Mathematical Models for the pH Dependence of Oxygen Evolution under Fluoride Inhibition and Effects of Nitrite on Oxygen Evolution in Photosystem II

UNCG Author/Contributor (non-UNCG co-authors, if there are any, appear on document)
Xuejin Chen (Creator)
The University of North Carolina at Greensboro (UNCG )
Web Site:
Alice Haddy

Abstract: Photosynthesis produces dioxygen at photosystem II (PSII), which is located in the thylakoid membrane of plant chloroplasts. Chloride (Cl-) is required for oxygen evolution activity and other anions are known to activate or inhibit O2 evolution. In this study, the inhibitory effects of fluoride (F-) and nitrite (NO2-) were investigated to gain insight into the requirement for Cl-. The way the pH dependence is affected might reveal the type of residue that the Cl- binds to. In the first part of the study, a simplified model followed by a comprehensive kinetic model for the pH dependence of O2 evolution under F- inhibition in intact PSII was built based on the data of T. Delaney Santoro from the same lab, which included several F- concentrations. The results show that the comprehensive model fit the experiment data quite well. The dissociation constants between enzyme E and its complexes EH+ and E(H+)2, pK1 and pK2, were found to be 5.1 and 7.2. The inhibition constants of F- binding to E, EH+ and E(H+)2 were 15.8 mM, 381 mM and 2.0 mM, respectively. In the second part of the study, the NO2- dependence of O2 evolution by PSII was characterized. The investigation showed that NO2- activated O2 evolution at low concentrations and inhibited at the higher concentrations in both NaCl-washed PSII and intact Cl- depleted PSII. This characteristic of nitrite is similar to that of iodide (I-) and nitrate (NO3-). The kinetics of NO2- action in O2 evolution was modeled as substrate inhibition, in which NO2- activates from the Cl- site and inhibits from a second site. It also suggested that the inhibition due to NO2- in PSII lacking extrinsic subunits PsbP and PsbQ was primarily uncompetitive with uncompetitive inhibition constant Ki' of 0.60 mM, while the NO2- inhibition in intact PSII was probably in an uncompetitive mode with inhibition constant Ki' of 14 mM. However, the competitive inhibition constants Ki for both types of PSII were not well determined due to large error in the data. The Michaelis constants Km for NO2-, Cl- and NO3- were found to be 0.33 mM, 0.54 mM and 0.16 mM for intact Cl- depleted PSII. The Km values for NO2-, Cl-, and NO3- without substrate inhibition were 2.1 mM, 5.0 mM, and 5.5 mM for NaCl-washed PSII, respectively. These values are fairly close and all are within the error range for each type of PSII.

Additional Information

Language: English
Date: 2008
Oxygen evolution, photosynthesis, nitrite inhibition and activation, pH dependence of fluoride inhibition, kinetic models for oxygen evolution, photosystem II
Photosynthesis--Molecular aspects.
Photosynthetic oxygen evolution.

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