Cytochrome substrate-enzyme comparison

A comparison of peroxidase and cytochrome P-450 illustrates the problems of comparing enzymes and their related catalysts such as synzymes. Peroxidase has low substrate specificity and a simple free-radical oxidation reaction. The substrate site is 10 A from the iron (H202 site) and is probably just an oily droplet region of the protein. This proteins has parallels with Professor Klotz s systems. Proximity is perhaps sufficient to explain the activation of the organic substrate (but not for that of H2Oz). 

A band of this type has been observed for an enzyme-substrate complex ES where the enzyme was represented by the oxidized form of peroxidase cytochrome c, cyt(Fe(III)) and the substrate was the reduced form of cytochrome c, cytj (Fe(II)) [298]. Indeed, on mixing the solution of cyt(Fe(I I)) and cytj (Fe(II)) there appeared a new absorption band with the absorption maximum at Emax = 1.4 eV, the extinction coefficient e = 0.35 M-1 cm-1, and the width a = 0.2 eV. This band was referred [298] to charge transfer via electron tunneling, [cyt(Fe(III))/ cyt, (Fe(II))] -> [cyt(Fe(II))/cytl(Fe(III))]. From a comparison of the data on the intensity of this band with the results of fluorescence measurements, the distance between the iron atoms Fe(III) and Fe (II) in the [cyt(Fe(III))/cyt1(Fe(II))] complex has been estimated to be R 15-20 A and the edge-to-edge tunneling distance Rt = 7 A. 

Fig. 4. A comparison of peroxidase, cytochrome c oxidase and cytochrome P-450 reaction mechanisms. Peroxidase mechanism adapted from that of Poulos[143], cytochrome P-450 mechanism adapted from that of Sligar [100], and cytochrome oxidase mechanism adapted from that of Babcock and Wikstrom[90]. RH, organic substrate for peroxidase and cytochrome P-450 (in the latter case the substrate is presumed to remain bound to the enzyme through most of the reaction cycle).

ESR investigations of microsomal drug oxidation have been reported. For example, the microsomal enzyme mixed function amine oxidase converts hindered hydroxyl-amines to the corresponding ESR-detectable nitroxides. From a comparison of rates of nitroxide and superoxide production, it has been concluded [177] that oxidation of the hydroxylamine is mediated solely by enzyme-generated superoxide radical. In addition, it appears that some oxidations mediated by cytochrome P-450 may occur, at least in part, by a one-electron mechanism. Oxidation of several dihydropyridine derivatives and some substituted hydrazines in the presence of spin traps has given spectra of spin-adducts consistent with radical production from the P-450 substrates [178]. Cumene hydroperoxide has been shown to support the P-450-catalyzed oxidation of aminopyrine to its radical cation [179]. 

---