Abstract
Tyrosyl radicals play key roles in the mechanisms of a wide range of enzymes. In the water-oxidizing enzyme, two redox-active tyrosines are present. While the tyrosine that is kinetically competent in electron transfer, TyrZ, may also have a role in the enzyme mechanism, the second tyrosine, TyrD, has a stable radical and is not directly involved in the redox chemistry associated with enzyme function. Despite homologous positions in subunits D1 and D2, TyrZ and TyrD exhibit extremely different kinetics, different redox potentials, and play different functional roles in the enzyme. Thus, they constitute an ideal system for understanding how the protein environment controls the reactivity of tyrosine radicals. In this work, site-directed mutagenesis has been used to modify the hydrogen bond network around TyrD. Then, both the formation of the TyrD radical and its properties have been studied by X-band and high-field EPR. Structural and mechanistic implications will be discussed.
