Differing responses of the two forms of photosystem II carbonic anhydrase to chloride, cations, and pH

The effects of Cl−, Mn2+, Ca2+, and pH on extrinsic and intrinsic photosystem II carbonic anhydrase activity were compared. Under the conditions of our in vitro experiments, extrinsic CA activity, located on the OEC33 protein, was optimum at about 30 mM Cl−, and strongly inhibited above this concentration. This enzyme is activated by Mn2+ and stimulated somewhat by Ca2+. The OEC33 showed dehydration activity that is optimum at pH 6 or below. In contrast, intrinsic CA activity found in the PSII complex after removal of extrinsic proteins was stimulated by Cl− up to 0.4 M. Ca2+ appears to be the required cofactor, which implies that the location of the intrinsic CA activity is in the immediate vicinity of the CaMn4 complex. Up to now, intrinsic CA has shown only hydration activity that is nearly pH independent.