title: Substrate inhibition competes with halide inhibition in polyphenol oxidase.
abstract: 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.
authors.affiliation: Department of Applied Biological Chemistry, School of Agricultural and Life Science, The University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo, 113-8657, Japan.
authors.forename: Giselle Grace Fernando
authors.initials: GG
authors.lastname: Lim
authors.forename: Yuki
authors.initials: Y
authors.lastname: Imura
authors.forename: Etsuro
authors.initials: E
authors.lastname: Yoshimura
mesh_terms: D000838:Anions / Q000737:chemistry / Q000494:pharmacology; D001665:Binding Sites; D001965:Bromides / Q000737:chemistry / Q000494:pharmacology; D004156:Catechol Oxidase / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000378:metabolism; D002396:Catechols / Q000032:analysis / Q000378:metabolism*; D007700:Kinetics; D011485:Protein Binding; D012965:Sodium Chloride / Q000737:chemistry / Q000494:pharmacology; D017670:Sodium Compounds / Q000737:chemistry / Q000494:pharmacology; D012974:Sodium Iodide / Q000737:chemistry / Q000494:pharmacology; D013379:Substrate Specificity
chemical_list: D000838:Anions; D001965:Bromides; D002396:Catechols; D017670:Sodium Compounds; D012965:Sodium Chloride; D004156:Catechol Oxidase; D012974:Sodium Iodide; C027938:sodium bromide; C034221:catechol