Nonheme enzymes

Iron centers that are more electron-deficient than iron(III) compounds are used for efficient and highly specific oxidation reactions in, for example, heme and nonheme enzymes [166-172]. Most iron(IV)-complexes found in biological reaction cycles possess terminal or bridging 0x0 groups as is known from a large number of structural and spectroscopic investigations. With the exception of iron(IV)-nitrido groups, nonoxo iron(IV) centers very rarely take part in such reactions. 

Iron. Iron plays a vital role during development and growth and is an important factor in many metabolic reactions, including protein synthesis as a co factor of both heme and nonheme enzymes, and in the development of neuronal processes. However, free iron, particularly Fe2+, is highly toxic by virtue of its ability to trigger cellular deleterious effects, including the Fenton reaction, which generates free radical species and lipid peroxidation (Ch. 32). 

Another related nonheme enzyme is the selenoprotein glutathione peroxidase. It reacts by a mechanism very different (Eq. 15-59) from those discussed... 

On the basis of the structure of their active site, catalases may be classified as heme or nonheme enzymes. Those that contain heme iron are efficient catalysts, operating close to the diffusion limit, 108M 1sec 1. In iron catalases the metal is coordinated by four heme nitrogens and a proximal Tyr residue, which occupies the fifth coordination site. A catalytically required His is found on the distal side of the heme. In addition, a water molecule has also been observed close to the iron sixth coordination site. 

For nonheme enzymes that fiuther activate dioxygen, it is apparent that the diferrous forms also bind O2 to eventually generate the active species responsible for the oxidative transformations. In the case of MMO, the first intermediate has been labeled compound P (Scheme 2), which subsequently converts to compound Q both are kinetically competent to hydroxylate methane. In the case of RNR, compound X (Scheme 1) is responsible for the one-electron oxidation of a tyrosine residue to generate a tyrosyl radical. Based on chemical considerations and its Mossbauer properties, it has been proposed that compound P is a diferric peroxide species. To date, three model complexes of compound P, with comparable spectroscopic properties, have been structurally characterized (Figure g). In two of these models O2 is bound in a cis... 

The catalases catalyze the disproportionation of hydrogen peroxide (equations). Most catalases contain the iron-protoporphyrin IX prosthetic group see Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases). However, some bacteria are able to synthesize catalases that are not inhibited even by millimolar concentrations of azide and cyanide, suggesting that some catalases are nonheme enzymes it is now known that these enzymes possess a dinuclear Mn active site. 

FIGURE 3.11 High -valent intermediates detected in nonheme enzymes. 

Keywords Nonheme enzymes Heme enzymes Hydroxylation Epoxidation Sulfoxidation Iron-oxo Valence bond theory Density functional theory. 

Heme monojg genases such as the P450s undergo a catalytic cycle that is very different from the one shown above for nonheme enzymes, like TauD. Figure 3 shows the catalsrtic cycle of P450 (29-31). Thus, in the resting state (structure G), a water... 

Computational Studies of the Catalytic Activity of Heme and Nonheme Enzymes... 

Fe(IV) is an important intermediate in such heme enzymes as cytochrome c oxidase and cytochrome P450, as well as the nonheme enzymes methane monooxygenase and ribonucleotide reductase. Voltammetric studies on yeast cytochrome... 

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