title: Differential response of chloride binding sites to elevated temperature: a comparative study in spinach thylakoids and PSII-enriched membranes.
abstract: A study of heat effects was performed in thylakoids and photosystem II (PSII)-enriched membranes isolated from spinach in relation to Cl(-)-induced activation of PSII catalyzed oxygen evolution and the retention of Cl(-) in the PSII complex. For this, Cl(-)-sufficient membranes and low-Cl(-) membranes were used. The presence of Cl(-) in the reaction medium did accelerate oxygen evolution, which remained unaffected by heat treatment up to 40 degrees C in PSII membranes and up to 42.5 degrees C in thylakoids. Heat resistance of Cl(-)-induced activation of oxygen evolution was found to be independent of the presence of 'bound Cl(-)' in the preparations. However, the functional stability of the PSII complex during heat treatment showed a marked dependence on the presence of bound Cl(-) in PSII. Electron paramagnetic resonance study of manganese (Mn) release per reaction center/Y (D) (+) showed that there was little loss of Mn(2+) up to 42 degrees C in our preparations, although the PSII activity was significantly lowered. These observations together with data from steady state chlorophyll a fluorescence imply that the site of action of Cl(-) causing direct activation of oxygen evolution was different from the site of primary heat damage. A differential response of chloride binding sites to heat stress was observed. The high-affinity (tightly bound, slow exchanging) site of chloride is affected earlier ( approximately 37 degrees C) while low-affinity (loosely bound, fast exchanging) site gets affected at higher temperatures (42.5 degrees C in thylakoids and 40 degrees C in the case of PSII-enriched membranes).
authors.affiliation: School of Life Sciences, Devi Ahilya University, Life Science Annex Building, Khandwa Road, Indore 452017, India.
authors.forename: Arjun
authors.initials: A
authors.lastname: Tiwari
authors.forename: Anjana
authors.initials: A
authors.lastname: Jajoo
authors.forename: Sudhakar
authors.initials: S
authors.lastname: Bharti
authors.forename: Prasanna
authors.initials: P
authors.lastname: Mohanty
mesh_terms: D001665:Binding Sites; D002712:Chlorides / Q000494:pharmacology*; D002734:Chlorophyll / Q000378:metabolism; D000077194:Chlorophyll A; D004578:Electron Spin Resonance Spectroscopy; D007425:Intracellular Membranes / Q000187:drug effects / Q000378:metabolism*; D008345:Manganese / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D018724:Spinacia oleracea / Q000187:drug effects / Q000378:metabolism*; D013696:Temperature; D020524:Thylakoids / Q000187:drug effects* / Q000378:metabolism*
chemical_list: D002712:Chlorides; D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D008345:Manganese; D010100:Oxygen; D000077194:Chlorophyll A