TYROSYL RADICALS IN ENZYME CATALYSIS - SOME PROPERTIES AND A FOCUS ONPHOTOSYNTHETIC WATER OXIDATION

Enzymes that require a redox-active amino acid for catalysis or functi on have emerged as a distinct class of proteins. For the tyrosine-base d radical enzymes, we show that the spin-density distribution in the r adical follows an odd alternate pattern that is invariant to within 10 % across the class. General properties of the radical enzymes are summ arized from which we conclude that their essential role in catalysis i s to initiate substrate metabolism by hydrogen-atom abstraction. These ideas are extended to the Y-Z and Y-D tyrosines in Photosystem II and a radical-based hydrogen-atom abstraction model for water oxidation i s discussed. Differences in rates of oxidation of Y-Z and Y-D by the r eaction-center chlorophyll, P680(+), under various conditions, are con sidered and rationalized on the basis of changes in reorganization ene rgy induced by the local protein structure and by the presence or abse nce of the (Mn)(4) cluster that binds substrate water.