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Substrate Inhibition Competes with Halide Inhibition in Polyphenol Oxidase


Polyphenol oxidase (PPO) is a ubiquitous enzyme important in the food industry. Although PPO activity followed Michaelis–Menten kinetics at catechol concentrations of up to 1 mM, it slowly decreased at catechol concentrations above 2 mM. This result indicated that in addition to the active site (site A), the enzyme possesses a second catechol-binding site (site B) that exerts an inhibitory effect on PPO activity. Halides inhibit PPO activity in such a way that substrate inhibition is lessened when halide concentration is increased. Furthermore, elevated concentrations of catechol diminished the degree of inhibition by halides. These findings suggest that halides also bind to site B to inhibit PPO activity. A steady-state kinetic analysis demonstrated that the dissociation constant between catechol and PPO depended on the binding of halides to site B. The dissociation constants were greatest when chloride bound to the site. Bromide and iodide yielded lower dissociation constants, in that order. These data indicate that the binding of halide to site B modulated the structure of site A, thereby exerting an inhibitory effect.

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EoH :

PPO Empty at site A and bound to halide at site B


Free PPO


PPO Bound to substrate at site A and to halide at site B


PPO Bound to substrate at site A and empty at site B


PPO Bound to substrates at both sites A and B


Polyphenol oxidase

Site A:

First substrate-binding site of PPO

Site B:

Second substrate-binding site of PPO


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This work was supported by a fund (11D3) from The Salt Science Research Foundation, Japan. G.G.F.L was supported by a scholarship from The Ajinomoto Scholarship Foundation.

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Correspondence to Etsuro Yoshimura.

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Lim, G.G.F., Imura, Y. & Yoshimura, E. Substrate Inhibition Competes with Halide Inhibition in Polyphenol Oxidase. Protein J 31, 609–614 (2012).

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  • Enzyme kinetics
  • Polyphenol oxidase
  • Substrate inhibition
  • Halide
  • Inhibition
  • Catechol