4XIX: Carbonic anhydrase Cah3 from Chlamydomonas reinhardtii in complex with phosphate

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 alga Chlamydomonas 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-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 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.
PDB ID: 4XIXDownload
MMDB ID: 126991
PDB Deposition Date: 2015/1/8
Updated in MMDB: 2017/09
Experimental Method:
x-ray diffraction
Resolution: 2.7  Å
Source Organism:
Similar Structures:
Biological Unit for 4XIX: dimeric; determined by author and by software (PISA)
Molecular Components in 4XIX
Label Count Molecule
Proteins (2 molecules)
Carbonic Anhydrase, Alpha Type
Molecule annotation
Chemicals (8 molecules)
* Click molecule labels to explore molecular sequence information.

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