PROTEIN SECONDARY STRUCTURE AND CONFORMATIONAL-CHANGES OF PHOTOSYSTEM-II DURING HEAT DENATURATION STUDIED BY FOURIER TRANSFORM-INFRARED SPECTROSCOPY

Fourier transform-infrared (FT-IR) spectroscopy was used to study seco ndary structure and conformational changes of a photosystem II (PS II) membrane and core complex during heat denaturation. In light of the D SC and thermal gel analysis studies of PS II by Thompson et al, (Bioch emistry, 25 (1986) 6161-6169; 28 (1989) 6686-6695), we tried to assign the global conformational changes obtained in this study to the denat uration of the individual protein or smaller parts of the complex. The conformational changes below 40 degrees C were temporarily assigned t o the extrinsic proteins and/or Cyt b(559). The loss of oxygen-evoluti on activity occurred in a temperature range which contained no remarka ble conformational changes, The core of PS II denatured from 55 to 65 degrees C with transitions of secondary structure from alpha-helix and turns to beta-sheet and random coils. The denaturation of light-harve sting chlorophyll protein (LHCP) began at 65 degrees C and was charact erized by a rapid increase of the turns. Our results indicate that the inactivation of the O-2-evolving apparatus is not associated with any major protein secondary structural changes. The conformational change s at lower temperature may be responsible for the heat sensitivity of the apparatus. (C) 1998 Elsevier Science.