Analysis of heat-induced disassembly process of three different monomeric forms of the major light-harvesting chlorophyll a/b complex of photosystem II.

The temperature-dependent disassembly process of three monomeric isoforms, namely Lhcb1, Lhcb2, and Lhcb3, of the major light-harvesting chlorophyll (Chl) a/b complexes of photosystem II (LHCIIb) were characterized by observing the changes of absorption spectra, circular dichroism (CD), and dissociation processes of the bound pigments to the in vitro reconstituted complexes subjected to high temperatures. Our results suggest that the three isoforms of LHCIIb undergo conformational rearrangements, structural changes, and dissociations of the bound pigments when the ambient temperature increases from 20 to 90°C. The conformation of the complexes changed sensitively to the changing temperatures because the absorption peaks in the Soret region (436 and 471 nm) and the Qy region (650-660 and 680 nm) decreased immediately upon elevating the ambient temperatures. Analyzing temperature-dependent denaturing and pigment dissociation process, we can divide the disassembly process into three stages: The first stage, appeared from 20°C to around 50-60°C, was characterized by the diminishment of the absorption around 650-660 and 680 nm, accompanied by the blue-shift of the peak at 471 nm and disappearance of the absorbance at 436 nm, which is related to changes in the transition energy of the Chl b cluster, and the red-most Chl a cluster in the LHCIIb. The second stage, beginning at about 50-60°C, was signified by the diminishment of the CD signal between (+)483 nm and (-)490 nm, which implied the disturbance of dipole-dipole interaction of pigments, and the onset of the pigment dissociation. The last stage, beginning at about 70-80°C, indicates the complete dissociation of the pigments from the complex. The physiological aspects of the three stages in the denaturing process are also discussed.