Characterization of the coral allene oxide synthase active site with UV-visible absorption, magnetic circular dichroism, and electron paramagnetic resonance spectroscopy: Evidence for tyrosinate ligation to the ferric enzymeheme iron

Coral allene oxide synthase (AOS) a hemoprotein with weak sequence homology to catalase, I is the N-terminal domain of a naturally occurring fusion pr otein with an 8R-lipoxygenase. AOS converts 8R-hydroperoxyeicosatetraenoic acid to the corresponding allene oxide. The UV-visible absorption and magne tic circular dichroism spectra of ferric AOS and of its cyanide and azide c omplexes, and the electron paramagnetic resonance spectra of native AOS thi gh-spin, g = 6.56, 5.22, 2.00) and of its cyanide adduct (low-spin, g = 2.8 6, 2.24, 1.60) closely resemble the corresponding spectra of bovine liver c atalase (BLC). These results provide strong evidence for tyrosinate ligatio n to the heme iron of AOS as has been established for catalases. On the oth er hand, the positive circular dichroism bands in the Soret region for all three derivatives of ferric AOS are almost the mirror image of those in cat alase. In addition, the cyanide affinity of native AOS (K-d = 10 mM at pH 7 ) is about 3 orders of magnitude lower than that of BLC. Thus, while these results conclusively support a common tyrosinate-ligated heme in AOS as in catalase, significant differences exist in the interaction between their re spective heme prosthetic groups and protein environments, and in the access of small molecules to the heme iron.