Dioxygen binding

Scheme 10.27 Catalytic cycle of HppE. Dashed arrows indicate electron transport. In this scheme HPP binds to iron1". After a one-electron reduction, dioxygen binds and reoxidizes the iron center. The peroxide radical is capable of stereospecifically abstracting the (pro-R) hydrogen. Another one-electron reduction is required to reduce one peroxide oxygen to water. Epoxide formation is mediated by the resulting ironlv-oxo species.

On carbon monoxide and dioxygen binding by iron(II) porphyrinato systems. G. B. Jameson and... 

Spin-pairing model of dioxygen binding and its application to various transition metal systems as well as hemoglobin cooperativity. R. S. Drago and B. B. Corden, Acc. Chem. Res., 1980, 13, 353-360 (39). 

As briefly mentioned above, the reduced form of MMO reacts with oxygen to initiate substrate oxygenation. To further analyze the protein effects on this reaction, the dioxygen-binding step was treated with two-layer ONIOM (B3LYP Amber) [25], The overall setup was similar to the one used for evaluating active-site geometries. 

Figure 2.3 Structures of two globin compounds capable of reversible dioxygen binding. From Lippard and Berg, 1994. Reproduced by permission of University Science Books.

In a subsequent study of this type (Durand, Bencosme, Collman Anson, 1983), dimers of type (143) were investigated as potential redox catalysts for the four-electron reduction of dioxygen to water (via peroxide). The Co(ii)/Co(ii) dimer is an effective catalyst for the electrochemical reduction of dioxygen once again the dioxygen binds... 

Even though the iron atoms are separated in haemoglobin by about 25 A, communication between them is still able to occur and this has been postulated to involve a trigger mechanism (Perutz, 1971). The trigger is the movement of the proximal histidine as dioxygen binds to (or is released from) the Fe(n) and results in interconversion between the T and R structures. This movement causes a conformational change which is transmitted through the protein to the other iron sites. X-ray studies indicate that relative shifts of up to 6 A at subunit interfaces occur between the T and R states (Perutz, 1978). 

Hemoglobin s dioxygen binding is regulated by local concentrations of H+ (known as the Bohr effect), CO2 concentration, and organic phosphates such as diphosphoglycerate (DPG), whose structure is shown in Figure 4.2.17... 

Figure 4.2 Diphosphoglycerate (DPG), regulator of heme dioxygen binding.

Figure 4.9 Dioxygen binding curves for myoglobin and hemoglobin. (Reprinted with permission from Figure 4.4 of Cowan, J. A. Inorganic Biochemistry, An Introduction, 2nd ed., Wiley-VCH, New York, 1997. Copyright 1997, Wiley-VCH.)...

Figure 4.13 Co(acacen), an early model compound exhibiting reversible dioxygen binding at low temperatures.

A thorough kinetic and thermodynamic analysis of this model system (small positive or negative enthalpies of formation are canceled by more negative entropies of formation) led Karlin s group to conclude that the stability of dioxygen binding is driven by favorable enthalpies, but unfavorable reaction entropies preclude observation of Cu2-02 at room temperatures.412... 

Another gradient-corrected density functional study of reversible dioxygen binding and reversible 0-0 bond cleavage has been carried out for Cu2(p-T 2 r 2... 

Dioxo-L-gulonic acid, 25 751 Dioxygen, reversible binding of, 14 554 Dioxygen binding, by iron, 24 47... 

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