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Publication Abstract from PubMed

In oxygenic photosynthesis, light energy is stored in the form of chemical energy by converting CO2 and H2O 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 alga Chlamydomonas reinhardtii a luminal carbonic anhydrase, CrCAH3, (Cr), 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 (MIMS) and X-ray crystallography. MIMS 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 A 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 in alpha-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 wt 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.

Crystal structure and functional characterization of photosystem II-associated carbonic anhydrase CAH3 in Chlamydomonas reinhardtii.,Benlloch R, Shevela D, Hainzl T, Grundstrom C, Shutova T, Messinger J, Samuelsson G, Sauer-Eriksson AE Plant Physiol. 2015 Jan 23. pii: pp.114.253591. PMID:25617045^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.