D2 protein

The Quinone Acceptors. - The two plastoquinones (PQ-9) QA and QB act as sequential electron acceptors in PS II, QA being a one- and QB a two-electron acceptor. Both quinones are coupled to a high-spin Fe2+ (S = 2) that is coordinated by four histidines (two from D1 and two from D2 protein). The 5th and 6th coordination position are occupied by bicarbonate. A number of small molecules can replace bicarbonate (OH-, CN-, NO etc.) by which the spin/charge state of the complex and the ET rate can be influenced. 

The Tyrosine Radicals. - In PS II two tyrosine radicals Y,D and Y z are known. The position of YD and Yz within PS II has been determined by X-ray crystallography 7,19,341 see Fig. 1. Yz is located in between P680 and the tetranuclear Mn-cluster (Mm) in the D1 protein. It is involved in ET between these 2 components and is only observed as a transient radical or in the form of a split signal (see below). YD is found in a symmetry equivalent position to Yz in a hydrophobic pocket of the D2 protein. The Y D is stable in the dark for hours. Y D undergoes slow redox reactions with the lower S states of the OEC (see 4.7) in the dark but is not involved in the main ET pathway that leads to water oxidation. Its detailed function is not fully understood. 

One problem associated with the yeast 2-hybrid system is that protein interactions that are dependent on posttranslational modifications that can occur only in mammalian cells, would not be detected. For example, monoubiquitinylation and serine phosphorylation of a Fanconi anemia complementation group D2 protein (FANC-D2) are both required for mediating cellular resistance to ionizing radiation (122), an interaction that would be missed using this approach. 

Striatum Many postmortem studies have focused on DA receptor expression in the striatum, due to its high expression of D and D2 receptors, the importance of D2 receptors in the treatment of schizophrenia, and the involvement of the striatum in generating extrapyramidal side effects. Several studies of striatal expression of both Dr and D2-type receptors in postmortem brain did not find altered expression of transcripts for any of these receptors in schizophrenia ( Table 4.2-1) (Roberts et al., 1994, 1996 Harrington et al., 1995 Meador-Woodruff et al., 1997 Stefanis et al., 1998). In addition, one study measured D2 protein expression, reporting unaltered levels of this receptor in the caudate and putamen (Dean et al., 2004). 

J Xiong, S Subramaniam and Govindjee (1996) Modeling of the D1/D2 proteins and cofactors ofthe photosystem II reaction center. Implications for herbicide and bicarbonate binding. Protein Sci 5 2054-2073 FI Michel and J Deisenhofer (1988) Relevance ofthe photosynthetic reaction centers from purple bacteria to the structure of photosystem II. Biochemistry 27 1-7... 

In photosynthetic bacteria, the four histidine residues coordinated to the iron atom are supplemented by a bidendate ligand. However, a corresponding glutamyl residue is not present in the D2 protein. Michel and Deisenhofer reasoned that since the D- and E-helices in Dl and D2 are of equal length, it is rather... 

Fig. 2. Pattern showing removal and restoration of cofactors in the oxygen-evolving complex in a PS-II particle from spinach. The PS-II complex consists of the D1/D2 proteins, as shown In Fig. 1. Legend for the extrinsic proteins is shown at upper left Manganese atoms are represented by four small dots. See text for other details. Scheme adapted from Yamamoto (1989) Molecular organization of oxygen-evolution system in chioropiasts. Bot Mag (Tokyo) 102 572.

WFJ Vermass, AW Rutherford and Flansson (1988) Site-directed mutagenesis in photosystem II of the cyanobacterium Synechocystis sp. PCC 6803 DonorD is a tyrosine residue in the D2 protein. Proc Nat Acad Sci, USA 85 8477-8481... 

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