The antibrowning agent sulfite inactivates Agaricus bisporus tyrosinase through covalent modification of the copper‐B site

Abstract

Sulfite salts are widely used as antibrowning agents in food processing. Nevertheless, the exact mechanism by which sulfite prevents enzymatic browning has remained unknown. Here, we show that sodium hydrogen sulfite (NaHSO 3 ) irreversibly blocks the active site of tyrosinase from the edible mushroom Agaricus bisporus, and that the competitive inhibitors tropolone and kojic acid protect the enzyme from NaHSO 3 inactivation. LC-MS analysis of pepsin digests of NaHSO 3 -treated tyrosinase revealed two peptides showing a neutral loss corresponding to the mass of SO 3 upon MS 2 fragmentation. These peptides were found to be homologous peptides containing two of the three histidine residues that form the copper-B-binding site of mushroom tyrosinase isoform PPO3 and mushroom tyrosinase isoform PPO4, which were both present in the tyrosinase preparation used. Peptides showing this neutral loss behavior were not found in the untreated control.Comparison of the effects of NaHSO 3 on apo-tyrosinase and holo-tyrosinase indicated that inactivation is facilitated by the active site copper ions. These data provide compelling evidence that inactivation of mushroom tyrosinase by NaHSO 3 occurs through covalent modification of a single amino-acid residue, probably via addition of HSO 3 À to one of the copper-coordinating histidines in the copper-B site of the enzyme.

Boosting LPMO-driven lignocellulose degradation by polyphenol oxidase-activated lignin building blocks

“…4; Additional file 4: Table S1). Different from Mt PPO7, in previous research Ab PPO has been reported to be highly active towards these types compounds [19, 24, 25].

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“…We choose Mt PPO7, which originates like the Mt LPMO9B employed here, from the thermophilic filamentous fungus Myceliophthora thermophila C1. As a reference, the well-studied tyrosinase Ab PPO from the edible mushroom Agaricus bisporus was used [19, 25–28].

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“…5). Based on the model, the large distance (10.5 Å) between the sulfur atom of Cys302 and C ɛ -atom of His110 prevents formation of a thioether bond, which is present in other PPOs such as Ab PPO3 and Ab PPO4 [25]. Characteristics of the amino acid sequence and structural model of Mt PPO7 are further described in the “Discussion” section.

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Aurone synthase is a catechol oxidase with hydroxylase activity and provides insights into the mechanism of plant polyphenol oxidases

“…Irreversible inactivation of tyrosinase from Agaricus bisporus (PPO3; abTYR) has been reported to occur by incubation with sulfite ions (33). The authors evidenced the sulfation of a CuBcoordinating histidine and speculated that the sulfation (accompanied by the loss of the CuB atom) presumably occurred at His263 (His256 in AUS1).…”

Cambios Físicos Y Fisicoquímicos Durante El Almacenamiento en Plátano Impregnado Al Vacío Con Soluciones Antioxidantes

“…La PPV impregnada con la SI con sulfitos cumplió con el L* crítico durante los 15 días de almacenamiento en ambas condiciones de envasado (CV: 76,3 ± 0,9; SV: 73,3 ± 0,7). Los iones S 2 O 5 = actúan como agentes blanqueadores sobre algunos pigmentos que al reaccionar producen formas sulfónicas en diversas posiciones que son incoloras; por otro lado, inhiben especialmente las reacciones de oscurecimiento producidas por la enzima PFO, reduciendo las quinonas y transformándolas en compuestos incoloros (quinona-sulfito) [30,31] que evitan que la quinona se polimerice; otro mecanismo indica que el sulfito inhibe directamente la enzima alterando su estructura proteica [32]. Por otro lado, los tratamientos con las mezclas de ácidos y el patrón en condiciones CV también fueron efectivos en el control del pardeamiento durante los 15 días.…”