Synergistic Effect of Distal Polar Interactions in Myoglobin and Their Structural Consequences

Functional Myoglobin Model Composed of a Strapped Porphyrin/Cyclodextrin Supramolecular Complex with an Overhanging COOH That Increases O2/CO Binding Selectivity in Aqueous Solution

A water-soluble strapped iron(III)tetraarylporphyrin (Fe^IIIPor-1) bearing two propylpyridinium groups at the side chains and a carboxylic acid group at the overhanging position of the strap was synthesized to mimic the function of myoglobin with the distal polar functionality in aqueous solution. Fe^IIIPor-1 forms a stable 1:1 inclusion complex with a per-O-methylated β-cyclodextrin dimer having a pyridine linker (Py3OCD), providing a hydrophobic environment and a proximal fifth ligand to stabilize the O₂-complex. The ferrous complex (Fe^IIPorCD-1) binds both O₂ and CO in aqueous solution. The O₂ and CO binding affinities (P_1/2^O2 and P_1/2^CO) and half-life time (t_1/2) of the O₂ complex of Fe^IIPorCD-1 are 6.3 and 0.021 Torr, and 7 h, respectively, at pH 7 and 25 °C. The control compound without the strap structure (Fe^IIPorCD-2) has similar oxygen binding characteristics (P_1/2^O2 = 8.0 Torr), but much higher CO binding affinity (P_1/2^CO = 3.8 × 10^-4 Torr), and longer t_1/2 (30 h). The O₂ and CO kinetics indicate that the strapped structure in Fe^IIPorCD-1 inhibits the entrance of these gaseous ligands into the iron(II) center, as evidenced by lower k_on^O2 and k_on^CO values. Interestingly, the CO complex of Fe^IIPorCD-1 is significantly destabilized (relatively larger k_off^CO), while the k_off^O2 value is much smaller than that of Fe^IIPorCD-2, resulting in significantly increased O₂/CO selectivity (reduced M value, where M = P_1/2^O2/P_1/2^CO = 320) in Fe^IIPorCD-1 compared to Fe^IIPorCD-2 (M = 21000).

Effect of the Electron Density of the Heme Fe Atom on the Nature of Fe-O2 Bonding in Oxy Myoglobin

Mössbauer spectroscopy has been used to characterize oxygenated myoglobins (oxy Mbs) reconstituted with native and chemically modified ⁵⁷Fe-enriched heme cofactors with different electron densities of the heme Fe atom (ρ_Fe) and to elucidate the effect of a change in the ρ_Fe on the nature of the bond between heme Fe and oxygen (O₂), i.e., the Fe-O₂ bond, in the protein. Quadrupole splitting (ΔE_Q) was found to decrease with decreasing ρ_Fe, and the observed ρ_Fe-dependent ΔE_Q confirmed an increase in the contribution of the ferric-superoxide (Fe³⁺-O₂^-) form to the resonance hybrid of the Fe-O₂ fragment with decreasing ρ_Fe. These observations explicitly accounted for the lowering of O₂ affinity of the protein due to an increase in the O₂ dissociation rate and a decrease in the autoxidation reaction rate of oxy Mb through decreasing H⁺ affinity of the bound ligand with decreasing ρ_Fe. Therefore, the present study demonstrated the mechanism underlying the electronic control of O₂ affinity and the autoxidation of the protein through the heme electronic structure. Carbon monoxide (CO) adducts of reconstituted Mbs (CO-Mbs) were similarly characterized, and we found that the resonance between the two canonical forms of the Fe-CO fragment was also affected by a change in ρ_Fe. Thus, the nature of the Fe-ligand bond in the protein was found to be affected by the ρ_Fe.

Towards the understanding of the enzymatic cleavage of polyisoprene by the dihaem-dioxygenase RoxA

Utilization of polyisoprene (natural rubber) as a carbon source by Steroidobacter cummioxidans 35Y (previously Xanthomonas sp. strain 35Y) depends on the formation and secretion of rubber oxygenase A (RoxA). RoxA is a dioxygenase that cleaves polyisoprene to 12-oxo-4,8-dimethyl-trideca-4,8-diene-1-al (ODTD), a suitable growth substrate for S. cummioxidans. RoxA harbours two non-equivalent, spectroscopically distinguishable haem centres. A dioxygen molecule is bound to the N-terminal haem of RoxA and identifies this haem as the active site. In this study, we provide insights into the nature of this unusually stable dioxygen-haem coordination of RoxA by a re-evaluation of previously published together with newly obtained biophysical data on the cleavage of polyisoprene by RoxA. In combination with the meanwhile available structure of RoxA we are now able to explain several uncommon and previously not fully understood features of RoxA, the prototype of rubber oxygenases in Gram-negative rubber-degrading bacteria.

Catalase-Like Antioxidant Activity is Unaltered in Hypochlorous Acid Oxidized Horse Heart Myoglobin

Activated neutrophils release myeloperoxidase that produces the potent oxidant hypochlorous acid (HOCl). Exposure of the oxygen transport protein horse heart myoglobin (hhMb) to HOCl inhibits Iron III (Fe(III))-heme reduction by cytochrome b5 to oxygen-binding Iron II (Fe(II))Mb. Pathological concentrations of HOCl yielded myoglobin oxidation products of increased electrophoretic mobility and markedly different UV/Vis absorbance. Mass analysis indicated HOCl caused successive mass increases of 16 a.m.u., consistent serial addition of molecular oxygen to the protein. By contrast, parallel analysis of protein chlorination by quantitative mass spectrometry revealed a comparatively minor increase in the 3-chlorotyrosine/tyrosine ratio. Pre-treatment of hhMb with HOCl affected the peroxidase reaction between the hemoprotein and H₂O₂ as judged by a HOCl dose-dependent decrease in spin-trapped tyrosyl radical detected by electron paramagnetic resonance (EPR) spectroscopy and the rate constant of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid (ABTS) oxidation. By contrast, Mb catalase-like antioxidant activity remained unchanged under the same conditions. Notably, HOCl-modification of Mb decreased the rate of ferric-to-ferrous Mb reduction by a cytochrome b5 reductase system. Taken together, these data indicate oxidizing HOCl promotes Mb oxidation but not chlorination and that oxidized Mb shows altered Mb peroxidase-like activity and diminished rates of one-electron reduction by cytochrome b5 reductase, possibly affecting oxygen storage and transport however, Mb-catalase-like antioxidant activity remains unchanged.