Purification and Characterization of a Dark Red Skin Related Dimeric Polyphenol Oxidase from Huaniu Apples

Lily bulb polyphenol oxidase obtained via an optimized multi-stage separation strategy for structural analysis and browning mechanism elucidation

An optimized multi-stage separation strategy was developed to purify lily bulb polyphenol oxidase (PPO) for revealing its molecular structure. The PPO was purified 14.64-fold with high specific activity of 153,900 U/mg via optimized conditions of phosphate buffer pH (6.5), solid-liquid ratio (1:3), PVPP content (2 %), extraction time (4 h), followed by 30 %-50 % ammonium sulfate, diethylaminoethyl ion-exchange chromatography (0.1 M NaCl), and size exclusion chromatography. The PPO was identified as a dimeric protein with molecular weight of 135 kDa, containing 58.79 % random coil, 20.78 % α-helix, 17.41 % β-folding, and 3.02 % β-corner. The three-dimensional structure via homology modeling suggested that active center CuA bound to His151, His172, and His181, CuB bound to His307, His311, and His341. Furthermore, molecular docking indicated that its Phe337 and Tyr312 residues were catalytic cavity gates of catechol and 4-methylcatechol, respectively. Therefore, this study successfully analyzed purified PPO structure and further provided a theoretical foundation for its browning mechanism.

Recent Advances Regarding Polyphenol Oxidase in Camellia sinensis: Extraction, Purification, Characterization, and Application

Polyphenol oxidase (PPO) is an important metalloenzyme in the tea plant (Camellia sinensis). However, there has recently been a lack of comprehensive reviews on Camellia sinensis PPO. In this study, the methods for extracting PPO from Camellia sinensis, including acetone extraction, buffer extraction, and surfactant extraction, are compared in detail. The main purification methods for Camellia sinensis PPO, such as ammonium sulfate precipitation, three-phase partitioning, dialysis, ultrafiltration, ion exchange chromatography, gel filtration chromatography, and affinity chromatography, are summarized. PPOs from different sources of tea plants are characterized and systematically compared in terms of optimal pH, optimal temperature, molecular weight, substrate specificity, and activators and inhibitors. In addition, the applications of PPO in tea processing and the in vitro synthesis of theaflavins are outlined. In this review, detailed research regarding the extraction, purification, properties, and application of Camellia sinensis PPO is summarized to provide a reference for further research on PPO.

Optimization of the Fermentation Conditions of Huaniu Apple Cider and Quantification of Volatile Compounds Using HS-SPME-GC/MS

The fermentation process and composition of volatile compounds play a crucial role in the production of Huaniu apple cider. This study aimed to optimize the fermentation conditions of Huaniu apple cider and quantify its volatile compounds using headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC/MS). The optimal fermentation parameters were determined using response surface methodology (RSM). The optimal fermentation temperature was 25.48 °C, initial soluble solids were 18.90 degrees Brix, inoculation amount was 8.23%, and initial pH was 3.93. The fermentation rate was determined to be 3.0, and the predicted value from the verification test was 3.014. This finding demonstrated the excellent predictability of a RSM-optimized fermentation test for Huaniu apple cider, indicating the reliability of the process conditions. Moreover, the analysis of volatile compounds in the optimized Huaniu cider identified 72 different ingredients, including 41 esters, 16 alcohols, 6 acids, and 9 other substances. Notably, the esters exhibited high levels of ethyl acetate, ethyl octanoate, and ethyl capricate. Similarly, the alcohols demonstrated higher levels of 3-methyl-1-butanol, phenethylethanol, and 2-methyl-1-propanol, while the acids displayed increased concentrations of acetic acid, caproic acid, and caprylic acid. This study provides the essential technical parameters required for the preparation of Huaniu apple cider while also serving as a valuable reference for investigating its distinct flavor profile.

Substrate specificity of polyphenol oxidase and its selectivity towards polyphenols: Unlocking the browning mechanism of fresh lotus root (Nelumbo nucifera Gaertn.)

Polyphenol oxidase (PPO) causes the browning of lotus roots (LR), negatively affecting their nutrition and shelf-life. This study aimed to explore the specific selectivity of PPO toward polyphenol substrates, thus unlocking the browning mechanism of fresh LR. Results showed that two highly homologous PPOs were identified in LR and exhibited the highest catalytic activity at 35 ℃ and pH 6.5. Furthermore, the substrate specificity study revealed (-)-epigallocatechin had the lowest Km among the polyphenols identified in LR, while (+)-catechin showed the highest Vmax. The molecular docking further clarified that (-)-epigallocatechin exhibited lower docking energy and formed more hydrogen bonds and Pi-Alkyl interactions with LR PPO than (+)-catechin, while (+)-catechin entered the active cavity of PPO more quickly due to its smaller structure, both of which enhance their affinity to PPO. Thus, (+)-catechin and (-)-epigallocatechin are the most specific substrates responsible for the browning mechanism of fresh LR.