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  • Title: Uncoupling oxygen transfer and electron transfer in the oxygenation of camphor analogues by cytochrome P450-CAM. Direct observation of an intermolecular isotope effect for substrate C-H activation.
    Author: Kadkhodayan S, Coulter ED, Maryniak DM, Bryson TA, Dawson JH.
    Journal: J Biol Chem; 1995 Nov 24; 270(47):28042-8. PubMed ID: 7499289.
    Abstract:
    The hydroxylation of (1R)-camphor by cytochrome P450-CAM involves almost complete coupling of electron to oxygen transfer. Modifications at C-5 of camphor, the normal site of hydroxylation by P450-CAM, lead to as much as 98% uncoupling of electron and oxygen transfer as well as to decreases in the rate of electron uptake (up to 10-fold) and the rate of oxygenated product formation (up to 210-fold). Two modes of uncoupling are seen: (a) two-electron uncoupling in which the decrease in oxygenated product formation is balanced by increases in H2O2 formation and (b) four-electron "oxidase" uncoupling where the NADH/O2 ratio has changed from one to nearly two and relatively little H2O2 is formed. Both enantiomers of 5-methylenylcamphor are two-electron uncouplers, while (1R)- and (1S)-5,5-difluorocamphor and (1R)-9,9,9-d3-5,5-difluorocamphor are four-electron uncouplers. An intermolecular isotope effect of 11.7 is observed for oxygenation of C-9 in (1R)-5,5-difluorocamphor. With this substrate, the significant decrease in the rate of oxygenated product formation combined with the large isotope effect suggest that the rate-limiting step has switched from electron to oxygen transfer.
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