Water may inhibit oxygen binding in hemoprotein models

Abstract

Three distal imidazole pickets in a cytochrome c oxidase (CcO) model form a pocket hosting a cluster of water molecules. The cluster makes the ferrous heme low spin, and consequently the O2 binding slow. The nature of the rigid proximal imidazole tail favors a high spin/low spin cross-over. The O2 binding rate is enhanced either by removing the water, increasing the hydrophobicity of the gas binding pocket, or inserting a metal ion that coordinates to the 3 distal imidazole pickets.spin cross-over ͉ tris-imidazole pocket ͉ water cluster H emoglobin (Hb), myoglobin (Mb) and cytochrome c oxidase (CcO) are hemoproteins that bind O 2, subsequently transporting or reducing it in a 4e-/4Hϩ process (1, 2). The kinetics of oxygen binding to the active sites of these biomolecules are tuned by stabilizing interactions between the oxygen complex and the immediate environment (3, 4). In monometallic proteins such as myoglobin or hemoglobin, a distal histidine stabilizes the oxygen complex by hydrogen bonding (5, 6). A distal copper tris-imidazole ligand in CcO also provides enthalpic stabilization (7). In addition to structural factors affecting oxygen adduct stability in Mb and Hb, it has been suggested that the molecules of water present in the binding pocket could contribute to the relative oxygen affinities (4, 8, 9) in a model dubbed the ''water displacement model.'' This proposed model could logically be extended to CcO where water is the product of the 4e-/4Hϩ reduction of O 2 . Water is expected to be present in larger quantities in CcO than in Hb or Mb, and it is removed from the binding site by water channels (10-13). Here, we describe well-defined biomimetic hemoprotein models 1-4 ( Fig. 1) that demonstrate the role of water in slowing the binding of O 2 . The rates of reaction correlate with the hydrophobicity of the distal pocket (tris-pivalamido-or picket fence in 2, tris-imidazole in 3) and the presence or absence of a distal bound metal [Cu(I) or Zn(II) in 4ab]. Results and DiscussionThe reaction of oxygen with ferrous porphyrins 1-4 was carried out by injecting an anaerobic solution of porphyrin into O 2 -saturated dichloromethane solution (10 mM) under 1 atm of O 2 . Under the initial conditions of the reaction it is assumed, based on earlier studies (15), that the ligand association rate k on (O 2 ) is several orders of magnitude faster than the ligand dissociation rate k off (O 2 ) and consequently the dissociation rate can be assumed negligeable. The O 2 binding rates were obtained by monitoring the change in of the Soret and Q bands in the UV/Vis spectrum (Fig. 2) that shifted from 426 nm to 421 nm, and from 535 nm to 550 nm, respectively. The oxygen complex was characterized at various stages of the reaction by 2 resonance Raman stretches: (i) an oxygen isotope sensitive band observed at 570/544 cm Ϫ1 that corresponds to the Fe-O stretch and (ii) a 4-band (a spin state and redox state marker band) at 1,370 cm Ϫ1 that is typical of a ferric-superoxo species (16)(17)(18)(19). Second order k on (O 2 ) ra...

Role of a distal pocket in the catalytic O 2 reduction by cytochrome c oxidase models immobilized on interdigitated array electrodes

“…electron transfer rate ͉ oxygen binding rates ͉ water cluster ͉ oxygen complex stabilization ͉ partially reduced oxygen species A recent report showed that water may inhibit oxygen binding in hemoprotein models (1). This echoes the ''water displacement model'' proposed for oxygen binding in myoglobin (2)(3)(4).…”

“…However, the amount of PROS formed with the picket fence 4 is twice the amount formed with the Trisimidazole 3. Note that the measure of PROS is a percentage of the total current generated and, hence, does not reflect the number of turnovers associated with slower oxygen binding rates as was found in the Tris-imidazole model 3 (1). Altogether these facts suggest that the nature of the superstructure may play a significant but surprising role in preventing the release of the oxygen complex (or other partially reduced oxygen species), making 3 more stable to hydrolytic formation of PROS than the picket fence 4.…”

Electrocatalytic O2-Reduction by Synthetic Cytochrome c Oxidase Mimics: Identification of a “Bridging Peroxo” Intermediate Involved in Facile 4e–/4H+ O2-Reduction

“…38,40 Note that imidazole and water bound LS Fe II species, although proposed in some synthetic systems, are yet to be structurally characterized. 47,48 …”

“…6,28 The slower reactivity of those synthetic heme-Cu systems may be due to a slower rate of O 2 binding to the 6C LS reduced iron center. 28,47,51 …”

Transition Metal Complexes and the Activation of Dioxygen

“…This conclusion is largely supported on the basis of comparison to 46a, which reacts slower than its dinuclear counterpart [136]. In aqueous systems, the presence of copper has also been suggested to play a role in protecting the iron center against the coordination of water, which has been shown to significantly inhibit the binding of O 2 by effecting a high-to low-spin crossover [137].…”