Adelroth P, Lindberg K, Andréasson L E
Department of Biochemistry and Biophysics, Göteborg University, Sweden.
Biochemistry. 1995 Jul 18;34(28):9021-7. doi: 10.1021/bi00028a010.
The Ca(2+)-binding properties of photosystem II were investigated with radioactive 45Ca2+. PS II membranes, isolated from spinach grown on a medium containing 45Ca2+, contained 1.5 Ca2+ per PS II unit. Approximately half of the incorporated radioactivity was lost after incubation for 30 h in nonradioactive buffer. About 1 Ca2+/PS II bound slowly to Ca(2+)-depleted membranes in the presence of the extrinsic 16- and 23-kDa polypeptides in parallel with restoration of oxygen-evolving activity. The binding was heterogeneous with dissociation constants of 60 microM (0.7 Ca2+/PS II) and 1.7 mM (0.3 Ca2+/PS II), respectively, which could reflect different affinities of the dark-stable S-states for Ca2+. The reactivation of oxygen-evolving activity closely followed the binding of Ca2+, showing that a single exchangeable Ca2+ per PS II is sufficient for the water-splitting reaction to function. In PS II, depleted of the 16- and 23-kDa polypeptides, about 0.7 exchangeable Ca2+/PS II binds with a dissociation constant of 26 microM, while 0.3 Ca2+ binds with a much weaker affinity (Kd > 0.5 mM). The rate of binding of Ca2+ in the absence of the two extrinsic polypeptides was significantly higher than with the polypeptides bound. The rate of dissociation of bound Ca2+ in the dark, which had a half-time of about 80 h in intact PS II, increased in the absence of the 16- and 23-kDa polypeptides and showed a further increase after the additional removal of the 33-kDa protein and manganese. The rate of dissociation was also significantly faster in weak light than in the dark regardless of the presence or absence of the 16- and 23-kDa polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS)
利用放射性45Ca2+研究了光系统II的Ca(2+)结合特性。从在含有45Ca2+的培养基上生长的菠菜中分离出的PS II膜，每个PS II单位含有1.5个Ca2+。在非放射性缓冲液中孵育30小时后，约一半掺入的放射性消失。在存在外在的16 kDa和23 kDa多肽的情况下，约1个Ca2+/PS II缓慢结合到Ca(2+)耗尽的膜上，同时伴随着放氧活性的恢复。这种结合是异质性的，解离常数分别为60 microM（0.7个Ca2+/PS II）和1.7 mM（0.3个Ca2+/PS II），这可能反映了暗稳定S态对Ca2+的不同亲和力。放氧活性的重新激活紧密跟随Ca2+的结合，表明每个PS II一个可交换的Ca2+足以使水裂解反应发挥作用。在缺乏16 kDa和23 kDa多肽的PS II中，约0.7个可交换的Ca2+/PS II以26 microM的解离常数结合，而0.3个Ca2+以弱得多的亲和力结合（Kd>0.5 mM）。在没有这两种外在多肽的情况下，Ca2+的结合速率明显高于有多肽结合时。在完整的PS II中，结合的Ca2+在黑暗中的解离速率半衰期约为80小时，在缺乏16 kDa和23 kDa多肽时增加，在进一步去除33 kDa蛋白和锰后进一步增加。无论是否存在16 kDa和23 kDa多肽，在弱光下的解离速率也明显快于黑暗中。（摘要截断于250字）
