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The reactivity of heme in biological systems: autocatalytic formation of both tyrosine-heme and tryptophan-heme covalent links in a single protein architecture.

We have previously shown that introduction of an engineered Met160 residue in ascorbate peroxidase (S160M variant) leads to the formation of a covalent link between Met160 and the heme vinyl group [Metcalfe, C. L., et al. (2004) J. Am. Chem. Soc. 126, 16242-16248]. In this work, we have used electronic spectroscopy, HPLC, and mass spectrometry to show that the introduction of a tyrosine residue at the same position (S160Y variant) leads, similarly, to the formation of a heme-tyrosine covalent link in an autocatalytic reaction that also leads to formation of a second covalent link from the heme to Trp41 [Pipirou, Z., et al. (2007) Biochemistry 46, 2174-2180]. Stopped-flow and EPR data implicate the involvement of a tyrosyl radical in the reaction mechanism. The results indicate that the heme can support the formation of different types of covalent links under appropriate conditions. The generality of this idea is discussed in the context of other heme enzymes.