Chemical composition of purified oxygen-evolving complexes from the thermophilic cyanobacterium Synechococcus sp.

Chemical constituents of a highly purified oxygen-evolving complex isolated from the thermophilic cyanobacterium Synechococcus sp. (Satoh, K., Ohno, T. and Katoh, S. (1985) FEBS Lett. 180, 326–330) was determined. Oxygen-evolving activity of the complex was considerably increased by improvements of purification procedures and the maximum rate of 1300 μmol O 2 per mg Chl per h was obtained at 40°C with ferricyanide as electron acceptor. Electrophoresis in polyacrylamide gradient gels resolved seven polypeptides of 47, 40, 35, 31, 28, 9 and 8 kDa from the complex. The light-harvesting pigment system of the complex consisted of 50 chlorophyll a and 7 β-carotene molecules but no xanthophylls. The complex contained two plastoquinone molecules, which may be attributed to the secondary electron carrier Q A and possibly Z, because there were only low levels of Q B remained unextracted. The complex contained 3.8 atoms Mn. A simple method was developed to determine bound Ca 2+ in the complex without pretreatments to eliminate contaminating Ca 2+ from chemicals and wares. The Ca content determined by this method varied from 0.46 to 0.93 atom with preparations. A good correlation found between the activity and the Ca content suggests that one bound Ca atom is essential for oxygen evolution. Five Fe atoms were present in the complex, of which two are explained by cytochrome b -559. In addition, a significant amount of fatty acid was associated with the complex. By assuming that each one of 47, 40, 35, 31, 28 and 8 kDa polypeptides and two 9 kDa polypeptide are present, the molecular weight of the complex was estimated to be 3 · 10 5 .