Kinetic Parameters for Dehydration Carbonic Anhydrase Activity of Photosystem 2 Core Complex from Pisum sativum

Photosystem II (PSII) is a key factor in the productivity of photosynthetic organisms, including crops. $${\text{HCO}}_{3}^{ - }$$ ions are known to affect its efficiency. The acceleration of reactions involving $${\text{HCO}}_{3}^{ - }$$ , by carbonic anhydrase (CA) activity, has been shown to be important and interconnected with the photosynthetic activity of PSII. However, the location of the active center(s) of this activity has not yet been established. Therefore, it is of interest to obtain a minimal complex of PSII with high CA activity. This work presents manganese-depleted PSII core particles that were obtained from Pisum sativum L. leaves. The $${\text{HCO}}_{3}^{ - }$$ dehydration activity of PSII core (as measured by pH indicator pyranine) was found to be 778 ± 138 Wilbur-Anderson (W-A) units/mg of chlorophyll (Chl) or (2.46 ± 0.50) × 1010 W-A un./mol of photosynthetic reaction centers of PSII, which is close to that of α-CAII from bovine erythrocytes—one of the most active CAs. The kinetic parameters of the activity in the PSII core are: Vmax = 7.8 × 10–3 mM/s, kcat = 4.0 × 102 s–1, Km = 7.16 mM, kcat/Km = 5.7 × 104 M–1 s–1. The Vmax and kcat, in the PSII core, are close to those in the manganese-depleted PSII particles, but the Km of the latter is lower (4.5 mM) indicating its higher affinity for $${\text{HCO}}_{3}^{ - }$$ compared to that in the PSII core. We propose that non-heme iron is the metal for the active center of CA in the PSII core. Further, we present here possible mechanisms involving CA activity in the electron transfer near the non-heme iron.