A new group of photosynthetic inhibitors is described. Thiazolyliden-ketonitriles are blocking electron flow at the acceptor side of photosystem II. They displace radioactive metribuzin from the membrane. Functional properties indicate that the new inhibitors share the binding site on the herbicide binding protein D-1 identical to that of the classical triazinone/urea-type inhibitors. However, fine details in the functional inhibition pattern, like lag in the inhibition and no loss of activity in tris-treated membranes, group them more specifically among the phenol-type family of photosystem II inhibitors. By photoaffinity labeling and immunoblotting it is shown that an azido- derivative of the thiazolyliden-ketonitriles tags the 32 kDa polypeptide subunit of photosystem II. This shows that also the phenol group inhibitors interact with the D-1 protein.
MO calculations show that the predominant tautomeric form of the new ketonitrile inhibitors when bound to the membrane is the hydroxy-form, whereas cyanoacrylates with a comparable essential element are bound in the keto-form. As the first is a phenol-type, the second an urea- type inhibitor, this allows to define more clearly the essential chemical element in the phenol-type inhibitors responsible for effective binding to the D-1 protein, probably oriented in the binding niche towards histidine 215, rather than towards serine 264, as triazine/urea and the cyanoacrylate derivatives do.
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