Galactose oxidase: Probing radical mechanism with ultrafast radical probe

Abstract

The detection of radical intermediates in reaction mechanisms can be difficult. Many radicals are highly reactive species and have very short lifetimes. Many well established methods exist to detect radical intermediates, including direct electron spin resonance (ESR) detection and spin-trapping methods. Still, there are many problems in enzyme reaction mechanisms where such methods are not effective in addressing mechanistic questions. A radical at an enzyme active site will be short-lived if it is generated in the active site in close proximity to other radicals. This is not an uncommon situation. One selected example that illustrates this is in the reaction mechanism of flavin-containing monoamine oxidase enzymes. Electron transfer from the amine substrate to the flavin coenzyme generates a transient amine radical cation and flavin semiquinone radical. Further reactions can then lead to nonradical products and a normal catalytic turnover. This mechanism was determined through the use of radical clock radical-probing substrates to detect radical intermediates in the radical reaction mechanism. Ultrafast, hypersensitive radical-probing radical rearrangements represent the ultimate in the detection of radical intermediates in enzyme-catalyzed reactions. Such probes can detect radical intermediates that cannot be detected by any other expelfmental technique. The ultrafast, hypersensitive trans -2-phenylcyclopropylcarbinyl radical rearrangement, 5 to 6, has been applied to the study of several enzymes including galactose oxidase, the chloroperoxidase from Caldariomyces fumago , an alkane monooxygenase from Pseudomonas oleovorans , and cytochromes P450.