrometry. Additional bromination reaction of 1 with 8 equiv of NBS in a chloroform/methanol mixture led (after the two-sided ring opening) to nonmetalated brominated bi(pyrrole) product, C36H34N2O4Br4 (6). The detailed structures of complexes 1, 2, 3, and 6, available in a single crystal form, have been confirmed by X-ray diffraction analysis.

A review on hemeoxygenase-2: focus on cellular protection and oxygen response

Hemeoxygenase (HO) system is responsible for cellular heme degradation to biliverdin, iron, and carbon monoxide. Two isoforms have been reported to date. Homologous HO-1 and HO-2 are microsomal proteins with more than 45% residue identity, share a similar fold and catalyze the same reaction. However, important differences between isoforms also exist. HO-1 isoform has been extensively studied mainly by its ability to respond to cellular stresses such as hemin, nitric oxide donors, oxidative damage, hypoxia, hyperthermia, and heavy metals, between others. On the contrary, due to its apparently constitutive nature, HO-2 has been less studied. Nevertheless, its abundance in tissues such as testis, endothelial cells, and particularly in brain, has pointed the relevance of HO-2 function. HO-2 presents particular characteristics that made it a unique protein in the HO system. Since attractive results on HO-2 have been arisen in later years, we focused this review in the second isoform. We summarize information on gene description, protein structure, and catalytic activity of HO-2 and particular facts such as its cellular impact and activity regulation. Finally, we call attention on the role of HO-2 in oxygen sensing, discussing proposed hypothesis on heme binding motifs and redox/thiol switches that participate in oxygen sensing as well as evidences of HO-2 response to hypoxia.

Introduction of water into the heme distal side by Leu65 mutations of an oxygen sensor, YddV, generates verdoheme and carbon monoxide, exerting the heme oxygenase reaction

The globin-coupled oxygen sensor, YddV, is a heme-based oxygen sensor diguanylate cyclase. Oxygen binding to the heme Fe(II) complex in the N-terminal sensor domain of this enzyme substantially enhances its diguanylate cyclase activity which is conducted in the C-terminal functional domain. Leu65 is located on the heme distal side and is important for keeping the stability of the heme Fe(II)-O2 complex by preventing the entry of the water molecule to the heme complex. In the present study, it was found that (i) Escherichia coli-overexpressed and purified L65N mutant of the isolated heme-bound domain of YddV (YddV-heme) contained the verdoheme iron complex and other modified heme complexes as determined by optical absorption spectroscopy and mass spectrometry; (ii) CO was generated in the reconstituted system composed of heme-bound L65N and NADPH:cytochrome P450 reductase as confirmed by gas chromatography; (iii) CO generation of heme-bound L65N in the reconstituted system was inhibited by superoxide dismutase and catalase. In a concordance with the result, the reactive oxygen species increased the CO generation; (iv) the E. coli cells overexpressing the L65N protein of YddV-heme also formed significant amounts of CO compared to the cells overexpressing the wild type protein; (v) generation of verdoheme and CO was also observed for other mutants at Leu65 as well, but to a lesser extent. Since Leu65 mutations are assumed to introduce the water molecule into the heme distal side of YddV-heme, it is suggested that the water molecule would significantly contribute to facilitating heme oxygenase reactions for the Leu65 mutants.

Trypanosomatid essential metabolic pathway: new approaches about heme fate in Trypanosoma cruzi

Trypanosoma cruzi, the causal agent of Chagas disease, has a complex life cycle and depends on hosts for its nutritional needs. Our group has investigated heme (Fe-protoporphyrin IX) internalization and the effects on parasite growth, following the fate of this porphyrin in the parasite. Here, we show that epimastigotes cultivated with heme yielded the compounds α-meso-hydroxyheme, verdoheme and biliverdin (as determined by HPLC), suggesting an active heme degradation pathway in this parasite. Furthermore, through immunoprecipitation and immunoblotting assays of epimastigote extracts, we observed recognition by an antibody against mammalian HO-1. We also detected the localization of the HO-1-like protein in the parasite using immunocytochemistry, with antibody staining primarily in the cytoplasm. Although HO has not been described in the parasite's genome, our results offer new insights into heme metabolism in T. cruzi, revealing potential future therapeutic targets.

Nucleophilic ring opening of meso-substituted 5-oxaporphyrin by oxygen, nitrogen, sulfur, and carbon nucleophiles

Nucleophilic ring opening of 23H-[21,23-didehydro-10,15,20-tris(4-methoxycarbonylphenyl)-5-oxaporphyrinato](trifluoroacetato)zinc(II) with various nucleophiles such as alkoxide, amine, thiolate, and enolate gave 19-substituted bilinone zinc complexes, and they were isolated as free base bilinones. An X-ray crystallographic study demonstrated that the product of 5-oxaporphyrin with sodium methoxide was 21H,23H-(4Z,9Z,15Z)-1,21-dihydro-19-methoxy-5,10,15-tris(4-methoxycarbonylphenyl)bilin-1-one with a helicoidal conformation. The structure of the product of 5-oxaporphyrin with an enolate of ethyl acetoacetate was 21H,22H,24H-(4Z,9Z,15Z,19E)-19-(1-ethoxycarbonyl-2-oxopropylidene)-5,10,15-tris(4-methoxycarbonylphenyl)-1,19,21,24-tetrahydrobilin-1-one, with three inner NH groups. The product with SH(-) was also the same tautomer, 21H,22H,24H-19-thioxo-bilin-1-one, with three NH groups, while the products with RO(-), RNH2, and RS(-) nucleophiles were 21H,23H-bilin-1-ones with two inner NH groups. The first-order rate constants of the ring opening reaction of 5-oxaporphyrin with 1 M BnOH and BnSH in toluene at 303 K were 3.0 × 10(-4) and 6.1 × 10(-4) s(-1), respectively. The ratio of the rate of alcohol to thiol was much higher than that with methyl iodide, suggesting that 5-oxaporphyrin reacted as a hard electrophile in comparison to methyl iodide. UV-visible spectra of 19-substituted bilinones in CHCl3 at 298 K showed that the absorption maximum of the lower energy band was red-shifted in increasing order of O-substituted (645 nm), S-substituted (668 nm), N-substituted (699 nm), and C-substituted bilinones (706 nm).

Quinoline drug-heme interactions and implications for antimalarial cytostatic versus cytocidal activities

Historically, the most successful molecular target for antimalarial drugs has been heme biomineralization within the malarial parasite digestive vacuole. Heme released from catabolized host red blood cell hemoglobin is toxic, so malarial parasites crystallize heme to nontoxic hemozoin. For years it has been accepted that a number of effective quinoline antimalarial drugs (e.g., chloroquine, quinine, amodiaquine) function by preventing hemozoin crystallization. However, recent studies over the past decade have revealed a surprising molecular diversity in quinoline-heme molecular interactions. This diversity shows that even closely related quinoline drugs may have quite different molecular pharmacology. This paper reviews the molecular diversity and highlights important implications for understanding quinoline antimalarial drug resistance and for future drug design.