Cofactor and structural role of СО2 in chloroplasts

Abstract

The rate of photochemical reactions in chloroplasts of higher plants depends on the presence of inorganic carbon (C i)-carbon dioxide and/or anions of carbonic acid in the medium. This relationship is known as the «bicarbonate effect» studied at the level of the intact leaf, isolated thylakoids, photosystems and reaction centers (RC) and is associated with the presence in the photosystem II (PSII) of tightly bound НСО 3-. The total amount of bound inorganic carbon is about 1 micromoles per mg of chlorophyll, but only one or two tightly bound to the RC molecules control PSII activity. The thylakoid membranes of chloroplasts also contain less tightly bound pool of bicarbonate (with a concentration close to the concentration of chlorophyll), which can be removed without significant consequences for the activity of PSII. The review summarizes data on the cofactor role of the tightly bound inorganic carbon, the removal of which completely inhibits the photochemical reactions in chloroplasts. It is assumed that weakly bound bicarbonate is involved in the proton transport and the regulation of phosphorylation. Interconversion of the C i forms occurs in chloroplasts with participation of several carbonic anhydrases (CA). The suppression of their activity leads to a slowing of photochemical reactions and causes significant changes in the ultrastructural organization of the thylakoid membranes. These data indicate a structural role of C i in the chloroplasts.