Nonheme iron-dependent

H6H, which has been cloned and expressed heterologously (162), is a nonheme, iron-dependent, oxoglutarate-dependent protein. It seems that the epoxidation reaction occurs much more slowly than the hydroxylation reaction. The tropane alkaloids seem to be formed in the roots and then transported to the aerial parts of the plant (163). 

The initial step of most aromatic degradation pathways is the oxidation of an arene to the corresponding f-dihydro diol. This remarkable reaction is catalyzed by a family of nonheme iron-dependent multicomponent dioxygenases, which have been reviewed. The best-characterized example of this family is naphthalene dioxygenase (Figure 11). 

The other major class of nonheme iron-dependent dioxygenases are the a-ketoglutarate-dependent dioxygenases, which catalyze the oxidative decarboxylation of cosubstrate a-ketoglutarate to form succinate and an iron(IV)-oxo intermediate, which is then used to carry out a range of hydroxylation, desaturation, and other oxidative reactions. While the majority of reactions catalyzed by this family of enzymes are involved in biosynthetic pathways, enzymes such as HPPD (see Section 8.16.2.1) are involved in degradation pathways, therefore it is appropriate to discuss this family of enzymes, and contrast them with the nonheme iron-dependent dioxygenases described in Section 8.16.1. 

A large family of these enzymes is now known, and their enzymology and structures have been reviewed. A number of crystal structures have been obtained for enzymes in this family, and in each case the mononuclear iron(II) center is coordinated by a His,His,Glu motif, also observed in the extradiol catechol dioxygenases, and in other nonheme iron-dependent enzymes. Structural studies on clavaminic acid synthase have indicated the structural basis for the separate hydroxylation and oxidative cyclization/ desaturation reactions catalyzed by this enzyme. ... 

Acinetohacter johnsonii is able to degrade acetylacetone (2,4-pentanedione), utilizing a novel dioxygenase enzyme, as shown in Figure 40. The purified enzyme is dependent upon Fe for activity, implying that it is a nonheme iron-dependent dioxygenase. labeling studies and structure—activity relationships have been carried out... 

In summary, a range of oxidative cleavage reactions, catalyzed by dioxygenase enzymes, have been identified, that are involved in catabolic and biosynthetic pathways. While the majority of these enzymes require nonheme iron as cofactor, several examples of copper-dependent or cofactor-independent dioxygenases have come to light, whose mechanisms for oxygen activation and catalytic mechanism share several features with the nonheme iron-dependent dioxygenases. 

Nonheme iron-dependent monooxygenases, which use two iron atoms as cofactors, are bacterial multicomponent monooxygenases. They mainly consist of three components, including a monooxygenase, a reductase component, and a small regulatory... 

Although many nonheme iron-dependent monooxygenases have been purified and cloned from hydrocarbon-utilizing bacteria, the complicated nature of this enzyme family often places a considerable synthetic burden on the host cells during heterologous expression. Therefore, the whole cells of the native strains or recombinant E. coli, instead of free enzymes, are often applied in biocatalytic reactions. 

Reactions Catalyzed with Nonheme Iron-Dependent Monooxygenases... 

Microorganisms that use gaseous olefins as a carbon source are widely exist in nature, and they typically produce nonheme iron-dependent alkene/alkane monooxygenases. Although in a lot of cases, the identification of the key enzyme hasn t been attempted many of those alkene-utilizing bacteria have been used in the asymmetric epoxidation of aliphatic alkenes with high stereoselectivities [26]. 

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