Elucidation of the heme active site electronic structure affecting the unprecedented nitrite dismutase activity of the ferriheme b proteins, the nitrophorins

The H-NOX protein structure adapts to different mechanisms in sensors interacting with nitric oxide

Some classes of bacteria within phyla possess protein sensors identified as homologous to the heme domain of soluble guanylate cyclase, the mammalian NO-receptor. Named H-NOX domain (Heme-Nitric Oxide or OXygen-binding), their heme binds nitric oxide (NO) and O2 for some of them. The signaling pathways where these proteins act as NO or O2 sensors appear various and are fully established for only some species. Here, we investigated the reactivity of H-NOX from bacterial species toward NO with a mechanistic point of view using time-resolved spectroscopy. The present data show that H-NOXs modulate the dynamics of NO as a function of temperature, but in different ranges, changing its affinity by changing the probability of NO rebinding after dissociation in the picosecond time scale. This fundamental mechanism provides a means to adapt the heme structural response to the environment. In one particular H-NOX sensor the heme distortion induced by NO binding is relaxed in an ultrafast manner (∼15 ps) after NO dissociation, contrarily to other H-NOX proteins, providing another sensing mechanism through the H-NOX domain. Overall, our study links molecular dynamics with functional mechanism and adaptation.

Cytochrome P460 Cofactor Maturation Proceeds via Peroxide-Dependent Post-translational Modification

Cytochrome P460s are heme enzymes that oxidize hydroxylamine to nitrous oxide. They bear specialized "heme P460" cofactors that are cross-linked to their host polypeptides by a post-translationally modified lysine residue. Wild-type N. europaea cytochrome P460 may be isolated as a cross-link-deficient proenzyme following anaerobic overexpression in E. coli. When treated with peroxide, this proenzyme undergoes maturation to active enzyme with spectroscopic and catalytic properties that match wild-type cyt P460. This maturation reactivity requires no chaperones─it is intrinsic to the protein. This behavior extends to the broader cytochrome c'β superfamily. Accumulated data reveal key contributions from the secondary coordination sphere that enable selective, complete maturation. Spectroscopic data support the intermediacy of a ferryl species along the maturation pathway.

The effect of pH and nitrite on the haem pocket of GLB-33, a globin-coupled neuronal transmembrane receptor of Caenorhabditis elegans

Out of the 34 globins in Caenorhabditis elegans, GLB-33 is a putative globin-coupled transmembrane receptor with a yet unknown function. The globin domain (GD) contains a particularly hydrophobic haem pocket, that rapidly oxidizes to a low-spin hydroxide-ligated haem state at physiological pH. Moreover, the GD has one of the fastest nitrite reductase activity ever reported for globins. Here, we use a combination of electronic circular dichroism, resonance Raman and electron paramagnetic resonance (EPR) spectroscopy with mass spectrometry to study the pH dependence of the ferric form of the recombinantly over-expressed GD in the presence and absence of nitrite. The competitive binding of nitrite and hydroxide is examined as well as nitrite-induced haem modifications at acidic pH. Comparison of the spectroscopic results with data from other haem proteins allows to deduce the important effect of Arg at position E10 in stabilization of exogenous ligands. Furthermore, continuous-wave and pulsed EPR indicate that ligation of nitrite occurs in a nitrito mode at pH 5.0 and above. At pH 4.0, an additional formation of a nitro-bound haem form is observed along with fast formation of a nitri-globin.

Interaction of nitrite with ferric protoglobin from Methanosarcina acetivorans - an interesting model for spectroscopic studies of the haem-ligand interaction

Protoglobin from Methanosarcina acetivorans (MaPgb) is a dimeric globin belonging to the same lineage of the globin superfamily as globin-coupled sensors. A putative role in the scavenging of reactive nitrogen and oxygen species has been suggested as a possible adaptation mechanism of the host organism to different gaseous environments in the course of evolution. A combination of optical absorption, electronic circular dichroism (ECD), resonance Raman (rRaman), and electron paramagnetic resonance (EPR) reveal the unusual in vitro reaction of ferric MaPgb with nitrite. In contrast to other globins, a large excess of nitrite did not induce the formation of a nitriglobin form in MaPgb. Surprisingly, the addition of nitrite in mildly acidic pH led to the formation of a stable nitric-oxide ligated ferric form of the protein (MaPgb-NO). Furthermore, the 300-700 nm ECD spectrum of ferric MaPgb is for the first time reported and discussed, showing strong differences in the Soret and Q ellipticity compared to ferric myoglobin, in line with the unusually strongly ruffled haem group of MaPgb and the related quantum-mechanical admixture of the S = 5/2 and S = 3/2 state of its ferric form. The Soret and Q ellipticity change strongly upon formation of MaPgb-NO, revealing a significant effect of the nitric-oxide ligation on the haem group and pocket. The related changes in the asymmetric pyrrole half-ring stretching vibration modes observed in the rRaman spectra give experimental support to earlier theoretical models, in which an important role of the in-plane breathing modes of the haem was predicted for the stabilization of the binding of diatomic gases to MaPgb.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Impact of the dynamics of the catalytic arginine on nitrite and chlorite binding by dimeric chlorite dismutase

Chlorite dismutases (Clds) are heme b containing oxidoreductases able to decompose chlorite to chloride and molecular oxygen. This work analyses the impact of the distal, flexible and catalytic arginine on the binding of anionic angulate ligands like nitrite and the substrate chlorite. Dimeric Cld from Cyanothece sp. PCC7425 was used as a model enzyme. We have investigated wild-type CCld having the distal catalytic R127 hydrogen-bonded to glutamine Q74 and variants with R127 (i) being arrested in a salt-bridge with a glutamate (Q74E), (ii) being fully flexible (Q74V) or (iii) substituted by either alanine (R127A) or lysine (R127K). We present the electronic and spectral signatures of the high-spin ferric proteins and the corresponding low-spin nitrite complexes elucidated by UV-visible, circular dichroism and electron paramagnetic resonance spectroscopies. Furthermore, we demonstrate the impact of the dynamics of R127 on the thermal stability of the respective nitrite adducts and present the X-ray crystal structures of the nitrite complexes of wild-type CCld and the variants Q74V, Q74E and R127A. In addition, the molecular dynamics (MD) and the binding modi of nitrite and chlorite to the ferric wild-type enzyme and the mutant proteins and the interaction of the oxoanions with R127 have been analysed by MD simulations. The findings are discussed with respect to the role(s) of R127 in ligand and chlorite binding and substrate degradation.

The Heme-Lys Cross-Link in Cytochrome P460 Promotes Catalysis by Enforcing Secondary Coordination Sphere Architecture

Cytochrome (cyt) P460 is a c-type monoheme enzyme found in ammonia-oxidizing bacteria (AOB) and methanotrophs; additionally, genes encoding it have been found in some pathogenic bacteria. Cyt P460 is defined by a unique post-translational modification to the heme macrocycle, where a lysine (Lys) residue covalently attaches to the 13' meso carbon of the porphyrin, modifying this heme macrocycle into the enzyme's eponymous P460 cofactor, similar to the cofactor found in the enzyme hydroxylamine oxidoreductase. This cross-link imbues the protein with unique spectroscopic properties, the most obvious of which is the enzyme's green color in solution. Cyt P460 from the AOB Nitrosomonas europaea is a homodimeric redox enzyme that produces nitrous oxide (N₂O) from 2 equiv of hydroxylamine. Mutation of the Lys cross-link results in spectroscopic features that are more similar to those of standard cyt c' proteins and renders the enzyme catalytically incompetent for NH₂OH oxidation. Recently, the necessity of a second-sphere glutamate (Glu) residue for redox catalysis was established; it plausibly serves as proton relay during the first oxidative half of the catalytic cycle. Herein, we report the first crystal structure of a cross-link deficient cyt P460. This structure shows that the positioning of the catalytically essential Glu changes by approximately 0.8 Å when compared to a cross-linked, catalytically competent cyt P460. It appears that the heme-Lys cross-link affects the relative position of the P460 cofactor with respect to the second-sphere Glu residue, therefore dictating the catalytic competency of the enzyme.

Nitrosyl Myoglobins and Their Nitrite Precursors: Crystal Structural and Quantum Mechanics and Molecular Mechanics Theoretical Investigations of Preferred Fe -NO Ligand Orientations in Myoglobin Distal Pockets

The globular dioxygen binding heme protein myoglobin (Mb) is present in several species. Its interactions with the simple nitrogen oxides, namely, nitric oxide (NO) and nitrite, have been known for decades, but the physiological relevance has only recently become more fully appreciated. We previously reported the O-nitrito mode of binding of nitrite to ferric horse heart wild-type (wt) MbIII and human hemoglobin. We have expanded on this work and report the interactions of nitrite with wt sperm whale (sw) MbIII and its H64A, H64Q, and V68A/I107Y mutants whose dissociation constants increase in the following order: H64Q < wt < V68A/I107Y < H64A. We also report their X-ray crystal structures that reveal the O-nitrito mode of binding of nitrite to these derivatives. The MbII-mediated reductions of nitrite to NO and structural data for the wt and mutant MbII-NOs are described. We show that their FeNO orientations vary with distal pocket identity, with the FeNO moieties pointing toward the hydrophobic interiors when the His64 residue is present but toward the hydrophilic exterior when this His64 residue is absent in this set of mutants. This correlates with the nature of H-bonding to the bound NO ligand (nitrosyl O vs N atom). Quantum mechanics and hybrid quantum mechanics and molecular mechanics calculations help elucidate the origin of the experimentally preferred NO orientations. In a few cases, the calculations reproduce the experimentally observed orientations only when the whole protein is taken into consideration.