Amine binding and oxidation at the catalytic site for photosynthetic water oxidation

Photosynthetic water oxidation occurs at the Mn-containing catalytic site of photosystem II (PSII). By the use of (14)C-labeled amines and SDS-denaturing PAGE, covalent adducts derived from primary amines and the PSII subunits, CP47, D2/D1, and the Mn-stabilizing protein, can be observed. When PSII contains the 18- and 24-kDa extrinsic proteins, which restrict access to the active site, no (14)C labeling is obtained. NaCl, but not Na(2)SO(4), competes with (14)C labeling in Mn-containing PSII preparations, and the concentration dependence of this competition parallels the activation of oxygen evolution. Formation of (14)C-labeled adducts is observed in the presence or in the absence of a functional manganese cluster. However, no significant Cl(−) effect on (14)C labeling is observed in the absence of the Mn cluster. Isolation and quantitation of the (14)C-labeled aldehyde product, produced from [(14)C]benzylamine, gives yields of 1.8 ± 0.3 mol/mol PSII and 2.9 ± 0.2 mol/mol in Mn-containing and Mn-depleted PSII, respectively. The corresponding specific activities are 0.40 ± 0.07 μmol(μmol PSII-hr)(−1) and 0.64 ± 0.04 μmol(μmol PSII-hr)(−1). Cl(−) suppresses the production of [(14)C]benzaldehyde in Mn-containing PSII, but does not suppress the production in Mn-depleted preparations. Control experiments show that these oxidation reactions do not involve the redox-active tyrosines, D and Z. Our results suggest the presence of one or more activated carbonyl groups in protein subunits that form the active site of PSII.