Evidence for an Unprecedented Histidine Hydroxyl Modification
on D2-His336 in Photosystem II of <i>Thermosynechoccocus vulcanus</i> and <i>Thermosynechoccocus elongatus</i>
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Abstract
The electron density map of the 3D
crystal of Photosystem II from Thermosynechococcus
vulcanus with a 1.9 Å resolution
(PDB: 3ARC)
exhibits, in the two monomers in the asymmetric unit cell, an, until
now, unidentified and uninterpreted strong difference in electron
density centered at a distance of around 1.5 Å from the nitrogen
Nδ of the imidazole ring of D2-His336. By MALDI-TOF/MS upon
tryptic digestion, it is shown that ∼20–30% of the fragments
containing the D2-His336 residue of Photosystem II from both Thermosynechococcus vulcanus and Thermosynechococcus
elongatus bear an extra mass of +16 Da. Such an extra
mass likely corresponds to an unprecedented post-translational or
chemical hydroxyl modification of histidine