Cytochromes P450 : from model compounds towards artificial hemoproteins

Cytochromes P450 are heme-thiolate proteins abundant in nature. These monooxygenases catalyse a variety of reactions including alkene epoxixation, N-, O-, and S-dealkylation, C-C bond cleavage and hydrocarbon hydroxylation. This arsenal of interesting transformations, together with the importance of their biological functions has rendered them subject of intensive studies. In this context, two projects were pursued in the present work. In a first project, two members of a new family of P450 model compounds, 28 and 29 (figure 37) were synthesized and characterized. The coordination of SO3- as a fifth ligand to iron in both compounds resembles the reduced charge density on the cystein thiolate coordinating in the enzyme case. This concept was confirmed by calculation results and X-ray crystallography as well as redox potentials of 28 and 29 and their reactivity in P450-catalysed reactions.
Both model compounds have been shown to generate an analogue of CpdI (the natural active species) upon shunt pathway reaction with external oxidant (mCPBA). Model compound 29 was studied closely in its alkene epoxidation reaction in collaborationwith van Eldik et al, wherefrom the complete catalytic cycle of this reaction could be
visualised. Both compounds have been applied to biomimetic demethylation of amines, therein showing the influence of their different porphyrin substituents upon reactivity. Appliance of the upper systems to cleavage of vicinal diols has been accomplished by employing N-Oxides as 'O'-donors. Diol cleavage represents the last step in C-C bond cleavage by P450s, one of the reactions taht illustrate best the power of P450 oxidative
transformations. With the above findings, a simple system for future studies on this interesting reaction has been elaborated. Furthermore the findings for this system suggest CpdI as the active species in diol cleavage reaction in similarity to alkene epoxidation and hydrocarbon
hydroxylation, thereby confirming the unique role of this iron-oxo species in the field of P450 catalysts.
The field of appliance of the new model compounds has been further expanded in collaboration with van Eldik et al towards sutdies on binding of nitric oxide (NO). Binding of NO to 28 in toluene has therein been found to be an appropriate model for NO coordination of substrate bound P450cam studied by the van Eldik group earlier, displaying completely reversible binding of NO in mechanistic analogy to the enzyme case. Comparison of thermodynamic and kinetic factors for the two cases emphasise the role of the enzyme pocket in NO coordination of heme proteins. In summary, the obtained results establish 28 and 29 as valuable models for diverse aspects of P450 research.
In a second project, modification of the natural cofactor of cytochromes P450 (heme b) was investigated. Additional substituents were introduced into the originally free meso-positions to alter the properties towards higher reactivity. By this strategy the tetrachlorinated cofactor 92 and its Dimethylester 93 (figure 38) were obtained and characterized.
The reactivity of 93 compared to its nonchlorinated counterpart 96 towards alkene epoxidation in solution indeed supported the applied strategy by ~ 2.6fold lincrease of product formation for chlorinated 93. With these findings, a new cofactor is avaliable, which has promising properties for biotechnological applications once successfully incorporated into a cofactor free “apoprotein”.