Characterization of the secondary structure and thermostability of the extrinsic 16 kilodalton protein of spinach photosystem II by Fourier transforminfrared spectroscopy

The secondary structure and thermostability of the extrinsic 16 kDa protein of the spinach photosystem II (OEC16) were characterized in solution betwe en 25 and 75 degrees C using Fourier transform infrared(FTIR) spectroscopy. Quantitative analyses of the amide I band (1700-1600 cm(-1)) showed that t he OEC16 subunit contains 34% alpha-helix, 28% beta-sheet, 6% turn, and 32% disorder structures at 25 degrees C. This structural feature differs signi ficantly from that of OEC23 as we had reported previously (H. Zhang, Y. Ish ikawa, Y. Yamamoto, R. Carpentier, FEES Letters 426 (1998) 347-351), althou gh both the OEC subunits are involved in regulating Ca2+ and Cl- requiremen ts. In addition, it was observed that the structure of OEC16 is stable at < 60 degrees C. At increased temperatures, however, thermal denaturation due to an irreversible protein aggregation occurs with a conformational transi tion at similar to 65 degrees C. This transitional temperature is considera bly higher than that (42.5 degrees C) of the PSII reaction centers as also determined by FTIR spectroscopy (J. De Las Rivas, J. Barber, Biochemistry 3 6 (1997) 8897-8903). The higher thermostability of OEC16 may indicate the r ole of OEC16 against heat inactivation of PSII in vivo. (C) 1999 Elsevier S cience B.V. All rights reserved.