Nonclinical Characterization of the Hypoxia-Inducible Factor Prolyl Hydroxylase Inhibitor Roxadustat, a Novel Treatment of Anemia of Chronic Kidney Disease.

Anemia of chronic kidney disease (CKD) is a multifactorial disorder caused by impaired erythropoietin (EPO) production and altered iron homeostasis associated with inflammation. Hypoxia-inducible factor (HIF) is a transcription factor that stimulates erythropoiesis via a coordinated response involving increased EPO production and enhanced iron availability for Hb synthesis. HIF degradation is regulated by HIF-prolyl hydroxylase (HIF-PH) enzymes. We hypothesized that roxadustat, an orally available small-molecule inhibitor of HIF-PH, would increase EPO production and promote erythropoiesis in animal models of anemia. In cells, roxadustat increased both HIF-1α and HIF-2α proteins, leading to an increase in EPO production, even in the presence of EPO-suppressing inflammatory cytokines. Roxadustat administered intermittently to healthy rats and cynomolgus monkeys increased circulating EPO levels, reticulocytes, blood Hb, and hematocrit in a dose-dependent manner. Roxadustat corrected anemia in a rat model of CKD after five-sixth nephrectomy and in a rat model of anemia of inflammation with impaired iron metabolism induced by peptidoglycan-polysaccharide (PG-PS). In the PG-PS model, roxadustat significantly decreased hepatic expression of hepcidin, a hormone responsible for iron sequestration and functional iron deficiency, and increased expression of two genes involved in duodenal iron absorption: divalent metal transporter 1 and duodenal cytochrome b. In conclusion, by activating the HIF pathway, roxadustat increased EPO production, elevated Hb, corrected anemia, and improved iron homeostasis. The coordinated erythropoietic response stimulated by roxadustat, involving both EPO production and mobilization of iron stores, makes this compound a promising treatment of anemia of CKD and anemia associated with functional iron deficiency. SIGNIFICANCE STATEMENT: Roxadustat is a novel orally available small-molecule inhibitor of HIF prolyl hydroxylase enzymes that reversibly stabilizes HIF-α, thus activating transcription of HIF-dependent genes, including EPO and regulators of iron homeostasis. Activation of the HIF pathway by roxadustat induces erythropoiesis in healthy rats and monkeys and corrects experimentally induced anemia in rats. The coordinated erythropoietic response that increases EPO production and mobilizes iron stores makes roxadustat a promising treatment for anemia of chronic kidney disease and anemia associated with functional iron deficiency.

Mechanism of chloride inhibition of bilirubin oxidases and its dependence on potential and pH.

Bilirubin oxidases (BODs) belong to the multi-copper oxidase (MCO) family and efficiently reduce O2 at neutral pH and in physiological conditions where chloride concentrations are over 100 mM. BODs were consequently considered to be Cl- resistant contrary to laccases. However, there has not been a detailed study on the related effect of chloride and pH on the redox state of immobilized BODs. Here, we investigate by electrochemistry the catalytic mechanism of O2 reduction by the thermostable Bacillus pumilus BOD immobilized on carbon nanofibers in the presence of NaCl. The addition of chloride results in the formation of a redox state of the enzyme, previously observed for different BODs and laccases, which is only active after a reductive step. This behavior has not been previously investigated. We show for the first time that the kinetics of formation of this state is strongly dependent on pH, temperature, Cl- concentration and on the applied redox potential. UV-visible spectroscopy allows us to correlate the inhibition process by chloride with the formation of the alternative resting form of the enzyme. We demonstrate that O2 is not required for its formation and show that the application of an oxidative potential is sufficient. In addition, our results suggest that the reactivation may proceed thought the T3 ß.

Two-Electron Reduction versus One-Electron Oxidation of the Type 3 Pair in the Multicopper Oxidases.

Multicopper oxidases (MCOs) utilize an electron shuttling Type 1 Cu (T1) site in conjunction with a mononuclear Type 2 (T2) and a binuclear Type 3 (T3) site, arranged in a trinuclear copper cluster (TNC), to reduce O2 to H2O. Reduction of O2 occurs with limited overpotential indicating that all the coppers in the active site can be reduced via high-potential electron donors. Two forms of the resting enzyme have been observed in MCOs: the alternative resting form (AR), where only one of the three TNC Cu's is oxidized, and the resting oxidized form (RO), where all three TNC Cu's are oxidized. In contrast to the AR form, we show that in the RO form of a high-potential MCO, the binuclear T3 Cu(II) site can be reduced via the 700 mV T1 Cu. Systematic spectroscopic evaluation reveals that this proceeds by a two-electron process, where delivery of the first electron, forming a high energy, metastable half reduced T3 state, is followed by the rapid delivery of a second energetically favorable electron to fully reduce the T3 site. Alternatively, when this fully reduced binuclear T3 site is oxidized via the T1 Cu, a different thermodynamically favored half oxidized T3 form, i.e., the AR site, is generated. This behavior is evaluated by DFT calculations, which reveal that the protein backbone plays a significant role in controlling the environment of the active site coppers. This allows for the formation of the metastable, half reduced state and thus the complete reductive activation of the enzyme for catalysis.

Mechanism of the reduction of the native intermediate in the multicopper oxidases: insights into rapid intramolecular electron transfer in turnover.

The multicopper oxidases (MCOs) are the family of enzymes that catalyze the 4-electron reduction of O2 to H2O coupled to the four 1-electron oxidations of substrate. In the catalytic cycle electrons are transferred intramolecularly over ∼13 Šfrom a Type 1 (T1) Cu site that accepts electrons from substrate to a trinuclear Cu cluster (TNC) where O2 is reduced to H2O at rapid rates consistent with turnover (560 s(-1)). The oxygen reduction mechanism for the MCOs is well-characterized, whereas the rereduction is less understood. Our initial study of Rhus vernicifera Laccase (Heppner et al. J. Am. Chem. Soc. 2013, 135, 12212) experimentally established that the native intermediate (NI), the species formed upon O-O bond cleavage, is reduced with an IET rate >700 s(-1) and is the catalytically relevant fully oxidized form of the enzyme, rather than the resting state. In this report, we present kinetic and spectroscopic results coupled to DFT calculations that evaluate the mechanism of the 3 e(-)/3 H(+) reduction of NI, where all three catalytically relevant intramolecular electron transfer (IET) steps are rapid and involve three different structural changes. These three rapid IET processes reflect the sophisticated mechanistic control of the TNC to enable rapid turnover. All three IET processes are fast due to the associated protonation of the bridging oxo and hydroxo ligands, generated by O-O cleavage, to form water products that are extruded from the TNC upon full reduction, thereby defining a unifying mechanism for oxygen reduction and rapid IET by the TNC in the catalytic cycle of the MCOs.

Spectroscopic and computational insight into the activation of O2 by the mononuclear Cu center in polysaccharide monooxygenases.

Strategies for O2 activation by copper enzymes were recently expanded to include mononuclear Cu sites, with the discovery of the copper-dependent polysaccharide monooxygenases, also classified as auxiliary-activity enzymes 9-11 (AA9-11). These enzymes are finding considerable use in industrial biofuel production. Crystal structures of polysaccharide monooxygenases have emerged, but experimental studies are yet to determine the solution structure of the Cu site and how this relates to reactivity. From X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies, we observed a change from four-coordinate Cu(II) to three-coordinate Cu(I) of the active site in solution, where three protein-derived nitrogen ligands coordinate the Cu in both redox states, and a labile hydroxide ligand is lost upon reduction. The spectroscopic data allowed for density functional theory calculations of an enzyme active site model, where the optimized Cu(I) and (II) structures were consistent with the experimental data. The O2 reactivity of the Cu(I) site was probed by EPR and stopped-flow absorption spectroscopies, and a rapid one-electron reduction of O2 and regeneration of the resting Cu(II) enzyme were observed. This reactivity was evaluated computationally, and by calibration to Cu-superoxide model complexes, formation of an end-on Cu-AA9-superoxide species was found to be thermodynamically favored. We discuss how this thermodynamically difficult one-electron reduction of O2 is enabled by the unique protein structure where two nitrogen ligands from His1 dictate formation of a T-shaped Cu(I) site, which provides an open coordination position for strong O2 binding with very little reorganization energy.

Molecular origin of rapid versus slow intramolecular electron transfer in the catalytic cycle of the multicopper oxidases.

Kinetic measurements on single-turnover processes in laccase established fast type-1 Cu to trinuclear Cu cluster (TNC) intramolecular electron transfer (IET) in the reduction of the native intermediate (NI), the fully oxidized form of the enzyme formed immediately after O-O bond cleavage in the mechanism of O2 reduction. Alternatively, slow IET kinetics was observed in the reduction of the resting enzyme, which involves proton-coupled electron transfer on the basis of isotope measurements. The >10(3) difference between the IET rates for these two processes confirms that the NI, rather than the resting enzyme that has been defined by crystallography, is the fully oxidized form of the TNC in catalytic turnover. Computational modeling showed that reduction of NI is fast because of the larger driving force associated with a more favorable proton affinity of its µ3-oxo moiety generated by reductive cleavage of the O-O bond. This defines a unifying mechanism in which reductive cleavage of the O-O bond is coupled to rapid IET in the multicopper oxidases.

Modified reactivity toward O2 in first shell variants of Fet3p: geometric and electronic structure requirements for a functioning trinuclear copper cluster.

Multicopper oxidases (MCOs) carry out the most energy efficient reduction of O2 to H2O known, i.e., with the lowest overpotential. This four-electron process requires an electron mediating type 1 (T1) Cu site and an oxygen reducing trinuclear Cu cluster (TNC), consisting of a binuclear type 3 (T3)- and a mononuclear type 2 (T2) Cu center. The rate-determining step in O2 reduction is the first two-electron transfer from one of the T3 Cu's (T3ß) and the T2 Cu, forming a bridged peroxide intermediate (PI). This reaction has been investigated in T3ß Cu variants of the Fet3p, where a first shell His ligand is mutated to Glu or Gln. This converts the fast two-electron reaction of the wild-type (WT) enzyme to a slow one-electron oxidation of the TNC. Both variants initially react to form a common T3ß Cu(II) intermediate that converts to the Glu or Gln bound resting state. From spectroscopic evaluation, the nonmutated His ligands coordinate linearly to the T3ß Cu in the reduced TNCs in the two variants, in contrast to the trigonal arrangement observed in the WT enzyme. This structural perturbation is found to significantly alter the electronic structure of the reduced TNC, which is no longer capable of rapidly transferring two electrons to the two perpendicular half occupied π*-orbitals of O2, in contrast to the WT enzyme. This study provides new insight into the geometric and electronic structure requirements of a fully functional TNC for the rate determining two-electron reduction of O2 in the MCOs.

Spectroscopic and crystallographic characterization of "alternative resting" and "resting oxidized" enzyme forms of bilirubin oxidase: implications for activity and electrochemical behavior of multicopper oxidases.

While there is broad agreement on the catalytic mechanism of multicopper oxidases (MCOs), the geometric and electronic structures of the resting trinuclear Cu cluster have been variable, and their relevance to catalysis has been debated. Here, we present a spectroscopic characterization, complemented by crystallographic data, of two resting forms occurring in the same enzyme and define their interconversion. The resting oxidized form shows similar features to the resting form in Rhus vernicifera and Trametes versicolor laccase, characterized by "normal" type 2 Cu electron paramagnetic resonance (EPR) features, 330 nm absorption shoulder, and a short type 3 (T3) Cu-Cu distance, while the alternative resting form shows unusually small A(||) and high g(||) EPR features, lack of 330 nm absorption intensity, and a long T3 Cu-Cu distance. These different forms are evaluated with respect to activation for catalysis, and it is shown that the alternative resting form can only be activated by low-potential reduction, in contrast to the resting oxidized form which is activated via type 1 Cu at high potential. This difference in activity is correlated to differences in redox states of the two forms and highlights the requirement for efficient sequential reduction of resting MCOs for their involvement in catalysis.

Bilirubin oxidase from Magnaporthe oryzae: an attractive new enzyme for biotechnological applications.

A novel bilirubin oxidase (BOD), from the rice blast fungus Magnaporthe oryzae, has been identified and isolated. The 64-kDa protein containing four coppers was successfully overexpressed in Pichia pastoris and purified to homogeneity in one step. Protein yield is more than 100 mg for 2 L culture, twice that of Myrothecium verrucaria. The k(cat)/K(m) ratio for conjugated bilirubin (1,513 mM⁻¹ s⁻¹) is higher than that obtained for the BOD from M. verrucaria expressed in native fungus (980 mM⁻¹ s⁻¹), with the lowest K(m) measured for any BOD highly desirable for detection of bilirubin in medical samples. In addition, this protein exhibits a half-life for deactivation >300 min at 37 °C, high stability at pH 7, and high tolerance towards urea, making it an ideal candidate for the elaboration of biofuel cells, powering implantable medical devices. Finally, this new BOD is efficient in decolorizing textile dyes such as Remazol brilliant Blue R, making it useful for environmentally friendly industrial applications.

Copper dioxygen (bio)inorganic chemistry.

Cu/O2 intermediates in biological, homogeneous, and heterogeneous catalysts exhibit unique spectral features that reflect novel geometric and electronic structures that make significant contributions to reactivity. This review considers how the respective intermediate electronic structures overcome the spin-forbidden nature of O2 binding, activate O2 for electrophilic aromatic attack and H-atom abstraction, catalyze the 4 e- reduction of O2 to H2O, and discusses the role of exchange coupling between Cu ions in determining reactivity.

Enzymatic versus inorganic oxygen reduction catalysts: comparison of the energy levels in a free-energy scheme.

In this paper, we present a method to directly compare the energy levels of intermediates in enzymatic and inorganic oxygen reduction catalysts. We initially describe how the energy levels of a Pt(111) catalyst, operating at pH = 0, are obtained. By a simple procedure, we then convert the energy levels of cytochrome c oxidase (CcO) models obtained at physiological pH = 7 to the energy levels at pH = 0, which allows for comparison. Furthermore, we illustrate how different bias voltages will affect the free-energy landscapes of the catalysts. This allows us to determine the so-called theoretical overpotential of each system, which is shown to be significantly lower for the enzymatic catalysts compared to the inorganic Pt(111) catalyst. Finally, we construct theoretical polarization curves for the CcO models, in order to illustrate the effect of the low overpotentials on turnover rates per site.