Cytochrome EXAFS

EXAFS data indicated an Fe=0 bond length of 1.65 0.05 A, similar to that found for compound I intermediates in several enzymes such as catalases, horseradish peroxidase, cytochrome c peroxidase, and chloroperoxidases. 

Dawson, J. H., et al. (1986) Oxygenated cytochrome P450cam and chloroperoxidase direct evidence for sulfur donor ligation trans todioxygen and structural characterization using EXAFS spectroscopy. J. Am. Chem. Soc. 108, 8114-8116. 

EXAFS data on the carbonmonoxy cytochrome a3 indicate1310 that the first shell structure is identical to that of hemoglobin, a point of some mechanistic interest. 

However, spectroscopic studies of activated BLM indicate that it is not an Fev=0 species. It exhibits an S - 1/2 EPR spectrum with g values at 2.26, 2.17, and 1.94 [15], which is typical of a low-spin Fe111 center. This low-spin Fem designation is corroborated by Mossbauer and x-ray absorption spectroscopy [16,19], Furthermore, EXAFS studies on activated BLM show no evidence for a short Fe—0 distance, which would be expected for an iron-oxo moiety [19], These spectroscopic results suggest that activated BLM is a low-spin iron(III) peroxide complex, so the two oxidizing equivalents needed for the oxidation chemistry would be localized on the dioxygen moiety, instead of on the metal center. This Fe(III)BLM—OOH formulation has been recently confirmed by electrospray ionization mass spectrometry [20] and is supported by the characterization of related synthetic low-spin iron(III) peroxide species, e.g., [Fe(pma)02]+ [21] and [Fe(N4py)OOH]2+ [22], The question then arises whether the peroxide intermediate is itself the oxidant in these reactions or the precursor to a short-lived iron-oxo species that effects the cytochrome P-450-like transformations. This remains an open question and the subject of continuing interest. 

The Cytochrome c/cytochrome b5 Complex Structural Features. One important feature of the c/b5 system is the detailed structural information which is available for these proteins. The structure of cytc is nown at 1.5A resolution in both the oxidized and reduced states. EXAFS studies have also been reported, which show no observable changes i getal coordination geometry on oxidation/reduction. The cytochrome b5 s ucture has also been solved at high resolution by Matthews et al. 

The knowledge of these unique short iron-oxygen distances in ferryl compounds aids in identifying whether a compound has a ferryl structure. Thus the 580 nm compound formed upon addition of peroxide to mitochondrial cytochrome c oxidase has an iron-to-oxygen distance of 1.7 A, suggesting that it is a ferryl intermediate [117]. Of the two haem iron atoms in this molecule, one is unreactive to peroxide. Therefore it is possible to analyse an EXAFS spectrum of the peroxide-treated enzyme minus the spectrum of the untreated enzyme to determine this distance. However, clearly such difference EXAFS spectra will have increased errors associated with the estimate of iron-oxygen distances. 

Cytochrome c Oxidase. This is a ubiquitous enzyme that catalyzes the terminal step in the four-electron reduction of 02 to 2H20. It contains both heme iron and copper and there are two kinds of copper sites, CuA and CuB, in each of which there is an association of copper with heme iron. The CuB site seems to have one copper atom linked to a heme unit, but details are still obscure.37 From modeling studies and EXAFS data, opinion presently favors some sort of Fe—X—Cu bridge... 

These are similar to the mitochondrial enzyme, apart from the simpler subunit composition. EXAFS studies on the oxidases from Paracoccus denitrijicans and Thermus thermophilus have confirmed the metal centres to be similar to those in the mitochondrial enzyme. In the latter case, the ESR silent cytochrome Us in the reduced enzyme has been shown by the use of Mossbauer spectroscopy to involve five-coordinate, high-spin On addition of cyanide to the oxidized... 

---