光系统Ⅱ(PSⅡ)放氧复合物中钙结合状态的研究

STUDY ON THE BINDING STATE OF CALCIUM IN PHOTOSYSTEM Ⅱ(PSII) OXYGEN- EVOLUTION COMPLEX

根据分离介质、EGTA、NaCl 等洗涤处理和回加 CaCl2的实验结果表明:PSⅡ颗粒中松散结合的 Ca2+,一部分附着于膜的表面,一部分被17、23kD 多肽组成的屏障所掩盖,这两种结合状态的 Ca2+对保持光合作用放氧活性都是必需的。后一种结合状态的 Ca2+与 Mn2+有相同的结合位点和结合特性,故 Mn2+可取代 Ca2+的结合而产生竞争性抑制作用,从而影响光合放氧。

Washing spinach PSII oxygen-evolution complex (OEC) with 2 mmol/L EGTA or extraction medium caused a 28.4% and 25.0% loss of oxygen evolution activities respectively, but the loss of polypeptide components of OEC did not take place, whereas washing with 1 mol/L NaCl caused both a 90.0% loss of oxygen evolution activity and loss of 17, 23kD polypeptides. Adding 5—10 mmol/L CaCl_2 could restore oxygen evolution activities of OEC by various washing to a great extent, but had no effect on control OEC, whereas adding 5—10 mmol/L EGTA had no effect on the OEC by various' washing, but caused the loss of oxygen evolution mixtures, which could induce the release of of 17, 23kD polypeptides from OEC, caused 54.3% loss of oxygen evolution activity, under this circumstance, adding 2 mmol/L of EGTA could only maintain a weak oxygen evolution activity of OEC, but adding 10 mmol/L of CaCl_2 could restore oxygen evolution activity of OEC to the control level. These findings' suggest a two way loose binding of Ga^(2+) to PSII OEC in one way Ca^(2+) is loose bound to the surface of PSII OEC and in other, the Ca^(2+)-binding site is wrapped by 17, 23kD polypeptides. Both of them have effect on oxygen evolution activity of PSII OEC. By way, Mn^(2+) can antagonize the restoration of oxygen evolution activity by Ca^(2+) to the NaCl-washing PSII OEC.