Protective Effect of α-Carbonic Anhydrase CAH3 Against Photoinhibition and Thermal Inactivation of Photosystem II in Membrane Preparations as Compared with α-Carbonic Anhydrase CA4

Photosystem II (PSII) is one of the most vulnerable components of photosynthetic apparatus of the thylakoid membrane to the action of inhibitory factors. The donor side of PSII exhibits high sensitivity to photoinhibition and thermal inactivation, which leads to the loss of O₂-evolving function of the water-oxidizing complex (WOC). The data obtained in this study demonstrated increased stability of WOC activity in the PSII membrane preparations from the wild-type (WT) Chlamydomonas reinhardtii compared to the PSII preparations from the cia3 mutant, which lack α-carbonic anhydrase (CA) CAH3, under conditions of moderate photoinhibition and thermal inactivation. This effect was completely eliminated by adding a CA inhibitor to the PSII preparations from WT. At the same time, addition of active recombinant CAH3 (rCAH3) protein to the preparations from cia3 restored increased resistance of PSII to these factors. Under the same conditions of photoinhibition and thermal inactivation, the PSII preparations from Arabidopsis thaliana demonstrated very low loss of O₂-evolving activity, regardless of the presence or absence of carbonic anhydrase α-CA4, which is similar to CAH3. More pronounced suppression of the O₂-evolving activity in the PSII from A. thaliana mutants lacking CA4 was observed only when they were incubated at elevated temperature, indicating the possibility of more significant conformational changes in the WOC proteins of PSII. Despite the clear binding of the rCAH3 to PSII membrane preparations from A. thaliana, the enzyme had little effect on the WOC activity in these preparations, suggesting absence of functional interaction between the rCAH3 and PSII from A. thaliana. The obtained results indicate different mechanisms of involvement of CAH3 and CA4, both of which are assumed to exist in close association with PSII in live systems, in the PSII functioning.