Fast enzymatic HCO3- dehydration supports photosynthetic water oxidation in Photosystem II from pea

Carbonic anhydrase (CA) activity, associated with Photosystem II (PSII) from Pisum sativum , has been shown to enhance water oxidation. But, the nature of the CA activity, its origin and role in photochemistry has been under debate, since the rates of CA reactions, measured earlier, were less than the rates of photochemical reactions. Here, we demonstrate high CA activity in PSII from Pisum sativum , measured by HCO 3 - dehydration at pH 6.5 (i.e. under optimal condition for PSII photochemistry), with kinetic parameters K m of 2.7 mM; V max of 2.74·10 -2 mM·sec -1 ; k cat of 1.16·10 3 sec -1 and k cat /K m of 4.1·10 5 M -1 sec -1 , showing the enzymatic nature of this activity, which k cat exceeds by ∼13 times the rate of PSII, as measured by O 2 evolution. The similar dependence of HCO 3 - dehydration, of the maximal quantum yield of photochemical reactions and of O 2 evolution on the ratio of chlorophyll/photochemical reaction center II demonstrate the interconnection of these processes on the electron donor side of PSII. Since the removal of protons is critical for fast water oxidation, and since HCO 3 - dehydration consumes a proton, we suggest that CA activity, catalyzing very fast removal of protons, supports efficient water oxidation in PSII and, thus, photosynthesis in general.