Manganese-dependent carboanhydrase activity of photosystem II proteins

Four sources of carbonic anhydrase (CA) activity in submembrane preparations of photosystem II (PS II) isolated from pea leaves were examined. Three of them belong to the hydrophilic proteins of the oxygen-evolving complex of PS II with molecular mass 33 kDa (protein PsbO), 24 kDa (protein PsbP), and 18 kDa (protein PsbQ). The fourth source of CA activity is associated with a pigment-protein complex of PS II after removing three hydrophilic proteins by salt treatment. Except for protein PsbQ, the CA activity of all these proteins depends on the presence of Mn²⁺: the purified protein PsbO did not show CA activity before adding Mn²⁺ into the medium (concentration of Mn²⁺ required for 50% effect, EC₅₀, was 670 μM); CA activity of protein mixture composed of PsbP and PsbQ increased more than 5-fold upon adding Mn²⁺ (EC₅₀ was 45 μM). CA activity of purified protein PsbP increased 2-fold in the presence of 200 μM Mn ²⁺. As indicated for the mixture of two proteins (PsbP and PsbQ), Mg²⁺, Ca²⁺, and Zn²⁺, in contrast to Mn ²⁺, suppressed CA activity (both initial and Mn²⁺-induced activity). Since the found sources of CA activity demonstrated properties different from ones of typical CA (need for Mn²⁺, insensitivity or low sensitivity to acetazolamide or ethoxyzolamide) and such CA activity was found only among PS II proteins, we cannot exclude that they belong to the type of Mn-dependent CA associated with PS II.