Low CO Affinity Research Articles

Shape dependency of CO2 hydrogenation on ceria supported singly dispersed Ru catalysts

Harnessing the selectivity in catalytic hydrogenation of CO2 is important for producing valuable chemicals. Herein, the shape dependency of the CO2 hydrogenation on singly dispersed Ru/CeO2 catalysts is investigated. CeO2 of three shapes, including nanocube, nanooctahedron and nanorod, are prepared as the catalyst supports and assigned to the surface of (100), (111) and (110) crystal plane, respectively. The Ru/CeO2 (nanorod) exhibits the highest selectivity to CO (97.6 %) at 350 °C, while the Ru/CeO2 (nanocube) exhibits a mediocre selectivity to CO (41.4 %) but the highest CO2 conversion. The shape of the CeO2 support determines the electronic state of supported Ru atoms through the electronic metal-support interaction. The low CO affinity on cationic Ruβ sites leads to the higher selectivity to CO on the Ru/CeO2 (nanorod) and Ru/CeO2 (nanooctahedron) catalysts, while the strong dissociation ability of H2 on low valence Ruα sites results in the high activity on the Ru/CeO2 (nanocube) catalyst.

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

YC-1 Binding to the β Subunit of Soluble Guanylyl Cyclase Overcomes Allosteric Inhibition by the α Subunit

Soluble guanylate cyclase (sGC) is a heterodimeric heme protein and the primary nitric oxide receptor. NO binding stimulates cyclase activity, leading to regulation of cardiovascular physiology and making sGC an attractive target for drug discovery. YC-1 and related compounds stimulate sGC both independently and synergistically with NO and CO binding; however, where the compounds bind and how they work remain unknown. Using linked equilibrium binding measurements, surface plasmon resonance, and domain truncations in Manduca sexta and bovine sGC, we demonstrate that YC-1 binds near or directly to the heme-containing domain of the β subunit. In the absence of CO, YC-1 binds with a Kd of 9-21 μM, depending on the construct. In the presence of CO, these values decrease to 0.6-1.1 μM. Pfizer compound 25 bound ∼10-fold weaker than YC-1 in the absence of CO, whereas compound BAY 41-2272 bound particularly tightly in the presence of CO (Kd = 30-90 nM). Additionally, we found that CO binds much more weakly to heterodimeric sGC proteins (Kd = 50-100 μM) than to the isolated heme domain (Kd = 0.2 μM for Manduca β H-NOX/PAS). YC-1 greatly enhanced binding of CO to heterodimeric sGC, as expected (Kd ∼ 1 μM). These data indicate the α subunit induces a heme pocket conformation with a lower affinity for CO and NO. YC-1 family compounds bind near the heme domain, overcoming the α subunit effect and inducing a heme pocket conformation with high affinity. We propose this high-affinity conformation is required for the full-length protein to achieve high catalytic activity.

Dynamics of Carbon Monoxide Binding to CooA

CooA is a dimeric CO-sensing heme protein from Rhodospirillum rubrum. The heme iron in reduced CooA is six-coordinate; the axial ligands are His-77 and Pro-2. CO displaces Pro-2 and induces a conformation change that allows CooA to bind DNA and activate transcription of coo genes. Equilibrium CO binding is cooperative, with a Hill coefficient of n = 1.4, P(50) = 2.2 microm, and estimated Adair constants K(1) = 0.16 and K(2) = 1.3 microm(-1). The rates of CO binding and release are both strongly biphasic, with roughly equal amplitudes for the fast and slow phases. The association rates show a hyperbolic dependence on [CO], consistent with Pro-2 dissociation being rate-limiting. The kinetic characteristics of the transiently formed five-coordinate heme are probed via flash photolysis. These observations are integrated into a kinetic model, in which CO binding to one subunit decreases the rate of Pro-2 rebinding in the second, leading to a net increase in affinity for the second CO. The CO adduct exists in slowly interconverting "open" and "closed" forms. This interconversion probably involves the large-scale motions required to bring the DNA-binding domains into proper orientation. The combination of low CO affinity, slow CO binding, and slow conformational transitions ensures that activation of CooA only occurs at high (micromolar) and sustained (> or =1 min) levels of CO. When micromolar levels do occur, positive cooperativity allows efficient activation over a narrow range of CO concentrations.

Iron twin-coronet porphyrins as models of myoglobin and hemoglobin: amphibious electrostatic effects of overhanging hydroxyl groups for successful CO/O2 discrimination.

Inspired by the observation of polar interactions between CO and O(2) ligands and the peptide residues at the active site of hemoglobin and myoglobin, we synthesized two kinds of superstructured porphyrins: TCP-IM, which contains a linked imidazole ligand, and TCP-PY, which contains a linked pyridine ligand, and examined the thermodynamic, kinetic, and spectroscopic (UV/Vis, IR, NMR, and resonance Raman) properties of their CO and O(2) complexes. On both sides of each porphyrin plane, bulky binaphthyl bridges form hydrophobic cavities that are suitable for the binding of small molecules. In the proximal site, an imidazole or pyridine residue is covalently fixed and coordinates axially to the central iron atom. In the distal site, two naphtholic hydroxyl groups overhang toward the center above the heme. The CO affinities of TCPs are significantly lower than those of other heme models. In contrast, TCPs have moderate O(2) binding ability. Compared with reported model hemes, the binding selectivity of O(2) over CO in TCP-IM and TCP-PY complexes is greatly improved. The high O(2) selectivity of the TCPs is mainly attributable to a low CO affinity. The comparison of k(on)(CO) values of TCPs with those of unhindered hemes indicates the absence of steric hindrance to the intrinsically linear CO coordination to Fe(II) in TCP-IM and TCP-PY. The abnormally large k(off)(CO) values are responsible for the low CO affinities. In contrast, k(off)(O(2)) of TCP-PY is smaller than those of other pyridine-coordinated model hemes. For the CO adducts of TCPs, unusually low nu(Fe-CO) and unusually high nu(C-O) frequencies are observed. These results can be ascribed to decreased back-bonding from the iron atom to the bound CO. The lone pairs of the oxygen atoms of the hydroxyl groups prevent back-bonding by exertion of a strong negative electrostatic interaction. On the other hand, high nu(Fe-O(2)) frequencies are observed for the O(2) adducts of TCPs. In the resonance Raman (RR) spectrum of oxy-TCP-IM, we observed simultaneous enhancement of the Fe-O(2) and O-O stretching modes. Furthermore, direct evidence for hydrogen bonding between the hydroxyl groups and bound dioxygen was obtained by RR and IR spectroscopy. These spectroscopic data strongly suggest that O(2) and CO binding to TCPs is controlled mainly by the two different electrostatic effects exerted by the overhanging OH groups: destabilization of CO binding by decreasing back-bonding and stabilization of O(2) binding by hydrogen bonding.

Low carbon monoxide affinity allene oxide synthase is the predominant cytochrome P450 in many plant tissues

A cytochrome P450 with low affinity (2.9 x 10(3) M-1) for CO appears to be the major microsomal P450 in some plant tissues. The presence of low CO affinity cytochrome P450 correlates with its lack of NADPH reducibility and with the presence of high levels of 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoate peroxidase activity. This activity and low CO affinity are retained by purified tulip cytochrome P450, which appears to be catalytically identical to a flaxseed-derived fatty acid allene oxide synthase P450 described previously [Song, W.-C., & Brash, A.R. (1991) Science 253, 781-784]. Other heme-binding ligands, such as CN- and imidazoles, bind weakly to the allene oxide synthase P450s, suggesting that axial coordination in the heme distal pocket may be hindered. We conclude that low CO affinity is characteristic of the allene oxide synthase P450s and that these P450s constitute a major portion of the microsomal P450 in a variety of plant tissues, particularly from monocot species.

Stereochemistry of carbon monoxide binding to normal human adult and Cowtown haemoglobins

The structures of carbonmonoxyhaemoglobins A and Cowtown (His146β → Leu) have been refined at 2.2 Å (1 Å = 0.1 nm) and 2.3 Å resolution, respectively. The least squares fit to the FeCO line makes an angle to the haem normal of about 6 °. The FeCO group is bent from linearity by about 7 °. The porphyrins in the CO liganded haemoglobins are ruffled. This deformation of the haem and the distortion of the FeCO group may explain the low CO affinity of haemoglobin. The electron density for the C-terminal residues is low but sufficient to distinguish the histidyl and leucyl residues clearly. The similarity between these two structures, apart from 146β, means that the reduced alkaline Bohr effect is due solely to the replacement of histidine by a leucine.

The mechanisms of H2 activation and CO binding by hydrogenase I and hydrogenase II of Clostridium pasteurianum.

Hydrogenase I (bidirectional) and hydrogenase II (uptake) of Clostridium pasteurianum have been investigated by electron paramagnetic resonance (EPR) spectroscopy, in the presence and absence of the inhibitor, CO. These hydrogenases contain both a novel type of iron-sulfur cluster (H), which is the proposed site of H2 catalysis, and ferredoxin-type [4Fe-4S] clusters (F). The results show that the H clusters of these two hydrogenases have very different properties. The H cluster of oxidized hydrogenase II (Hox-II) exhibits three distinct EPR signals, two of which are pH-dependent. Hox-II binds CO reversibly to give a single, pH-independent species with a novel, rhombic EPR spectrum. The H cluster of reduced hydrogenase II (Hred-II) does not react with CO. In contrast, the EPR spectrum of Hox-I appears homogeneous and independent of pH. Hox-I has a much lower affinity for CO than Hox-II, and binds CO irreversibly to give an axial EPR signal. Hred-I also binds CO irreversibly. The EPR spectra of Fred-I and Fred-II show little or no change after CO treatment. Prior exposure to CO does not affect the catalytic activity of the reduced or oxidized hydrogenases when assayed in the absence of CO, but both enzymes are irreversibly inactivated if CO is present during catalysis. Mechanisms for H2 activation by hydrogenase I and hydrogenase II are proposed from the determined midpoint potentials (Em, pH 8.0) of H-I and H-II (Em approximately -400 mV, -CO; approximately -360 mV, +CO), F-I (Em = -420 mV, +/- CO), and F-II (Em = -180 mV, +/- CO). These allow one to rationalize the different modes of CO binding to the two hydrogenases and suggest why hydrogenase II preferentially catalyzes H2 oxidation. The results are discussed in light of recent spectroscopic data on the structures of the two H clusters.

Cytochrome c peroxidase activity of bovine heart cytochrome oxidase incorporated in liposomes and generation of membrane potential.

Cytochrome oxidase vesicles catalyzed the peroxidatic oxidation of ferrocytochrome c. The maximal peroxidase activity in the absence of an uncoupling agent was 9.8 mol ferrocytochrome c oxidized/(s X mol heme a), indicating a 5-fold activation compared with the soluble enzyme system. The peroxidase activity was further enhanced 1.2 to 2.1 times upon addition of an uncoupler, carbonyl cyanide p-trifluoromethoxyphenyl hydrazone. The stoichiometry of the reduction of hydrogen peroxide by ferrocytochrome c was established to be 1 : 2, indicating water formation. Potassium cyanide (0.14 mM) completely inhibited the peroxidase activity. The inhibition by 1 mM CO was 40-77% depending on the energized state of cytochrome oxidase vesicles, but in contrast, 85% inhibition was observed with the soluble enzyme. In the energized state the enzyme showed a slightly lower affinity for CO than in the deenergized state. Coupled with the peroxidase activity, a membrane potential of 72 mV was registered transiently; this may be physiologically significant in relation to the energy transduction mechanism.

Spectral and carbon monoxide binding properties of Fe(II) picket-fence porphyrin and protoheme bound to liposomes

Spectral and CO binding properties of liposome bound heme compounds, Fe(II) picket-fence porphyrin and protoheme, were examined. Phosphatidylethanolamine and phosphatidylcholine were used to make liposomes. Liposome-bound protoheme showed a very low CO affinity, whereas liposome-bound Fe(II) picket-fence porphyrin showed a high affinity. Addition of the ligand, 1-methylimidazole, modulated the CO affinities of both types of complexes to a level comparable to that of hemoglobin. However, autoxidation rates of the liposome bound heme compounds were still considerably high, and no stable oxygenated form could be observed.

Kinetics of ligation reactions of rabbit hemoglobin in quaternary R and T states.

Rabbit hemoglobin shows significantly lower affinity for CO than does human hemoglobin (Hb A). The overall ligand combination and dissociation rate constants reveal, however, only small differences between Hb A rabbit Hb; this is mainly due to the fact that beta chains in rabbit hemoglobin determine the kinetics of ligand dissociation and combination. The heme environment in these chains is probably not very different in rabbit Hb and human Hb A. Rabbit hemoglobin alpha chains, on the other hand, exhibit greatly reduced CO and O2 combination rates in the R state and are primarily responsible for the overall low CO affinity of rabbit Hb. We postulate that the low ligand affinity of alpha chains in rabbit Hb is due to the substitution of larger residues at positions B10(Leu leads to Val), Cd6(Leu leads to Phe), and CD7(Ser leads to Thr). The possible implication of such substitutions for the kinetic and equilibrium properties of rabbit Hb are discussed.

Pathways of CO2 Fixation in the CAM Plant Kalanchoe daigremontiana. II* Effects of O2 and CO2 Concentration on Light and Dark CO2 Fixation

Steady-state fixation of CO*2 by leaves of K. daigremontiana was examined at O*2 concentrations of 4, 20 and 36%. Oxygen concentration had no effect on the rate of dark CO*2 fixation, whereas CO*2 fixation in the light was substantially inhibited by 20 and 36% O*2. Dark CO*2 fixation showed a high affinity for CO*2 (half-saturated at about 3 �M at 16�C) whereas CO*2 fixation in the light showed a much lower affinity for CO*2 (half-saturated at about 9 �M at 16�C and 37 �M at 29�C). The inhibition by O*2 of CO*2 fixation in the light was observed at intercellular CO*2 pressures up to 850 �bar. It was associated with a positive CO*2 compensation point which was abolished in 4% O*2, and a post- illumination burst of CO*2 evolution which was not modified by low O*2. The response of dark and light CO*2 fixation in K. daigremontiana to O*2 concentration is discussed in comparison with photosynthetic CO*2 fixation in C*3 and C*4 plants.

Interaction of hydrogen and carbon monoxide on platinum group metals

The coadsorption and interaction of carbon monoxide and hydrogen on a series of unsupported noble metal catalysts was studied in the temperature range 100–200 °C. In the case of platinum, CO was preferentially adsorbed from mixtures with H 2 and the resulting chemisorbed CO could not be completely removed by reduction and evacuation at 150 °C. With rhodium and iridium there was some evidence of interaction between the two gases and small amounts of methane were detected at 200 °C. Ruthenium and its alloys showed quite different behavior. Chemisorbed CO could be completely removed by reduction and evacuation at 150 °C, the adsorption of H 2 was enhanced by the presence of CO, and methane was formed at an appreciable rate at temperatures above 100 °C. On ruthenium, the activation energies for the reactions between CO and H 2 or D 2 were about 9 kcal/mole. The highly specific behavior of ruthenium in the methanation reaction is probably due to a lower affinity for CO than with the other noble metals.