Carbonic anhydrase activity of different photosystem II preparations

Abstract

The requirement of the inorganic carbon (IC) for the proper electron transport activity of thylakoids has been notified far ago. The generally accepted view on the site of its action implies an involvement of IC in electron transfer on the acceptor side of Photosystem II [1]. However, there were some works demonstrating the involvement of IC in the maintaining of photochemistry at the donor side of PSII [2–6]. There were a number of works settled to reveal the «active species» of IC-effect on photochemistry, either bicarbonate ion or CO₂. The principally new approach had originated from work of Stemler [7] putting together the scheme of both IC forms participation in the development of the «bicarbonate effect». This concept implies a coupling of an electron transport event with an act of conversion between forms of inorganic carbon, i.e. the participation of CA reaction in the performance of primary photosynthetic machinery. Added CA has been found to activate the Hill reaction maintained by chloroplast fragments [8]. Effects of light on the CA activity of thylakoids, both inhibitory or stimulatory in different conditions [9–11], dependency of this enzyme activity on the surrounding redox-potential [12], and on proton gradient in thylakoids both in the dark [13] and in the light [14] have been found providing additional basis to the existence of such coupling. Concerning an existence of specific CA in thylakoids, the corresponding activity was found in number of algae cells and in mesophyll of both C3- [15] and C4- [9] plants. This activity is believed to be associated with Photosystem II [15,9] and is different from the known soluble form of CA [12,13]. The isolation and sequencing of a form of CA localized in PSII of Chlamydomonas reinhardtii was performed recently [16]. However, the question on the nature and function of CA-activity in thylakoids and PSII particles in higher plants remains open. We show here that CA-activity of PSII fragments is associated with such a deep structure as PSII core complexes.

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