kD protein, located on the lumen side of thylakoid membranes, is one of three extrinsic proteins of photosystemⅡ(PSⅡ). Previous study showed that NBS modification of W241, the only tryptophan in 33 kD protein, is he...kD protein, located on the lumen side of thylakoid membranes, is one of three extrinsic proteins of photosystemⅡ(PSⅡ). Previous study showed that NBS modification of W241, the only tryptophan in 33 kD protein, is helpful for understanding the function of W241 in main-taining functional conformation of 33 kD protein. In this paper, studies of both circular dichroism and fluorescence spectra showed that upon decreasing pH from 6.2 to 2.5, the conformation of soluble 33 kD protein changed significantly, with an increase or a decrease in percentage of random coil or a-helix and turns. The changes in secondary structures of this protein are pH reversible. After NBS modification at pH 2.5, the conformational change of 33 kD protein was kept fixed. The CD ellipticity at 200 nm for NBS-modified 33 kD protein is much lower than that for control, indicating that the unfolding degree of 33 kD protein was enhanced after the NBS modification. Moreover, the conformational flexibility is lost in NBS-modified 33 kD protein, and the conformational change becomes pH irreversible, indicating that NBS modi-fication blocked the reversibility of conformational change of 33 kD protein. The specific binding capability of NBS-modi- fied 33 kD protein is much lower than that of low pH-treated control. Furthermore, the rebinding of modified protein on PSⅡ membranes cannot restore the activity of oxygen evo-lution. We suggest that it is low pH but not NBS modification of W241 that leads to the conformational change of 33 kD protein from one functional to another non-functional state. The significant capability of proton transport of 33 kD pro-tein is discussed.展开更多
文摘kD protein, located on the lumen side of thylakoid membranes, is one of three extrinsic proteins of photosystemⅡ(PSⅡ). Previous study showed that NBS modification of W241, the only tryptophan in 33 kD protein, is helpful for understanding the function of W241 in main-taining functional conformation of 33 kD protein. In this paper, studies of both circular dichroism and fluorescence spectra showed that upon decreasing pH from 6.2 to 2.5, the conformation of soluble 33 kD protein changed significantly, with an increase or a decrease in percentage of random coil or a-helix and turns. The changes in secondary structures of this protein are pH reversible. After NBS modification at pH 2.5, the conformational change of 33 kD protein was kept fixed. The CD ellipticity at 200 nm for NBS-modified 33 kD protein is much lower than that for control, indicating that the unfolding degree of 33 kD protein was enhanced after the NBS modification. Moreover, the conformational flexibility is lost in NBS-modified 33 kD protein, and the conformational change becomes pH irreversible, indicating that NBS modi-fication blocked the reversibility of conformational change of 33 kD protein. The specific binding capability of NBS-modi- fied 33 kD protein is much lower than that of low pH-treated control. Furthermore, the rebinding of modified protein on PSⅡ membranes cannot restore the activity of oxygen evo-lution. We suggest that it is low pH but not NBS modification of W241 that leads to the conformational change of 33 kD protein from one functional to another non-functional state. The significant capability of proton transport of 33 kD pro-tein is discussed.
