Photoassembly of the photosystem II manganese cluster

Assembly of the photosynthetic water-oxidizing complex (WOC) is believed to involve the sequential oxidation and ligation of individual Mn atoms in a process called photoactivation. The kinetics with which photoactivation occurs has been interpreted to indicate a series mechanism, in which the product of the reaction of Mn{sup 2+} with the first PSII charge separation is unstable until processed by a second PSII charge separation. Because the decay of the unstable intermediate is accelerated in the presence of added reductants of Mn{sup 3+}, it has been hypothesized that the unstable intermediate is Mn{sup 3+} ligated with high affinity and stabilized by ligands supplied by WOC polypeptides. According to this hypothesis, photooxidation of a second Mn{sup 2+} to Mn{sup 3+}, followed by ligation of two more Mn{sup 2+} ions, results in formation of a stable Mn(II)2-Mn(III)2 complex. Several authors have reported a {le}l {mu}M K{sub m} value for a Mn{sup 2+} (or Mn{sup 3+}) binding site in PSII. In most of these determinations, H{sub 2}O{sub 2} was present. In another case, the K{sub m} was determined from the inhibition by Mn{sup 2+} of diphenylcarbazide (DPC) photooxidation. On the other hand, the K{sub m} for Mn{sup 2+} oxidation by Y{sub Z+} in the absence of any other additions, is {approximately}10 {mu}M. We have postulated that this 10 {mu}M value is the K{sub m} for Mn{sup 2+} photooxidation by Y{sub Z+} in the second photoact of the photoactivation mechanism, which occurs with low quantum efficiency. According to this view, the Mn{sup {le}}{sup 3+} ligated near Y{sub Z} with high efficiency on the first photoact is responsible for the inhibition of DPC photooxidation in the presence of Mn{sup 2+} and for the low quantum efficiency of steady-state Mn{sup 2+} photooxidation. Here, we provide evidence that this is the case