The case for chloroplast thylakoid carbonic anhydrase

Washed thylakoid membranes and photosystem II-enriched membrane fragments from cyanobacteria, green algae, and chloroplasts from both C₃ and C₄ plants possess the ability to reversibly hydrate CO₂. That is, the membranes have an intrinsic carbonic anhydrase activity. The present review outlines the discovery of thylakoid carbonic anhydrase and presents the evidence that it is a unique isozyme, distinct from other cellular carbonic anhydrases. It appears that at least some thylakoid carbonic anhydrase is closely associated with photosystem II and may be required for electron transport. This would explain why all inhibitors of carbonic anhydrase also inhibit photosystem II. Several speculative functions of thylakoid carbonic anhydrase are discussed. These include a possible role in carbon metabolism, in the protonation of plastoquinone, and/or in oxygen evolution.