Topography of human cytochrome b5/cytochrome b5 reductase interacting domain and redox alterations upon complex formation

The Use of Flavylium Salts as Dynamic Inhibitor Moieties for Human Cb5R

Cytochrome b₅ reductase (Cb₅R) is a flavoprotein that participates in the reduction of multiple biological redox partners. Co-localization of this protein with nitric oxide sources has been observed in neurons. In addition, the generation of superoxide anion radical by Cb₅R has been observed. A search for specific inhibitors of Cb₅R to understand the role of this protein in these new functions has been initiated. Previous studies have shown the ability of different flavonoids to inhibit Cb₅R. Anthocyanins are a subgroup of flavonoids responsible for most red and blue colors found in flowers and fruits. Although usually represented by the flavylium cation form, these species are only stable at rather acidic pH values (pH ≤ 1). At higher pH values, the flavylium cation is involved in a dynamic reaction network comprising different neutral species with the potential ability to inhibit the activities of Cb₅R. This study aims to provide insights into the molecular mechanism of interaction between flavonoids and Cb₅R using flavylium salts as dynamic inhibitors. The outcome of this study might lead to the design of improved specific enzyme inhibitors in the future.

Roles of Ferredoxin-NADP+ Oxidoreductase and Flavodoxin in NAD(P)H-Dependent Electron Transfer Systems

Distinct isoforms of FAD-containing ferredoxin-NADP⁺ oxidoreductase (FNR) and ferredoxin (Fd) are involved in photosynthetic and non-photosynthetic electron transfer systems. The FNR (FAD)-Fd [2Fe-2S] redox pair complex switches between one- and two-electron transfer reactions in steps involving FAD semiquinone intermediates. In cyanobacteria and some algae, one-electron carrier Fd serves as a substitute for low-potential FMN-containing flavodoxin (Fld) during growth under low-iron conditions. This complex evolves into the covalent FNR (FAD)-Fld (FMN) pair, which participates in a wide variety of NAD(P)H-dependent metabolic pathways as an electron donor, including bacterial sulfite reductase, cytochrome P450 BM3, plant or mammalian cytochrome P450 reductase and nitric oxide synthase isoforms. These electron transfer systems share the conserved Ser-Glu/Asp pair in the active site of the FAD module. In addition to physiological electron acceptors, the NAD(P)H-dependent diflavin reductase family catalyzes a one-electron reduction of artificial electron acceptors such as quinone-containing anticancer drugs. Conversely, NAD(P)H: quinone oxidoreductase (NQO1), which shares a Fld-like active site, functions as a typical two-electron transfer antioxidant enzyme, and the NQO1 and UDP-glucuronosyltransfease/sulfotransferase pairs function as an antioxidant detoxification system. In this review, the roles of the plant FNR-Fd and FNR-Fld complex pairs were compared to those of the diflavin reductase (FAD-FMN) family. In the final section, evolutionary aspects of NAD(P)H-dependent multi-domain electron transfer systems are discussed.

Transmembrane helices mediate the formation of a stable ternary complex of b5R, cyt b5, and SCD1

Mammalian cytochrome b₅ (cyt b₅) and cytochrome b₅ reductase (b₅R) are electron carrier proteins for membrane-embedded oxidoreductases. Both b₅R and cyt b₅ have a cytosolic domain and a single transmembrane (TM) helix. The cytosolic domains of b₅R and cyt b₅ contain cofactors required for electron transfer, but it is not clear if the TM helix has function beyond being an anchor to the membrane. Here we show that b₅R and cyt b₅ form a stable binary complex, and so do cyt b₅ and stearoyl-CoA desaturase-1 (SCD1). We also show that b₅R, cyt b₅ and SCD1 form a stable ternary complex. We demonstrate that the TM helices are required for the assembly of stable binary and ternary complexes where electron transfer rates are greatly enhanced. These results reveal a role of the TM helix in cyt b₅ and b₅R, and suggest that an electron transport chain composed of a stable ternary complex may be a general feature in membrane-embedded oxidoreductases that require cyt b₅ and b₅R.

The transmembrane domains of mammalian cytochrome b₅ (cyt b₅), cyt b₅ reductase (b₅R), and stearoyl-CoA desaturase-1 (SCD1) form stable binary complexes between cyt b₅/b₅R or cyt b₅/SCD1 and a ternary complex, which enhance electron transfer rates.

Ordered Motions in the Nitric-Oxide Dioxygenase Mechanism of Flavohemoglobin and Assorted Globins with Tightly Coupled Reductases

Nitric-oxide dioxygenases (NODs) activate and combine O₂ with NO to form nitrate. A variety of oxygen-binding hemoglobins with associated partner reductases or electron donors function as enzymatic NODs. Kinetic and structural investigations of the archetypal two-domain microbial flavohemoglobin-NOD have illuminated an allosteric mechanism that employs selective tunnels for O₂ and NO, gates for NO and nitrate, transient O₂ association with ferric heme, and an O₂ and NO-triggered, ferric heme spin crossover-driven, motion-controlled, and dipole-regulated electron-transfer switch. The proposed mechanism facilitates radical-radical coupling of ferric-superoxide with NO to form nitrate while preventing suicidal ferrous-NO formation. Diverse globins display the structural and functional motifs necessary for a similar allosteric NOD mechanism. In silico docking simulations reveal monomeric erythrocyte hemoglobin alpha-chain and beta-chain intrinsically matched and tightly coupled with NADH-cytochrome b₅ oxidoreductase and NADPH-cytochrome P450 oxidoreductase, respectively, forming membrane-bound flavohemoglobin-like mammalian NODs. The neuroprotective neuroglobin manifests a potential NOD role in a close-fitting ternary complex with membrane-bound NADH-cytochrome b₅ oxidoreductase and cytochrome b₅. Cytoglobin interfaces weakly with cytochrome b₅ for O₂ and NO-regulated electron-transfer and coupled NOD activity. The mechanistic model also provides insight into the evolution of O₂ binding cooperativity in hemoglobin and a basis for the discovery of allosteric NOD inhibitors.

Structural Features of Cytochrome b5–Cytochrome b5 Reductase Complex Formation and Implications for the Intramolecular Dynamics of Cytochrome b5 Reductase

Membrane cytochrome b₅ reductase is a pleiotropic oxidoreductase that uses primarily soluble reduced nicotinamide adenine dinucleotide (NADH) as an electron donor to reduce multiple biological acceptors localized in cellular membranes. Some of the biological acceptors of the reductase and coupled redox proteins might eventually transfer electrons to oxygen to form reactive oxygen species. Additionally, an inefficient electron transfer to redox acceptors can lead to electron uncoupling and superoxide anion formation by the reductase. Many efforts have been made to characterize the involved catalytic domains in the electron transfer from the reduced flavoprotein to its electron acceptors, such as cytochrome b₅, through a detailed description of the flavin and NADH-binding sites. This information might help to understand better the processes and modifications involved in reactive oxygen formation by the cytochrome b₅ reductase. Nevertheless, more than half a century since this enzyme was first purified, the one-electron transfer process toward potential electron acceptors of the reductase is still only partially understood. New advances in computational analysis of protein structures allow predicting the intramolecular protein dynamics, identifying potential functional sites, or evaluating the effects of microenvironment changes in protein structure and dynamics. We applied this approach to characterize further the roles of amino acid domains within cytochrome b₅ reductase structure, part of the catalytic domain, and several sensors and structural domains involved in the interactions with cytochrome b₅ and other electron acceptors. The computational analysis results allowed us to rationalize some of the available spectroscopic data regarding ligand-induced conformational changes leading to an increase in the flavin adenine dinucleotide (FAD) solvent-exposed surface, which has been previously correlated with the formation of complexes with electron acceptors.

Mechanism of electron transfers mediated by cytochromes c and b5 in mitochondria and endoplasmic reticulum: classical and murburn perspectives

We explore the mechanism of electron transfers mediated by cytochrome c, a soluble protein involved in mitochondrial oxidative phosphorylation and cytochrome b₅, a microsomal membrane protein acting as a redox aide in xenobiotic metabolism. We found minimal conservation in the sequence and surface amino acid residues of cytochrome c/b₅ proteins among divergent species. Therefore, we question the evolutionary logic for electron transfer (ET) occurring through affinity binding via recognition of specific surface residues/topography. Also, analysis of putative protein-protein interactions in the crystal structures of these proteins and their redox partners did not point to any specific interaction logic. A comparison of the kinetic and thermodynamic constants of wildtype vs. mutants did not provide strong evidence to support the binding-based ET paradigm, but indicated support for diffusible reactive species (DRS)-mediated process. Topographically divergent cytochromes from one species have been substituted for reaction with proteins from other species, implying the involvement of non-specific interactions. We provide a viable alternative (murburn concept) to classical protein-protein binding-based long range ET mechanism. To account for the promiscuity of interactions and solvent-accessible hemes, we propose that the two proteins act as non- specific redox capacitors, mediating one-electron redox equilibriums involving DRS and unbound ions.Communicated by Ramaswamy H. Sarma.

Human erythrocytes exposure to juglone leads to an increase of superoxide anion production associated with cytochrome b5 reductase uncoupling

Cytochrome b₅ reductase is an enzyme with the ability to generate superoxide anion at the expenses of NADH consumption. Although this activity can be stimulated by cytochrome c and could participate in the bioenergetic failure accounting in apoptosis, very little is known about other molecules that may uncouple the function of the cytochrome b₅ reductase. Naphthoquinones are redox active molecules with the ability to interact with electron transfer chains. In this work, we made an inhibitor screening against recombinant human cytochrome b₅ reductase based on naphthoquinone properties. We found that 5-hydroxy-1,4-naphthoquinone (known as juglone), a natural naphthoquinone extracted from walnut trees and used historically in traditional medicine with ambiguous health and toxic outcomes, had the ability to uncouple the electron transfer from the reductase to cytochrome b₅ and ferricyanide. Upon complex formation with cytochrome b₅ reductase, juglone is able to act as an electron acceptor leading to a NADH consumption stimulation and an increase of superoxide anion production by the reductase. Our results suggest that cytochrome b₅ reductase could contribute to the measured energetic failure in the erythrocyte apoptosis induced by juglone, that is concomitant with the reactive oxygen species produced by cytochrome b₅ reductase.

Ligand accessibility to heme cytochrome b5 coordinating sphere and enzymatic activity enhancement upon tyrosine ionization

Recently, we observed that at extreme alkaline pH, cytochrome b₅ (Cb₅) acquires a peroxidase-like activity upon formation of a low spin hemichrome associated with a non-native state. A functional characterization of Cb₅, in a wide pH range, shows that oxygenase/peroxidase activities are stimulated in alkaline media, and a correlation between tyrosine ionization and the attained enzymatic activities was noticed, associated with an altered heme spin state, when compared to acidic pH values at which the heme group is released. In these conditions, a competitive assay between imidazole binding and Cb₅ endogenous heme ligands revealed the appearance of a binding site for this exogenous ligand that promotes a heme group exposure to the solvent upon ligation. Our results shed light on the mechanism behind Cb₅ oxygenase/peroxidase activity stimulation in alkaline media and reveal a role of tyrosinate anion enhancing Cb₅ enzymatic activities on the distorted protein before maximum protein unfolding.

Molecular mechanism of metabolic NAD(P)H-dependent electron-transfer systems: The role of redox cofactors

NAD(P)H-dependent electron-transfer (ET) systems require three functional components: a flavin-containing NAD(P)H-dehydrogenase, one-electron carrier and metal-containing redox center. In principle, these ET systems consist of one-, two- and three-components, and the electron flux from pyridine nucleotide cofactors, NADPH or NADH to final electron acceptor follows a linear pathway: NAD(P)H → flavin → one-electron carrier → metal containing redox center. In each step ET is primarily controlled by one- and two-electron midpoint reduction potentials of protein-bound redox cofactors in which the redox-linked conformational changes during the catalytic cycle are required for the domain-domain interactions. These interactions play an effective ET reactions in the multi-component ET systems. The microsomal and mitochondrial cytochrome P450 (cyt P450) ET systems, nitric oxide synthase (NOS) isozymes, cytochrome b₅ (cyt b₅) ET systems and methionine synthase (MS) ET system include a combination of multi-domain, and their organizations display similarities as well as differences in their components. However, these ET systems are sharing of a similar mechanism. More recent structural information obtained by X-ray and cryo-electron microscopy (cryo-EM) analysis provides more detail for the mechanisms associated with multi-domain ET systems. Therefore, this review summarizes the roles of redox cofactors in the metabolic ET systems on the basis of one-electron redox potentials. In final Section, evolutionary aspects of NAD(P)H-dependent multi-domain ET systems will be discussed.

Cytochrome b5 reductase is the component from neuronal synaptic plasma membrane vesicles that generates superoxide anion upon stimulation by cytochrome c

In this work, we measured the effect of cytochrome c on the NADH-dependent superoxide anion production by synaptic plasma membrane vesicles from rat brain. In these membranes, the cytochrome c stimulated NADH-dependent superoxide anion production was inhibited by antibodies against cytochrome b5 reductase linking the production to this enzyme. Measurement of the superoxide anion radical generated by purified recombinant soluble and membrane cytochrome b5 reductase corroborates the production of the radical by different enzyme isoforms. In the presence of cytochrome c, a burst of superoxide anion as well as the reduction of cytochrome c by cytochrome b5 reductase was measured. Complex formation between both proteins suggests that cytochrome b5 reductase is one of the major partners of cytochrome c upon its release from mitochondria to the cytosol during apoptosis. Superoxide anion production and cytochrome c reduction are the consequences of the stimulated NADH consumption by cytochrome b5 reductase upon complex formation with cytochrome c and suggest a major role of this enzyme as an anti-apoptotic protein during cell death.