Light- and heat-induced denaturation of photosystem II core-antenna complexes CP43 and CP47

Abstract

Light- and heat-induced denaturation of the core-antenna complexes of Photosystem II, CP43 and CP47, purified from spinach, has been investigated using absorption, fluorescence and circular dichroism (CD) spectroscopy. Light is found to bring about considerable bleaching of chlorophyll a but no apparent change in the protein secondary structure, while heat induces significant unfolding of the protein secondary structure but no apparent destruction of the chlorophyll a molecule in the two antenna complexes. Both the destruction of chlorophyll a by light and the denaturation of the protein conformation by heat cause the loss of excitonic interaction of chlorophyll a in CP43 and CP47, as measured by visible CD activity. Light induces a larger decrease of the chlorophyll a fluorescence and the CD activity of CP47 than that of CP43, indicating that the native state of chlorophyll a of CP47 is more sensitive to light than that of CP43. The main thermal transitions of protein secondary structure occur at 50°C for CP43 and 63°C for CP47, while the half-loss of chlorophyll a excitonic interaction during heating occurs at 45°C for CP43 and 60°C for CP47, suggesting that CP47 is more thermally stable than CP43.