Cytochrome catalysis, mechanisms

Burka LT, Guengerich FP, Willard RJ, Macdonald TL. Mechanism of cytochrome P-450 catalysis. Mechanism of N-dealkylation and amine oxide deoxygenation. J Am Chem Soc 1985 107 2549-2551. 

Chemical mechanisms of catalysis by cytochromes P-450 towards a unified view. F. P. Guengerich andT. L. Macdonald, Acc. Chem. Res., 1984,17, 9-16 (68). 

The mechanism of cytochrome P450 catalysis is probably constant across the system. It is determined by the ability of a high valent formal (FeO) species to carry out one-electron oxidations through the abstraction of hydrogen atoms or electrons. The resultant substrate radical can then recombine with the newly created hydroxyl radical (oxygen rebound) to form the oxidized metabolite. Where a heteroatom is the (rich) source of the electron more than one product is possible. There can be direct recombination to yield the heteroatom oxide or radical relocalization within the... 

NADPH oxidation and NO synthesis by the enzyme, it supports a role for reduction of the heme iron in catalysis, and may explain why NOS functions only as an NADPH-dependent reductase in the absence of bound calmodulin (Klatt et ai, 1993). The mechanism of calmodulin gating is envisioned to involve a conformational change between the reductase and oxygenase domains of NOS, such that an electron transfer between the terminal flavin and heme iron becomes possible. Calmodulin may also have a distinct role within the NOS reductase domain, in that its binding dramatically increases reductase activity of the enzyme toward cytochrome c (Klatt et al., 1993 Heinzel et al., 1992). However, it is clear that several other NOS functions occur independent of calmodulin, including the binding of L-arginine and NADPH, and transfer of NADPH-derived electrons into the flavins (Abu-Soud and Stuehr, 1993). 

In relation to enzymic cytochrome P-450 oxidations, catalysis by iron porphyrins has inspired many recent studies.659 663 The use of C6F5IO as oxidant and Fe(TDCPP)Cl as catalyst has resulted in a major improvement in both the yields and the turnover numbers of the epoxidation of alkenes. 59 The Michaelis-Menten kinetic rate, the higher reactivity of alkyl-substituted alkenes compared to that of aryl-substituted alkenes, and the strong inhibition by norbornene in competitive epoxidations suggested that the mechanism shown in Scheme 13 is heterolytic and presumably involves the reversible formation of a four-mernbered Fev-oxametallacyclobutane intermediate.660 Picket-fence porphyrin (TPiVPP)FeCl-imidazole, 02 and [H2+colloidal Pt supported on polyvinylpyrrolidone)] act as an artificial P-450 system in the epoxidation of alkenes.663... 

Scheme 2.10 Consensus mechanism of cytochrome P450 catalysis (S = substrate) [61, 62, 72].

Using crystalline cytochrome-c peroxidase [38] and catalase [39], it is shown that intermediates are formed according to the general mechanism of acidic-basic catalysis ... 

Cytochrome P 450. The mechanism of P-450 catalysis is outlined in Figure 2 (3, 4). The mechanism is based on the stoichiometry, product analysis, and labeling experiments, quenching experiments, spectral evidence for the structures of intermediates involved, and model studies (enzyme models as well as oxidant models) of P-450 catalysis. 

One of the most intriguing reactions in the chytochrome P450 catalysis is the transfer of second electron and dioxygen activation, which appears to be a key step of the entire process. The chemical nature of reactive oxidizing species appears in the coordination sphere of heme iron and the mechanism of hydroxylation of organic compounds, saturated hydrocarbons in particular, is a much debated question in the field of the cytochrome P450 catalysis. To solve this problem, an entire arsenal of modern experimental and theoretical methods are employed. The catalytic pathway of cytochrome P450cam from Pseudomonas putida obtained on the basis of X-ray analysis at atomic resolution is presented in Fig. 3.10. 

Guengerich, P.P. and Macdonald, T.L. (1984) Chemical mechanism of catalysis by cytochrome P450 a unify view, Acc. Chem. Res. 17, 9-16. 

A mechanism has been proposed for NADH-cytochrome bs reductase based on these elegant studies (Fig. 17) (308, 355). The hydrogen which is stereospeciflcally and directly transferred has been printed in boldface. Catalysis proceeds in a clockwise direction, and after the first catalytic cycle the dissociation of oxidized pyridine nucleotide and the association of reduced pyridine nucleotide are shown as a single step since both are very rapid processes and this emphasizes that the thiol is not (kinetically speaking) exposed during catalysis. The interaction of the thiol and the... 

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