Spectroscopic Studies on pH-induced Changes in Secondary Structure of Pophenol Oxidase from Tobacco

In order to obtain that more information on the secondary and active site structure of polyphenol oxidase from nicotinan tobacco，FT-IR，synchronous fluorescence and Vis-UV have bee employed to study the secondary structure changes of polyphenol oxidase induced by pH. The results show that，compared with native PPO（pH=7），PPO at pH=2 contains less both a-helix and anti-parallel β-sheet，more random coil，and almost the same amount of β-turn，and that PPO at pH = 11 contains less a-helix，anti-parallel β-sheet，β-turn，and more random coil. The microenvironments of both Trp and Tyr residues in PPO undergo some changes with pH changing. With the decrease in pH values at the range of pH=7.0-3.38，the coordination between Cu2+ in the active site and imidazoles of histidine are strenghtened. Different LMCT bands of the PPO active site have been observed by changing pH，which result from the structure of Cu（Ⅱ）-imidazole and Cu（Ⅱ）-imidazolate.