Protein secondary structure and conformational changes of photosystem II during heat denaturation studied by Fourier transform-infrared spectroscopy

Abstract

Fourier transform-infrared (FT-IR) spectroscopy was used to study secondary structure and conformational changes of a photosystem II (PS II) membrane and core complex during heat denaturation. In light of the DSC and thermal gel analysis studies of PS II by Thompson et al. (Biochemistry, 25 (1986) 6161–6169; 28 (1989) 6686–6695), we tried to assign the global conformational changes obtained in this study to the denaturation of the individual protein or smaller parts of the complex. The conformational changes below 40°C were temporarily assigned to the extrinsic proteins and/or Cyt b 559. The loss of oxygen-evolution activity occurred in a temperature range which contained no remarkable conformational changes. The core of PS II denatured from 55 to 65°C with transitions of secondary structure from α-helix and turns to β-sheet and random coils. The denaturation of light-harvesting chlorophyll protein (LHCP) began at 65°C and was characterized 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 changes at lower temperature may be responsible for the heat sensitivity of the apparatus.