Iron catalysis hydroxylation

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

Iron catalysis in oxidation reactions with peroxides, both hydrogen peroxide and alkyl hydroperoxides, is frequently regarded as just a Haber-Weiss-type system where hydroxyl... 

Indeed, when present in concentrations sufficient to overwhelm normal antioxidant defences, ROS may be the principal mediators of lung injury (Said and Foda, 1989). These species, arising from the sequential one-electron reductions of oxygen, include the superoxide anion radical, hydrogen peroxide, hypochlorous ions and the hydroxyl radical. The latter species is thought to be formed either from superoxide in the ptesence of iron ions (Haber-Weiss reaction Junod, 1986) or from hydrogen peroxide, also catalysed by ferric ions (Fenton catalysis Kennedy et al., 1989). 

An intriguing puzzle in NOS catalysis is the precise role of H4B. The traditional function of H4B is in aromatic amino acid metabolism where H4B directly participates in the hydroxylation reaction via a nonheme iron. However, the NOS pterin site has no similarity to the pterin site in the hydroxylases, nor does NOS have a nonheme iron to assist pterin in substrate hydroxylation as in the amino acid hydroxylases 111). NOS more closely resembles pterin-containing enz5unes that have a redox function 81). In particular, N3 and the 03 amino group form H-bonds with either GIu or Asp residues in a series of pterin enzymes 112-116) similar to NOS, except that NOS utilizes the heme propionate (Fig. 6). 

Fig. 21. Detailed mechanism for HO-1 catalysis. In 1, oxygenation and electron transfer forms the ferric (Fe +)-peroxy complex. Steric factors and H-bonding help to bend the peroxide toward the a-meso-heme position for regio-selective hydroxylation. One proposed mode of forming verdoheme is shown in part 2. A key part of step 2 is the resonance structures between Fe + and Fe +/radical, which enable the porph3rrin ring to be oxygenated. Although the mechanism shown does not require any reducing equivalents (176), there remain experimental inconsistencies on the requirement of an additional electron in step 2. However, reduction of the verdoheme iron is necessary to prepare the substrate for step 3, verdoheme to bihverdin.

FIGURE 16-10 Iron-sulfur center in aconitase. The iron-sulfur center is in red, the citrate molecule in blue. Three Cys residues of the enzyme bind three iron atoms the fourth iron is bound to one of the carboxyl groups of citrate and also interacts noncovalently with a hydroxyl group of citrate (dashed bond). A basic residue ( B) on the enzyme helps to position the citrate in the active site. The iron-sulfur center acts in both substrate binding and catalysis. The general properties of iron-sulfur proteins are discussed in Chapter 19 (see Fig. 19-5). 

Because the three-dimensional structures of the peroxidase, its reductant cytochrome c, and the complex of the two (Fig. 16-9) are known, cytochrome c peroxidase is the subject of much experimental study. Other fungal peroxidases, some of which contain manganese rather than iron, act to degrade lignin (Chapter 25).218 A lignin peroxidase from the white wood-rot fungus Phanerochaete chrysosporium has a surface tryptophan with a specifically hydroxylated C(3 carbon atom which may have a functional role in catalysis.2183 0... 

Several diverse metal centres are involved in the catalysis of monooxygenation or hydroxylation reactions. The most important of these is cytochrome P-450, a hemoprotein with a cysteine residue as an axial ligand. Tyrosinase involves a coupled binuclear copper site, while dopamine jS-hydroxylase is also a copper protein but probably involves four binuclear copper sites, which are different from the tyrosinase sites. Putidamonooxin involves an iron-sulfur protein and a non-heme iron. In all cases a peroxo complex appears to be the active species. 

PAH, TH and TPH are highly homologous enzymes. These enzymes catalyze a hydroxylation reaction of aromatic amino acids that requires reduced pterin cofactor 43, molecular oxygen, and iron (Scheme 28). Iron is present at the active sites of the enzymes. Ferrous iron (Fe(II)) is essential for the catalysis, although, the iron was found to be in the ferric form (Fe(III)) when the enzymes were purified from tissues or cells. The ferric iron at the active site of the enzymes was found to be reduced to the ferrous form by BH4 [125]. Thus, BH4 serves a bi-functional role for aromatic amino acid hydroxylases one is the reduction of iron at the active sites from the ferric form to the ferrous form and the other is an electron donor for the hydroxylation reaction. 

Ferrihydrite catalysis of hydroxyl radical formation from peroxide has also shown experimental results consistent with a surface reaction [57]. The yield of hydroxyl radical formation was lower for ferrihydrite than for dissolved iron, resulting in a higher peroxide demand to degrade a given amount of pollutant. As mentioned above, although ferrihydrite exhibited a faster rate of peroxide decomposition than goethite or hematite, the rate of 2-chlorophenol degradation with these catalysts was fastest for hematite [55], In other studies, quinoline oxidation by peroxide was not observed when ferrihydrite was used as catalyst [53]. 

One of the most intriguing reactions in the chytochrome P450 catalysis is the transfer of second electron and dioxygen activation, which appears to be a key step of the entire process. The chemical nature of reactive oxidizing species appears in the coordination sphere of heme iron and the mechanism of hydroxylation of organic compounds, saturated hydrocarbons in particular, is a much debated question in the field of the cytochrome P450 catalysis. To solve this problem, an entire arsenal of modern experimental and theoretical methods are employed. The catalytic pathway of cytochrome P450cam from Pseudomonas putida obtained on the basis of X-ray analysis at atomic resolution is presented in Fig. 3.10. 

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