Effect of thermal treatment on secondary structure and conformational change of mushroom polyphenol oxidase (PPO) as food quality related enzyme: A FTIR study

In order to understand the conformational changes of polyphenol oxidase (PPO), which is a food quality related enzyme, after thermal treatment, secondary structure changes of the enzyme were analyzed by using Fourier Transform Infrared (FTIR) spectroscopy and compared with the change in enzyme activity in the temperature range of 25-80 °C. Fourier self-deconvolution, neural network (NN) and curve-fitting analysis were applied to the amide I band of FTIR spectra for detail analysis of secondary structure elements. FTIR analysis indicated that PPO is an α-helix dominating enzyme. Detail analysis revealed that, as temperature increased, α-helix and β-sheet decreased, but aggregated β-sheet, turns and random coil increased. The marked changes were noted at 40 °C with the occurrence of new bands due to aggregated β-sheet structures, all of which indicate protein denaturation. These aggregation bands were still observed when the temperature was reduced back to 25 °C, from 70 °C, demonstrating an irreversible change in the structure.