Scheme 2. Radical mechanism of COX reactions.
The hydrogen abstractions are represented with red arrows: 1) pro-S hydrogen abstraction from AA by tyrosyl radical Y385·; 2) hydrogen abstraction from Y385 by PPG2· radical. EPR lineshapes of the radical intermediates which have been freeze-trapped and characterized are marked in blue. The location of the alternative tyrosyl radical, with narrow singlet EPR lineshape, is not clear and represented in grey color. This alternative radical is likely either located on a surrogate tyrosine or on Y385 with a different rotational conformation. The delocalization of the spin is marked with dashed line.