Photosynthetic oxygen-evolution reaction

Electrochemical and Spectroscopic Studies of Manganese(II, III, IV) Complexes as Models for the Photosynthetic Oxygen-Evolution Reaction... 

In this experiment we flushed the reaction assay with nitrogen in order to achieve anaerobic conditions be gre supplementing the gas phase over the assay with the oxygen isotope 0. Under such conditions no or onj minimal photosynthetic oxygen evolution from the splitting of... 

PS I-activity TFig. 3) is stable over the period of ten days indicating that the drop in the photosynthetic oxygen evolution (Fig. 2) may result from the impairment of reactions around PS II. Indeed the analysis of PS II-HILL-activity reveals an immediate and complete loss during the first day of N-deprivation (data not shown), but this may be caused by the more drastic method of membrane preparation of N-deprived cells. 

The oxygen evolution rate was measured by using the photosynthetic activity measurement system (Fig. 1). When light was illuminated to the reaction vessel, algal cells began to evolve oxygen and the linearity between dissolved oxygen and time was observed just after a few... 

In 1937, Robin Hill succeeded in isolating chloroplasts from leaves which under suitable conditions still retained some form of photosynthetic activity. The material isolated by Hill actually consisted of thylakoid membranes which lacked the necessary enzymes for CO2 fixation as a result of the loss of the outer membrane envelope. When these chloroplasts were illuminated, no oxygen evolution could be observed. However, when an electron acceptor such as ferric oxalate, as was used by Hill initially, was added, oxygen evolution was observed upon illumination, accompanied by the reduction of the ferric oxalate to the ferrous form. Lurthermore, four equivalents of the oxidant were found to be photochemi-cally reduced for each mole of oxygen evolved. This reaction was called the Hill reaction. Later, ferricyanide and benzoquinone were commonly used as the oxidizing agents. 

The thylakoids and stroma are the sites of the so-called light and dark reactions of photosynthesis, respectively. This compartmentalization of photosynthetic functions was recognized by Park and Pon when they broke open the chloroplasts, separated the contents into thylakoid and stroma fractions and examined their properties. The specific activities of the thylakoids include photochemical reactions, electron transport, oxygen evolution, ATP synthesis and NADP reduction, while the stroma contains enzymes for CO2 fixation driven by ATP and NADPH and other biochemical reactions in the dark. Our understanding and appreciation of the detailed structure and organization of the thylakoid membranes has increased tremendously in recent years. Further discussion of thylakoid structure will be continued in section VII on page 26. 

Manganese plays a critical role in oxygen evolution catalyzed by the proteins of the photosynthetic reaction center. The superoxide dismutase of bacteria and mitochondria, as well as pyruvate carboxylase in mammals, are also manganese proteins. How the multiple manganese atoms of the photosynthetic reaction center participate in the removal of four electrons and protons from water is the subject of intense investigation by spectroscopists, synthetic inorganic chemists, and molecular biologists. ... 

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