The effect of citric acid on the activity, thermodynamics and conformation of mushroom polyphenoloxidase

Few reports have focused on the effect of citric acid on thermodynamics and conformation of polyphenoloxidase (PPO). In this study, variations on activity, thermodynamics and conformation of mushroom PPO induced by citric acid (1–60 mM) and relationships among these were investigated. It showed that with the increasing concentration of citric acid, the activity of PPO decreased gradually to an inactivity condition; inactivation rate constant (k) of PPO increased and the activation energy (Ea) as well as thermodynamic parameters (ΔG, ΔH, ΔS) decreased, which indicated that the thermosensitivity, stability and number of non-covalent bonds of PPO decreased. The conformation was gradually unfolded, which was reflected in the decrease of α-helix contents, increase of β-sheet and exposure of aromatic amino acid residuals. Moreover, two linear relationships of relative activities, enthalpies (ΔH) against α-helix contents were obtained. It indicated that changes of activity and thermodynamics might correlate to the unfolding of conformation.

► PPO activity was not affected by low citric acid but inhibited by high citric acid.
► Decreases in activation energy indicated its thermosensitivity decrease.
► Low thermodynamic parameters indicated its low stability and non-covalent bond.
► The conformations of PPO were gradually unfolded with a decrease of α-helix.
► Both activity and enthalpy against α-helix content gave linear relationships.