Crystal Structure and Functional Characterization of Photosystem II-Associated Carbonic Anhydrase CAH3 inChlamydomonas reinhardtii

In oxygenic photosynthesis, light energy is stored in the form of chemical energy by converting CO₂ and water into carbohydrates. The light-driven oxidation of water that provides the electrons and protons for the subsequent CO₂ fixation takes place in photosystem II (PSII). Recent studies show that...

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Published in:Plant physiology (Bethesda) 2015-03, Vol.167 (3), p.950-962
Main Authors: Benlloch, Reyes, Shevela, Dmitriy, Hainzl, Tobias, Grundström, Christin, Shutova, Tatyana, Messinger, Johannes, Samuelsson, Göran, Sauer-Eriksson, A. Elisabeth
Format: Article
Language:English
Subjects:
Active sites
Atoms
Biology
Crystal structure
Dimers
Enzymes
Hydrogen bonds
MEMBRANES, TRANSPORT, AND BIOENERGETICS
Monomers
Plants
Thylakoids
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Summary:In oxygenic photosynthesis, light energy is stored in the form of chemical energy by converting CO₂ and water into carbohydrates. The light-driven oxidation of water that provides the electrons and protons for the subsequent CO₂ fixation takes place in photosystem II (PSII). Recent studies show that in higher plants, HCO₃ – increases PSII activity by acting as a mobile acceptor of the protons produced by PSII. In the green algaChlamydomonas reinhardtii, a luminal carbonic anhydrase, CrCAH3, was suggested to improve proton removal from PSII, possibly by rapid reformation of HCO₃ – from CO₂. In this study, we investigated the interplay between PSII and CrCAH3 by membrane inlet mass spectrometry and x-ray crystallography. Membrane inlet mass spectrometry measurements showed that CrCAH3 was most active at the slightly acidic pH values prevalent in the thylakoid lumen under illumination. Two crystal structures of CrCAH3 in complex with either acetazolamide or phosphate ions were determined at 2.6- and 2.7-Å resolution, respectively. CrCAH3 is a dimer at pH 4.1 that is stabilized by swapping of the N-terminal arms, a feature not previously observed inα-type carbonic anhydrases. The structure contains a disulfide bond, and redox titration of CrCAH3 function with dithiothreitol suggested a possible redox regulation of the enzyme. The stimulating effect of CrCAH3 and CO₂/HCO₃ – on PSII activity was demonstrated by comparing the flash-induced oxygen evolution pattern of wild-type and CrCAH3-less PSII preparations. We showed that CrCAH3 has unique structural features that allow this enzyme to maximize PSII activity at low pH and CO₂ concentration.
ISSN:0032-0889
1532-2548