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Light‐ and Redox‐dependent Thermal Stability of the Reaction Center of the Photosynthetic Bacterium Rhodobacter sphaeroides ¶
Author(s) -
Tokaji Zsolt,
Tandori Júlia,
Maróti Péter
Publication year - 2002
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2002)0750605lardts2.0.co2
Subject(s) - rhodobacter sphaeroides , photosynthetic reaction centre , rhodospirillales , redox , rhodospirillaceae , photosynthesis , chemistry , photochemistry , purple bacteria , bacteria , biophysics , biology , biochemistry , inorganic chemistry , genetics
Irreversible loss of the photochemical activity and damage of the pigments (bacteriochlorophyll [Bchl] monomer, Bchl dimer [P] and bacteriopheophytin) by combined treatment with intense and continuous visible light and elevated temperature have been studied in a deoxygenated solution of reaction center (RC) protein from the nonsulfur purple photosynthetic bacterium Rhodobacter sphaeroides . Both the fraction of RC in the charge‐separated redox state (P + Q − , where Q is a quinone electron acceptor) and the degradation of the pigments showed saturation as a function of increasing light intensity up to 400 mW cm −2 (488/515 nm) or 1100 μE m −2 s −1 (white light). The thermal denaturation curves of the RC in the P + Q − redox state demonstrated broadening and 10–20°C shift to lower temperature (after 30–90 min heat treatment) compared with those in the PQ redox state. Similar but less striking behavior was seen for RC of other redox states (P + Q and PQ − ) generated either by light or by electrochemical treatment in the dark. These experiments suggest that it is not the intense light per se but the changes in the redox state of the protein that are responsible for the increased sensitivity to photo‐ and heat damage. The RC with a charge pair (P + Q − ) is more vulnerable to elevated temperature than the RC with (P + Q or PQ − ) or without (PQ) a single charge. To reveal both the thermodynamic and kinetic aspects of the denaturation, a simple three‐state model of coupled reversible thermal and irreversible kinetic transitions is presented. These effects may have relevance to the heat stability of other redox proteins in bioenergetics.