Coral allene oxide synthase (cAOS), a fusion protein with 8R-lipoxygenase in Plexaura homomalla, is a hemoprotein with sequence similarity to catalases. cAOS reacts rapidly with the oxidant peracetic acid to form heme compound I and intermediate II. Concomitantly, an electron paramagnetic resonance (EPR) signal with tyrosyl radical-like features, centered at a g-value of 2.004-2.005, is formed. The radical is identified as tyrosyl by changes in EPR spectra when deuterated tyrosine is incorporated in cAOS. The radical location in cAOS is determined by mutagenesis of Y193 and Y209. Upon oxidation, native cAOS and mutant Y209F exhibit the same radical spectrum, but no significant tyrosine radical forms in mutant Y193H, implicating Y193 as the radical site in native cAOS. Estimates of the side chain torsion angles for the radical at Y193, based on the beta-proton isotropic EPR hyperfine splitting, A(iso), are theta(1) = 21 to 30 degrees and theta(2) = -99 to -90 degrees. The results show that cAOS can cleave nonsubstrate hydroperoxides by a heterolytic path, although a homolytic course is likely taken in converting the normal substrate, 8R-hydroperoxyeicosatetraenoic acid (8R-HpETE), to product. Coral AOS achieves specificity for the allene oxide formed by selection of the homolytic pathway normally, while it inactivates by the heterolytic path with nonoptimal substrates. Accordingly, with the nonoptimal substrate, 13R-hydroperoxyoctadecadienoic acid (13R-HpODE), mutant Y193H is inactivated after turning over significantly fewer substrate molecules than required to inactivate native cAOS or the Y209F mutant because it cannot absorb oxidizing equivalents by forming a radical at Y193.