HISTIDINE OXIDATION IN THE OXYGEN-EVOLVING PHOTOSYSTEM-II ENZYME

THE evolution of oxygen as a result of light-driven water oxidation occurs in plants and is catalysed by photosystem-II (PS-II). A manganese-cluster probably acts both as the active site and as a charge-accumulating device (for a review, see ref. 1). The enzyme cycle involves five redox states which are denoted as S0-S4, depend-ing on the number of positive equivalents stored2. Oxygen is released after formation of the transient S4 state. Ca2+ is an obligatory cofactor in this process and its depletion inhibits the enzyme cycle at the step before water oxidation, that is, after formation of the S3 state3. In chelator-treated, Ca2+-depleted PS-II, a new electron paramagnetic resonance (EPR) signal arising from a formal S3 state has been reported4. It was suggested that the S3 EPR signal could originate from the oxidation of an amino acid, interacting magnetically with the manganese cluster4. There are only a few examples of amino-acid oxidation in enzyme chemistry and these are limited to tyrosine5 and tryptophan6. Here we report evidence that the S2 to S3 transition occurring in Ca2+-depleted PS-II corresponds to the oxidation of histidine. © 1990 Nature Publishing Group.