Dioxygenase model

From an economic point of view, biomimetic systems able to perform selective oxidations by 02 alone (dioxygenase models) would be much superior to monooxygenase models that perform similar oxidations but with the consumption of a reducing agent (according to the monooxygenase equation). However, as mentioned previously, dioxygenases only catalyze the oxidation of a limited number of substrates that are already relatively reactive by themselves and susceptible... 

Figure 10 Reaction of extradiol dioxygenase model compound with dioxygen.

CIS- and trans-isomers, 51 and 52, respectively) rather than cleavage of the double bond [226]. This has some mechanistic implications inasmuch as the side-on peroxometal complexes of this type (cf. Chapter 1) are not likely intermediates in tryptophan-2,3-dioxygenase model systems. 

DTBSQ occurs also without an added catalyst. This raises the question of how far this mechanism may be involved in catechol dioxygenase model systems. The semiquinone radical is supposed to be an intermediate of 3,5-DTBC oxidation to the quinone. Once there is a path for its formation the reactions of Scheme 11 may also start to occur and contribute to the overall diol cleavage process. Thus it seems that diagnostic tests are required for reactions of Scheme 11, perhaps via... 

Balogh-Hergovich, E. Kaizer, J. Speier, G. Kinetics and mechanism of the Cu(I) and Cu(II) flavonolate-catalyzed oxygenation of flavonols. Functional quercetin 2,3-dioxygenase models, J. Mol. Catal. A Chem., 2000,159, 215. 

Cobalt Schiff Base Complexes as Simple Dioxygenase Models... 

Cobalt Schiff base complexes as simple dioxygenase models 158... 

Fig. 2. Ribbon diagram of the structures of (a) the water-soluble Rieske fragment from bovine heart bci complex (ISF, left, PDB file IRIE), (b) the water-soluble Rieske fragment from spinach b f complex (RFS, middle, PDB file IRFS), and (c) the Rieske domain of naphthalene dioxygenase (NDO, right, PDB file INDO). The [2Fe-2S] cluster is shown in a space-filling representation, the ligands as ball-and-stick models, and residues Pro 175 (ISF)/Pro 142 (RFS)/Pro 118 (NDO) as well as the disulfide bridge in the ISF and RFS as wireframes.

X-ray absorption spectroscopy has been performed on the isolated Rieske protein from bovine heart mitochondrial bc complex 69) as well as on the Rieske-type cluster in Burkholderia cepacia phthalate dioxygenase (PDO) (72). The analysis performed by Powers et al. 69) was significantly hampered by the fact that the presence of two histidine ligands was not fully recognized therefore, only the results obtained with the dioxygenase where the mononuclear iron has been depleted will be considered here. Table VII gives a comparison of the distances obtained from the fit of the EXAFS spectra assuming an idealized Rieske model and of the distances in the crystal structures... 

Subsequent studies have confirmed that the reason for this discrepancy is that the rat is able to rapidly metabolise P-carotene to retinol in the intestine, through the action of intestinal dioxygenase. In contrast humans absorb P-carotene systemically such that plasma levels of P-carotene increase to levels not found in the rodent. A more appropriate animal model is the ferret, which shows a similar metabolism to humans. High levels of plasma P-carotene in the ferret induce the cellular transcription factors c-fos and c-jun, and squamous metaplasia is seen in the lung with or without exposure to cigarette smoke (SCF, 2000). Even after the investment of all these resources it has not been possible for the EU Scientific Committee on Food to set an ADI. 

Habu N, M Samejima, T Yoshimoto (1989a) A novel dioxygenase responsible for the Ca-Cp cleavage of lignin model compounds from Pseudomonas sp. TMY1009. Mokuzai Gakkaishi 35 26-29. 

Fig. 7. (a) Active site of Taurine dioxygenase TauD after activation by the taurine substrate and (b) a benzoylformato model [Fe(bdtbpza)(02CC(0)Ph)] (6). 

Fig. 7. Model for oxygen activation by intradiol dioxygenases. PCA, the substrate pro-tochatechuic acid.

Kruger, H.-J. Iron-containing models of catechol dioxygenases, Biomimetic Oxidations Catalyzed by Transition Metal Complexes , Ed. Meunier, B. Imperial College Press London, 2000, pp. 363—413. 

---