Oxygen Generation via Water Splitting by a Novel Biogenic Metal Ion-Binding Compound.

Methanobactins (MBs) are small (<1,300-Da) posttranslationally modified copper-binding peptides and represent the extracellular component of a copper acquisition system in some methanotrophs. Interestingly, MBs can bind a range of metal ions, with some being reduced after binding, e.g., Cu2+ reduced to Cu+. Other metal ions, however, are bound but not reduced, e.g., K+. The source of electrons for selective metal ion reduction has been speculated to be water but never empirically shown. Here, using H218O, we show that when MBs from Methylocystis sp. strain SB2 (MB-SB2) and Methylosinus trichosporium OB3b (MB-OB3) were incubated in the presence of either Au3+, Cu2, or Ag+, 18,18O2 and free protons were released. No 18,18O2 production was observed in the presence of either MB-SB2 or MB-OB3b alone, gold alone, copper alone, or silver alone or when K+ or Mo2+ was incubated with MB-SB2. In contrast to MB-OB3b, MB-SB2 binds Fe3+ with an N2S2 coordination and will also reduce Fe3+ to Fe2+. Iron reduction was also found to be coupled to the oxidation of 2H2O and the generation of O2. MB-SB2 will also couple Hg2+, Ni2+, and Co2+ reduction to the oxidation of 2H2O and the generation of O2, but MB-OB3b will not, ostensibly as MB-OB3b binds but does not reduce these metal ions. To determine if the O2 generated during metal ion reduction by MB could be coupled to methane oxidation, 13CH4 oxidation by Methylosinus trichosporium OB3b was monitored under anoxic conditions. The results demonstrate that O2 generation from metal ion reduction by MB-OB3b can support methane oxidation. IMPORTANCE The discovery that MB will couple the oxidation of H2O to metal ion reduction and the release of O2 suggests that methanotrophs expressing MB may be able to maintain their activity under hypoxic/anoxic conditions through the "self-generation" of dioxygen required for the initial oxidation of methane to methanol. Such an ability may be an important factor in enabling methanotrophs to not only colonize the oxic-anoxic interface where methane concentrations are highest but also tolerate significant temporal fluctuations of this interface. Given that genomic surveys often show evidence of aerobic methanotrophs within anoxic zones, the ability to express MB (and thereby generate dioxygen) may be an important parameter in facilitating their ability to remove methane, a potent greenhouse gas, before it enters the atmosphere.

Plant hemoglobins: a journey from unicellular green algae to vascular plants.

Globins (Glbs) are widely distributed in archaea, bacteria and eukaryotes. They can be classified into proteins with 2/2 or 3/3 α-helical folding around the heme cavity. Both types of Glbs occur in green algae, bryophytes and vascular plants. The Glbs of angiosperms have been more intensively studied, and several protein structures have been solved. They can be hexacoordinate or pentacoordinate, depending on whether a histidine is coordinating or not at the sixth position of the iron atom. The 3/3 Glbs of class 1 and the 2/2 Glbs (also called class 3 in plants) are present in all angiosperms, whereas the 3/3 Glbs of class 2 have been only found in early angiosperms and eudicots. The three Glb classes are expected to play different roles. Class 1 Glbs are involved in hypoxia responses and modulate NO concentration, which may explain their roles in plant morphogenesis, hormone signaling, cell fate determination, nutrient deficiency, nitrogen metabolism and plant-microorganism symbioses. Symbiotic Glbs derive from class 1 or class 2 Glbs and transport O2 in nodules. The physiological roles of class 2 and class 3 Glbs are poorly defined but could involve O2 and NO transport and/or metabolism, respectively. More research is warranted on these intriguing proteins to determine their non-redundant functions.

The role of the NADH-dependent nitrite reductase, Nir, from Escherichia coli in fermentative ammonification.

Nitrate and nitrite reduction are of paramount importance for nitrogen assimilation and anaerobic metabolism, and understanding the specific roles of each participating reductase is necessary to describe the biochemical balance that dictates cellular responses to their environments. The soluble, cytoplasmic siroheme NADH-nitrite reductase (Nir) in Escherichia coli is necessary for nitrate/nitrite assimilation but has also been reported to either "detoxify" nitrite, or to carry out fermentative ammonification in support of anaerobic catabolism. Theoretically, nitrite detoxification would be important for anaerobic growth on nitrate, during which excess nitrite would be reduced to ammonium. Fermentative ammonification by Nir would be important for maximization of non-respiratory ATP production during anaerobic growth in the presence of nitrite. Experiments reported here were designed to test the potential role of Nir in fermentative ammonification directly by growing E. coli along with mutant strains lacking Nir or the respiratory nitrite reductase (Nrf) under anaerobic conditions in defined media while monitoring nitrogen utilization and fermentation metabolites. To focus on the role of Nir in fermentative ammonification, pH control was used in most experiments to eliminate nitrite toxicity due to nitric acid formation. Our results demonstrate that Nir confers a significant benefit during fermentative growth that reflects fermentative ammonification rather than detoxification. We conclude that fermentative ammonification by Nir allows for the energetically favorable fermentation of glucose to formate and acetate. These results and conclusions are discussed in light of the roles of Nir in other bacteria and in plants.

Steady-State Kinetics of Phytoglobin-Catalyzed Reduction of Hydroxylamine to Ammonium.

Phytoglobins are plant hexacoordinate hemoglobins with reversible coordination of a histidine side chain to the ligand binding site of the heme iron. They mediate electron transfer reactions such as nitric oxide scavenging and are particularly efficient at reducing nitrite and hydroxylamine. Previous work with phytoglobins has focused only on single turnovers of these reactions and has not revealed whether structural features, such as histidine hexacoordination, play a prominent role in the complete catalytic cycle. This work characterizes steady-state phytoglobin catalysis of reduction of hydroxylamine to ammonium using two different chemical reductants. Km and kcat values were measured for rice phytoglobin, horse myoglobin, human neuroglobin, and a rice phytoglobin mutant protein in which the hexacoordinating histidine has been replaced with leucine (Phyt:H73L). The results demonstrate that phytoglobin catalysis driven by benzyl viologen is limited only by the dissociation rate constant for the distal histidine. This is consistent with the rate limit in single-turnover experiments and demonstrates that the kinetics of hydroxylamine binding, and not phytoglobin reduction, ultimately governs the reaction. Catalysis by the other proteins that either lack or have tighter hexacoordination is much slower, suggesting that facile reversibility of the bond between the distal histidine and the heme iron is needed to allow both substrate binding and heme iron reduction. On the other hand, catalysis driven by dithionite is limited by SO2•- concentrations and is similar for all of these proteins, suggesting that dithionite is not a good reducing agent for evaluation of the catalytic properties of hemoglobins.

Spectroscopic evidence supporting neutral thiol ligation to ferrous heme iron.

The binding of neutral thiol (ethanethiol, EtSH) or thioether (tetrahydrothiophene, THT) to two types of heme proteins in their ferrous state has been investigated with UV-visible (UV-Vis) absorption and magnetic circular dichroism spectroscopy. For the second GAF (cGMP-specific phosphodiesterases, adenylyl cyclases, and FhlA) domain from the sensory kinase MsmS (sGAF2), stepwise additions of these respective two sulfur-donor ligands to its dithionite-reduced ferrous form generate homogeneous six-coordinate low-spin ferrous complexes at both pHs 7.0 and 5.4. Similar complexes were partially formed for deoxyferrous soybean leghemoglobin with EtSH or THT within their solubility limits in water. The titrations cause significant UV-Vis spectra changes attributable to a five-coordinate to six-coordinate heme iron coordination change. For sGAF2, the resulting spectra are essentially identical for the both ligands, clearly indicating the direct binding of neutral thiol/thioether to ferrous heme iron as the distal ligand. On the other hand, the thiol EtSH binds to ferric sGAF2 in the anionic thiolate form, while thioether THT forms its ferric sGAF2 complex as a neutral ligand. These observations provide compelling evidence that neutral cysteine is a plausible ligand for ferrous heme proteins.

Role of Reversible Histidine Coordination in Hydroxylamine Reduction by Plant Hemoglobins (Phytoglobins).

Reduction of hydroxylamine to ammonium by phytoglobin, a plant hexacoordinate hemoglobin, is much faster than that of other hexacoordinate hemoglobins or pentacoordinate hemoglobins such as myoglobin, leghemoglobin, and red blood cell hemoglobin. The reason for differences in reactivity is not known but could be intermolecular electron transfer between protein molecules in support of the required two-electron reduction, hydroxylamine binding, or active site architecture favoring the reaction. Experiments were conducted with phytoglobins from rice, tomato, and soybean along with human neuroglobin and soybean leghemoglobin that reveal hydroxylamine binding as the rate-limiting step. For hexacoordinate hemoglobins, binding is limited by the dissociation rate constant for the distal histidine, while leghemoglobin is limited by an intrinsically low affinity for hydroxylamine. When the distal histidine is removed from rice phytoglobin, a hydroxylamine-bound intermediate is formed and the reaction rate is diminished, indicating that the distal histidine imidazole side chain is critical for the reaction, albeit not for electron transfer but rather for direct interaction with the substrate. Together, these results demonstrate that phytoglobins are superior at hydroxylamine reduction because they have distal histidine coordination affinity constants near 1, and facile rate constants for binding and dissociation of the histidine side chain. Hexacoordinate hemoglobins such as neuroglobin are limited by tighter histidine coordination that blocks hydroxylamine binding, and pentacoordinate hemoglobins have intrinsically lower hydroxylamine affinities.

Refractory severe idiopathic thrombocytopenia and treatment challenges.

Refractory idiopathic thrombocytopenia (ITP) is a disease that does not respond to or relapses after splenectomy, requires treatment to reduce the risk of clinically significant bleeding, and is a challenging case to treat. Presentation of the Case: A 39-year-old male with a history of chronic ITP presented with a platelet count of 1000/µl and prostatitis. He was started on Ciprofloxacin and started intravenous immunoglobulin along with intravenous methylprednisolone. Then Rituximab was started on day fourth. Since his platelet remained 0/µl, Mycophenolate mofetil (Cellcept) was started on day 14th. Next, a dose of Romiplostim on day 19th was given. Eltrombopag (Promacta) and Tavlesse were started on day 23th and platelets rose to 96×103/µl on day 26th and then 418×103/µl. Discussion: Normally, refractory ITP patients who do not respond to first-line treatments require a combination therapy of one to two medicines of the second line, like thrombopoietin receptor agonists. However, this patient's thrombocytopenia neither responded to first-line treatment nor second-line treatment with Promacta/Romiplostin plus immunosuppressives or Tavlesse. Conclusion: Refractory ITP, who has not responded to first-line and second-line treatments, requires treatment with a combination of all first-line and second-line treatments. Furthermore, Promacta, Tavlesse, and Romiplostim have a big role to play in helping the patient.

Plant and cyanobacterial hemoglobins reduce nitrite to nitric oxide under anoxic conditions.

The ability of ferrous hemoglobins to reduce nitrite to form nitric oxide has been demonstrated for hemoglobins from animals, including myoglobin, blood cell hemoglobin, neuroglobin, and cytoglobin. In all cases, the rate constants for the bimolecular reactions with nitrite are relatively slow, with maximal values of ~5 M(-1) s(-1) at pH 7. Combined with the relatively low concentrations of nitrite found in animal blood plasma (normally no greater than 13 µM), these slow reaction rates are unlikely to contribute significantly to hemoglobin oxidation, nitrite reduction, or NO production. Plants and cyanobacteria, however, must contend with much higher (millimolar) nitrite concentrations necessitated by assimilatory nitrogen metabolism during hypoxic growth, such as the conditions commonly found during flooding or in waterlogged soil. Here we report rate constants for nitrite reduction by a ferrous plant hemoglobin (rice nonsymbiotic hemoglobin 1) and a ferrous cyanobacterial hemoglobin from Synechocystis that are more than 10 times faster than those observed for animal hemoglobins. These rate constants, along with the relatively high concentrations of nitrite present during hypoxia, suggest that plant and cyanobacterial hemoglobins could serve as anaerobic nitrite reductases in vivo.

Hydroxylamine reduction to ammonium by plant and cyanobacterial hemoglobins.

Plants often face hypoxic stress as a result of flooding and waterlogged soils. During these periods, they must continue ATP production and nitrogen metabolism if they are to survive. The normal pathway of reductive nitrogen assimilation in non-legumes, nitrate, and nitrite reductase can be inhibited during low oxygen conditions that are associated with the buildup of toxic metabolites such as nitrite and nitric oxide, so the plant must also have a means of detoxifying these molecules. Compared to animal hemoglobins, plant and cyanobacterial hemoglobins are adept at reducing nitrite to nitric oxide under anaerobic conditions. Here we test their abilities to reduce hydroxylamine, a proposed intermediate of nitrite reductase, under anaerobic conditions. We find that class 1 rice nonsymbiotic hemoglobin (rice nsHb1) and the hemoglobin from the cyanobacterium Synechocystis (SynHb) catalyze the reduction of hydroxylamine to ammonium at rates 100-2500 times faster than animal hemoglobins including myoglobin, neuroglobin, cytoglobin, and blood cell hemoglobin. These results support the hypothesis that plant and cyanobacterial hemoglobins contribute to anaerobic nitrogen metabolism in support of anaerobic respiration and survival during hypoxia.

Crystal structures of Parasponia and Trema hemoglobins: differential heme coordination is linked to quaternary structure.

Hemoglobins from the plants Parasponia andersonii (ParaHb) and Trema tomentosa (TremaHb) are 93% identical in primary structure but differ in oxygen binding constants in accordance with their distinct physiological functions. Additionally, these proteins are dimeric, and ParaHb exhibits the unusual property of having different heme redox potentials for each subunit. To investigate how these hemoglobins could differ in function despite their shared sequence identity and to determine the cause of subunit heterogeneity in ParaHb, we have measured their crystal structures in the ferric oxidation state. Furthermore, we have made a monomeric ParaHb mutant protein (I43N) and measured its ferrous/ferric heme redox potential to test the hypothesized link between quaternary structure and heme heterogeneity in wild-type ParaHb. Our results demonstrate that TremaHb is a symmetric dimeric hemoglobin similar to other class 1 nonsymbiotic plant hemoglobins but that ParaHb has structurally distinct heme coordination in each of its two subunits that is absent in the monomeric I43N mutant protein. A mechanism for achieving structural heterogeneity in ParaHb in which the Ile(101(F4)) side chain contacts the proximal His(105(F8)) in one subunit but not the other is proposed. These results are discussed in the context of the evolution of plant oxygen transport hemoglobins, and other potential functions of plant hemoglobins.

Comparison of the dielectric response obtained from fluorescence upconversion measurements and molecular dynamics simulations for coumarin 153-apomyoglobin complexes and structural analysis of the complexes by NMR and fluorescence methods.

We present a comparison of the dielectric response obtained from fluorescence upconversion experiments and from molecular dynamics simulations of the complexes of coumarin 153 with five apomyoglobins (apoMbs): wild-type horse heart (HH-WT) and those of wild-type sperm whale (SW-WT); its two triple mutants, L29F/H64Q/V68F and H64L/V68F/P88A; and its double mutant, L29F/V68L. Comparisons between experimental and simulated solvation relaxation functions, C(t)s, for the wild-type proteins range from very good to excellent. For the three mutants we investigated, however, agreement between experiment and simulation was considerably inferior. Thus, an NMR study of the complex of the HH-WT complex apoMb, and fluorescence energy transfer and anisotropy studies of the five complexes, were performed to investigate the structures upon which the simulations were based. The NMR measurements confirm our earlier conclusions that the C153 lies in the heme pocket of the HH-WT apoMb. For the wild-type complexes, fluorescence energy transfer measurements provide two rise times, suggesting a definite spatial relationship between the two Trp donors and the C153 acceptor. These results confirm the structural integrity of the wild-type complexes and validate the initial structures used for the molecular dynamics simulations. On the other hand, the three mutants provided single exponential rise times for energy transfer, suggesting that the position of the C153 used in the simulations may have been in error or that the C153 is mobile on the time scale of the energy transfer experiment. Fluorescence anisotropy studies also suggest that the double mutant was not structurally intact. Furthermore, examination of these systems demonstrates the sensitivity of C153 to its environment and permits the observation of differences in the heme pockets. These results point to the importance of structural characterization of modified proteins used in studies of the dielectric response and suggest strategies for performing molecular dynamics simulations of modified proteins.

Trema and parasponia hemoglobins reveal convergent evolution of oxygen transport in plants.

All plants contain hemoglobins that fall into distinct phylogenetic classes. The subset of plants that carry out symbiotic nitrogen fixation expresses hemoglobins that scavenge and transport oxygen to bacterial symbiotes within root nodules. These "symbiotic" oxygen transport hemoglobins are distinct in structure and function from the nonoxygen transport ("nonsymbiotic") Hbs found in all plants. Hemoglobins found in two closely related plants present a paradox concerning hemoglobin structure and function. Parasponia andersonii is a nitrogen-fixing plant that expresses a symbiotic hemoglobin (ParaHb) characteristic of oxygen transport hemoglobins in having a pentacoordinate ferrous heme iron, moderate oxygen affinity, and a relatively rapid oxygen dissociation rate constant. A close relative that does not fix nitrogen, Trema tomentosa, expresses hemoglobin (TremaHb) sharing 93% amino acid identity to ParaHb, but its phylogeny predicts a typical nonsymbiotic hemoglobin with a hexacoordinate heme iron, high oxygen affinity, and slow oxygen dissociation rate constant. Here we characterize heme coordination and oxygen binding in TremaHb and ParaHb to investigate whether or not two hemoglobins with such high sequence similarity are actually so different in functional behavior. Our results indicate that the two proteins resemble nonsymbiotic hemoglobins in the ferric oxidation state and symbiotic hemoglobins in the ferrous oxidation state. They differ from each other only in oxygen affinity and oxygen dissociation rate constants, two factors key to their different functions. These results demonstrate distinct mechanisms for convergent evolution of oxygen transport in different phylogenetic classes of plant hemoglobins.

Nitrosyl hydride (HNO) as an O2 analogue: long-lived HNO adducts of ferrous globins.

Nitrosyl hydride, HNO or nitroxyl, is the one-electron reduced and protonated form of nitric oxide. HNO is isoelectronic to singlet O(2), and we have previously reported that deoxymyoglobin traps free HNO to form a stable adduct. In this report, we demonstrate that oxygen-binding hemoglobins from human, soy, and clam also trap HNO to form adducts which are stable over a period of weeks. The same species can be formed in higher yields by careful reduction of the ferrous nitrosyl adducts of the proteins. Like the analogous O(2)-Fe(II) adducts, the HNO adducts are diamagnetic, but with a characteristic HNO resonance in (1)H NMR at ca. 15 ppm that splits into doublets for H(15)NO adducts. The (1)H and (15)N NMR resonances, obtained by HSQC experiments, are shown to differentiate subunits and isoforms of proteins within mixtures. An apparent difference in the reduction rates of the NO adducts of the two subunits of human hemoglobin allows assignment of two distinct nitrosyl hydride peaks by a combination of UV-vis, NMR, and EPR analysis. The two peaks of the HNO-hHb adduct have a persistent 3:1 ratio during trapping reactions, demonstrating a kinetic difference between HNO binding at the two subunits. These results show NMR characterization of ferrous HNO adducts as a unique tool sensitive to structural changes within the oxygen-binding cavity, which may be of use in defining modes of oxygen binding in other heme proteins and enzymes.

Effect of curcumin supplementation on serum expression of select cytokines and chemokines in a female rat model of nonalcoholic steatohepatitis.

OBJECTIVE: We recently reported that curcumin supplementation in a metabolically (i.e., Western diet [WD]) and chemically (i.e., CCl4) induced female rat model of non-alcoholic steatohepatitis (NASH) was associated with lower liver pathology scores and molecular markers of inflammation. This occurred when curcumin was given during induction of disease (preventative arm; 8-week WD with or without curcumin [8WD + C vs. 8WD]) as well as when given after disease development (treatment arm; 12-week WD with or without curcumin during weeks 9-12 [12WD + C vs. 12WD]). Herein, we sought to extend our findings from that study by determining the effects of curcumin supplementation on cytokine/chemokine expression in serum collected from these same rats. RESULTS: 24 cytokines/chemokines were assayed. IL-2 (+ 80%) and IL-13 (+ 83%) were greater with curcumin supplementation in the prevention arm. IL-2 (+ 192%), IL-13 (+ 87%), IL-17A (+ 81%) and fractalkine (+ 121%) were higher while RANTES was lower (- 22%) with curcumin supplementation in the treatment arm (p < 0.05 for all). RANTES concentrations also correlated significantly with hepatic pathology scores of inflammation (r = 0.417, p = 0.008). Select serum cytokines/chemokines were affected with curcumin supplementation in this female rat model of NASH. Moreover, curcumin's effect(s) on RANTES and its association with liver disease pathogenesis and progression may warrant further investigation.

Measurement of distal histidine coordination equilibrium and kinetics in hexacoordinate hemoglobins.

The kinetics of ligand binding to hemoglobins has been measured for decades. Initially, these studies were confined to readily available pentacoordinate oxygen transport proteins like myoglobin, leghemoglobin, and red blood cell hemoglobin. Bimolecular ligand binding to these proteins is relatively simple, as ligand association is largely unimpeded at the heme iron. Although many techniques have been used to examine these reactions in the past, stopped-flow rapid mixing and flash photolysis are the most common ways to measure rate constants for ligand association and dissociation. Expression of recombinant proteins has allowed for examination of many newly discovered hemoglobins. The hexacoordinate hemoglobins are one such group of proteins that exhibit more complex binding kinetics than pentacoordinate hemoglobins due to reversible intramolecular coordination by a histidine side chain. Here, we describe methods for characterizing the kinetics of ligand binding to hexacoordinate hemoglobins with a focus on measurement of histidine coordination and exogenous ligand binding in both the ferrous and the ferric oxidation states.

Plant hemoglobins: a molecular fossil record for the evolution of oxygen transport.

The evolution of oxygen transport hemoglobins occurred on at least two independent occasions. The earliest event led to myoglobin and red blood cell hemoglobin in animals. In plants, oxygen transport "leghemoglobins" evolved much more recently. In both events, pentacoordinate heme sites capable of inert oxygen transfer evolved from hexacoordinate hemoglobins that have unrelated functions. High sequence homology between hexacoordinate and pentacoordinate hemoglobins in plants has poised them for potential structural analysis leading to a molecular understanding of this important evolutionary event. However, the lack of a plant hexacoordinate hemoglobin structure in the exogenously ligand-bound form has prevented such comparison. Here we report the crystal structure of the cyanide-bound hexacoordinate hemoglobin from barley. This presents the first opportunity to examine conformational changes in plant hexacoordinate hemoglobins upon exogenous ligand binding, and reveals structural mechanisms for stabilizing the high-energy pentacoordinate heme conformation critical to the evolution of reversible oxygen binding hemoglobins.

Neuroglobin and cytoglobin distribution in the anterior eye segment: a comparative immunohistochemical study.

This study provides a detailed description of immunolocalization of two oxygen-binding proteins, neuroglobin (Ngb) and cytoglobin (Cygb), in the anterior segment of healthy human and canine eyes. Specific antibodies against Ngb and Cygb were used to examine their distribution patterns in anterior segment structures including the cornea, iris, trabecular meshwork, canal of Schlemm, ciliary body, and lens. Patterns of immunoreactivity (IR) were imaged with confocal scanning laser and conventional microscopy. Analysis of sectioned human and canine eyes showed Ngb and Cygb IR in the corneal epithelium and endothelium. In the iris, Ngb and Cygb IR was localized to the anterior border and the stroma, iridal sphincter, and dilator muscle. In the iridocorneal angle, Ngb and Cygb were detected in endothelial cells of the trabecular meshwork and canal of Schlemm in human. In the ciliary body, Ngb and Cygb IR was localized to the non-pigmented ciliary epithelium of the pars plana and pars plicata and in ciliary body musculature. Ngb and Cygb distribution was similar and colocalized within the same structures of healthy human and canine anterior eye segments. Based on their immunolocalization and previously reported biochemical features, we hypothesize that Ngb and Cygb may function as scavengers of reactive oxygen species. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

The structure and function of plant hemoglobins.

Plants, like humans, contain hemoglobin. Three distinct types of hemoglobin exist in plants: symbiotic, non-symbiotic, and truncated hemoglobins. Crystal structures and other structural and biophysical techniques have revealed important knowledge about ligand binding and conformational stabilization in all three types. In symbiotic hemoglobins (leghemoglobins), ligand binding regulatory mechanisms have been shown to differ dramatically from myoglobin and red blood cell hemoglobin. In the non-symbiotic hemoglobins found in all plants, crystal structures and vibrational spectroscopy have revealed the nature of the structural transition between the hexacoordinate and ligand-bound states. In truncated hemoglobins, the abbreviated globin is porous, providing tunnels that may assist in ligand binding, and the bound ligand is stabilized by more than one distal pocket residue. Research has implicated these plant hemoglobins in a number of possible functions differing among hemoglobin types, and possibly between plant species.

Covalent heme attachment in Synechocystis hemoglobin is required to prevent ferrous heme dissociation.

Synechocystis hemoglobin contains an unprecedented covalent bond between a nonaxial histidine side chain (H117) and the heme 2-vinyl. This bond has been previously shown to stabilize the ferric protein against denaturation, and also to affect the kinetics of cyanide association. However, it is unclear why Synechocystis hemoglobin would require the additional degree of stabilization accompanying the His117-heme 2-vinyl bond because it also displays endogenous bis-histidyl axial heme coordination, which should greatly assist heme retention. Furthermore, the mechanism by which the His117-heme 2-vinyl bond affects ligand binding has not been reported, nor has any investigation of the role of this bond on the structure and function of the protein in the ferrous oxidation state. Here we report an investigation of the role of the Synechocystis hemoglobin His117-heme 2-vinyl bond on structure, heme coordination, exogenous ligand binding, and stability in both the ferrous and ferric oxidation states. Our results reveal that hexacoordinate Synechocystis hemoglobin lacking this bond is less stable in the ferrous oxidation state than the ferric, which is surprising in light of our understanding of pentacoordinate Hb stability, in which the ferric protein is always less stable. It is also demonstrated that removal of the His117-heme 2-vinyl bond increases the affinity constant for intramolecular histidine coordination in the ferric oxidation state, thus presenting greater competition for the ligand binding site and lowering the observed rate and affinity constants for exogenous ligands.

Influence of the protein matrix on intramolecular histidine ligation in ferric and ferrous hexacoordinate hemoglobins.

Present in most organisms, hexacoordinate hemoglobins (hxHbs) are proteins that have evolved the capacity for reversible bis-histidyl heme coordination. The heme prosthetic group enables diverse protein functionality, such as electron transfer, redox reactions, ligand transport, and enzymatic catalysis. The reactivity of heme is greatly effected by the coordination and noncovalent chemical environment imposed by its connate protein. Of considerable interest is how the hxHb globin fold achieves reversible intramolecular coordination while still enabling high-affinity binding of oxygen, nitric oxide, and other small ligands. Here we explore this question by examining the role of the protein matrix on coordination behavior in a group of hxHbs from animals, plants, and bacteria, including human neuroglobin and cytoglobin, a nonsymbiotic hemoglobin from rice, and a truncated hemoglobin from the cyanobacterium Synechocystis. This is done with a set of experiments measuring the reduction potentials of each wild-type hxHb and its corresponding mutant protein where the reversibly bound histidine (the distal His) has been replaced with a noncoordinating side chain. These reduction potentials, coupled with studies of the mutant proteins saturated with exogenous imidazole, enable us to assess the effects of the protein matrices on histidine coordination. Our results show significant variation among the hxHbs, demonstrating flexibility in the globin moiety's ability to regulate reversible coordination. This regulation is particularly evident in the plant nonsymbiotic hemoglobins, where ferric state histidine coordination affinity is substantially lowered by the protein matrix.