Debus R J, Barry B A, Babcock G T, McIntosh L
Michigan State University-Department of Energy, East Lansing 48824.
Proc Natl Acad Sci U S A. 1988 Jan;85(2):427-30. doi: 10.1073/pnas.85.2.427.
Photosynthetic oxygen evolution takes place in the thylakoid protein complex known as photosystem II. The reaction center core of this photosystem, where photochemistry occurs, is a heterodimer of homologous polypeptides called D1 and D2. Besides chlorophyll and quinone, photosystem II contains other organic cofactors, including two known as Z and D. Z transfers electrons from the site of water oxidation to the oxidized reaction center primary donor, P+.680, while D+. gives rise to the dark-stable EPR spectrum known as signal II. D+. has recently been shown to be a tyrosine radical. Z is probably a second tyrosine located in a similar environment. Indirect evidence indicates that Z and D are associated with the D1 and D2 polypeptides, respectively. To identify the specific tyrosine residue corresponding to D, we have changed Tyr-160 of the D2 polypeptide to phenylalanine by site-directed mutagenesis of a psbD gene in the cyanobacterium Synechocystis 6803. The resulting mutant grows photosynthetically, but it lacks the EPR signal of D+.. We conclude that D is Tyr-160 of the D2 polypeptide. We suggest that the C2 symmetry in photosystem II extends beyond P680 to its immediate electron donor and conclude that Z is Tyr-161 of the D1 polypeptide.
光合放氧发生在被称为光系统II的类囊体蛋白复合物中。该光系统的反应中心核心是发生光化学作用的地方，是由称为D1和D2的同源多肽组成的异二聚体。除了叶绿素和醌之外，光系统II还含有其他有机辅因子，包括两种称为Z和D的辅因子。Z将电子从水氧化位点转移到氧化的反应中心初级供体P+.680，而D+产生被称为信号II的暗稳定电子顺磁共振（EPR）光谱。最近已证明D+是一个酪氨酸自由基。Z可能是位于类似环境中的第二个酪氨酸。间接证据表明Z和D分别与D1和D2多肽相关。为了鉴定与D相对应的特定酪氨酸残基，我们通过对集胞藻6803中的psbD基因进行定点诱变，将D2多肽的Tyr-160突变为苯丙氨酸。所得突变体进行光合生长，但缺乏D+的EPR信号。我们得出结论，D是D2多肽的Tyr-160。我们认为光系统II中的C2对称性延伸到P680之外，直至其直接电子供体，并得出结论，Z是D1多肽的Tyr-161。
