D1/D2 heterodimer

Despite the many similarities between the L/M complex and the D1/D2 heterodimer in regard to the primary electron donor and electron acceptors, the corresponding reaction centers apparently have several significant differences. The bacterial reaction center consists of three subunits, L, M and H, but a subunit comparable to the H-subunit is not present in PS II. Also, no counterpart to Cyt b559 found tightly bound to the D1/D2 heterodimer had yet been found in the L/M complex. 

The formation of small protein cross-linking products is also caused by ROS-induced oxidation of amino acids in the Dl protein that may lead to subsequent covalent association with other PSII polypeptides. The three most-frequendy-observed cross-linking products could be detected in illuminated or ROS-treated PSII preparations the D1-D2 heterodimer the DTcytochrome b-559 adduct and the D1-CP43 cross-linking product. The first two are formed in an oxygen-dependent manner, both in vivo and in vitro, in illuminated PSII preparations. ... 

A recent study of D1-D2 heterodimer formation in illuminated spinach leaves has indicated the cross-linking site with the D2 protein between residues 226-244 of the Dl protein. ... 

The reaction center of photosystem II (PSII) is composed of two hydrophobic proteins, D1 and D2, which carry the redox components necessary for photochemical charge separation between tyr2 on the donor side and the secondary quinone acceptor, Qg. In addition it is likely that also the Mn-cluster involved in the oxidation of water is bound to the D1/D2 heterodimer. 

Trebst was the first to point out that there was a considerable degree ofhomology between the amino-acid sequence of D1 and that of the L-subunit as well as between that of D2 and that of the M-subunit (see Figs. 3 and 4). This observation together with the known three-dimensional structure of the bacterial reaction center, led Michel and Deisenhofer to suggest that D1 and D2 formed a heterodimer in photosystem II just as the L- and M-subunit do in the bacterial reaction center and, therefore, the D1/D2 complex is the structural and functional analogue of the L/M-complex. 

It is now generally accepted that a heterodimer of the D1 and D2 proteins of Photosystem II binds the primary photoreactants (1,2). 

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