Role of the photosystem II-associated CAH3 in the oxygen evolving machinery in Chlamydomonas reinhardtii

One of the most abundant proteins on the Earth is ribulose-1,5-biphosphate carboxylase/oxygenase (RUBISCO). RUBISCO is a CO2 fixing enzyme in oxygenic photosynthetic organisms that it has low affinity for CO2. When CO2 is the limiting factor in the environment, RUBISCO works inefficiently due to its oxygenase activity. Some higher plants and aquatic photosynthetic organisms, such as the green alga Chlamydomonas reinhardtii; therefore, evolved Carbon Concentrating Mechanisms to acquire and to accumulate inorganic carbon for RUBISCO, when the CO2 concentration is at or below the air level. Carbonic anhydrases are important components of the Carbon Concentrating Mechanisms and they are zinc-containing metalloenzymes that catalyze the reversible interconversion of CO2 and HCO3−. CAH3 is one of the α-Carbonic anhydrases that it is associated with the photosystem II donor side in Chlamydomonas reinhardtii. CAH3 supplies CO2 to RUBISCO as primary function by speeding up the interconversion of CO2 and HCO3− in the thylakoid lumen. A proposed additional function of CAH3 is to regulate photosystem II by providing HCO3− to remove protons from the reaction center of photosystem II. In this report, I further test for the first time the function of CAH3 by monitoring CO2 and O2 release simultaneously from cia3 and wild type thylakoid membranes using Membrane Inlet Mass Spectromete