Cytochrome P450 monooxygenases forms

Franssen et al. [24] pointed out an alternative method of production of nootkatone from valencene catalysed by (-i-)-germacrene A hydroxylase, an enzyme of the cytochrome P450 monooxygenase type that was isolated from chicory roots. In general, this enzyme appeared to accept a broad range of sesquiterpenes and hydroxylates exclusively at the side-chain s isopropenyl group. Valencene is an exception it was not hydroxylated at the side chain, but -nootkatol was formed in the first step (Scheme 22.5) it is not yet clear if the second step is enzyme-catalysed. 

Whereas most, if not all, of the enzymes involved in xenobiotic metabolism can form reactive metabolites (Table 8.1), the enzyme systems most frequently involved in the activation of xenobiotics are those which catalyze oxidation reactions. The cytochrome P450 monooxygenases (CYP) are by far the most important enzymes involved in the oxidation of xenobiotics. This is because of the abundance of CYP (especially in the liver), the numerous isozymes of CYP, and the ability of CYP to be induced by xenobiotic compounds. 

Cytochrome P450 monooxygenases, which are protoheme proteins, present an unusually red-shifted Soret band for the reduced CO complex at 450 nm [167]. Upon denaturation by various treatments, the cytochromes are always able to bind CO reversibly by losing their unique spectral properties as well as their catalytic activities. The spectral properties of the reduced carbonylated complexes of the denaturated form called P420 are identical to those of the dioxygen-carrying hemoproteins (Amax = 420 nm). These facts indicate that the unusual spectral properties of cytochrome P450 closely relate to its function. 

Another fate of arachidonic acid with potential relevance to atherosclerosis is cytochrome P450 monooxygenase-derived metabolism to epoxyeicosatrienoic acids (EETs) (Chapter 12), which may also be formed non-enzymatically by the interaction of arachidonic acid with free radicals (D.D. Gutterman, 2006). EET synthesis in cultured endothelial cells can be induced by LDL, and EETs are found both in LDL and in human atherosclerotic lesions. Biological effects of EETs include potentially anti-atherogenic effects, such as vasodilatation and prevention of platelet aggregation, and atherogenic responses, such as increased monocyte adhesion. 

Bios5mthetic pathways of naturally occurring cytokinins are illustrated in Fig. 29.5. The first step of cytokinin biosynthesis is the formation of A -(A -isopentenyl) adenine nucleotides catalyzed by adenylate isopentenyltransferase (EC 2.5.1.27). In higher plants, A -(A -isopentenyl)adenine riboside 5 -triphosphate or A -(A -isopentenyl)adenine riboside 5 -diphosphate are formed preferentially. In Arabidopsis, A -(A -isopentenyl)adenine nucleotides are converted into fraws-zeatin nucleotides by cytochrome P450 monooxygenases. Bioactive cytokinins are base forms. Cytokinin nucleotides are converted to nucleobases by 5 -nucleotidase and nucleosidase as shown in the conventional purine nucleotide catabolism pathway. However, a novel enzyme, cytokinin nucleoside 5 -monophosphate phosphoribo-hydrolase, named LOG, has recently been identified. Therefore, it is likely that at least two pathways convert inactive nucleotide forms of cytokinin to the active freebase forms that occur in plants [27, 42]. The reverse reactions, the conversion of the active to inactive structures, seem to be catalyzed by adenine phosphoiibosyl-transferase [43] and/or adenosine kinase [44]. In addition, biosynthesis of c/s-zeatin from tRNAs in plants has been demonstrated using Arabidopsis mutants with defective tRNA isopentenyltransferases [45]. 

The vast majority of cytochrome P450 monooxygenases catalyze the reductive scission of dioxygen, which requires the consecutive delivery of two electrons to the heme iron. P450s utilize reducing equivalents (electrons in the form of hydride ions) ultimately derived from the pyridine cofactors NADH or NADPH and transferred to the heme via special redox proteins [38, 39]. 

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