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Studying on the Interaction Between Zn~(2+) and Polyphenol Oxidase from Apple

XIAO Hou-rong,WANG Zhong-feng,YANG Hong,WU Qian-qian,ZHANG Jie,CAI Jing-min (Department of Biological and Environmental Engineering,Hefei University,Hefei 230022,China)  
The effect of exo—Zn~(2+) on the structure and activity of Polyphenol Oxidase(PPO) is studied by employing enzymatic activity assay,differential absorption spectrum,synchronous fluorescence spectra and far—UV CD spectra, which could be used to explore the relations between their structure and function.The results show that the activity of PPO is enhanced by trial Zn~(2+),reaching its maxium at[Zn~(2+)]/[PPO]2.0,but inhibited when Zn~(2+) is further added. Trial Zn~(2+) can also increase and theα—helix content.Zn~(2+) may coordinate with histidine(s) in PPO,and the absorptions of(ImH)σ_N+σ_N→Zn~(2+),(ImH)σ→Zn~(2+),(ImH)π_2→Zn~(2+),(ImH)π_1→Zn~(2+) occur at 227,241,265,335nm,respectively. Synchronous fluorescence spectra exhibit that microenvironment of Trp residue in PPO is more hydrophobic than that of free Trp in water,and that both fluorescences of Trp and Tyr are quenched by Zn~(2+),but the microenvironments of both Trp and Tyr residues in PPO undergo no changes with Zn~(2+) addition.
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