STRUCTURE AND THERMAL-STABILITY OF PHOTOSYSTEM-II REACTION CENTERS STUDIED BY INFRARED-SPECTROSCOPY

The secondary structure of photosystem II reaction centers isolated fr om pea has been deduced from quantitative analysis of the component ba nds of the infrared amide I spectral region, determined by FTIR spectr oscopy. The analysis shows the isolated complex to consist of 40% alph a-helix, 10% beta-sheet, 14% beta-strands (or extended chains), 17% tu rns, 15% loops, and 3% nonordered segments. These structural protein e lements were determined for samples in H2O, in D2O, and in dried films . The isolated reaction center, composed of proteins D1,D2,cytochrome b(559), and PsbI, has been predicted to contain a total of 13 transmem brane alpha-helices, which conveys a percentage of this type of struct ure congruent with the structural determination deduced from FTIR spec tra. The process of thermal destabilization of the reaction centers ha s also been studied by FTIR spectroscopy, showing a clear main conform ational transition at 42 degrees C, which indicates a high thermal sen sitivity of the secondary structure of this protein complex. Such ther mal instability may correlate with the well-described high sensitivity of photosystem II to damage and may relate to the process of rapid pr otein degradation that photosystem II suffers during photoinhibition o f plants.