Hysteresis and Positive Cooperativity of Iceberg Lettuce Polyphenol Oxidase

Oxidative discolouration in whole-head and cut lettuce: biochemical and environmental influences on a complex phenotype and potential breeding strategies to improve shelf-life

“…In lettuce, Chazarra et al (2001) have shown that activation can occur slowly in the presence of the enzyme's substrate (in this case chlorogenic acid) and similar substrate activations have been demonstrated for a range of monophenols in PPO from red clover (Winters et al 2008). This has been suggested as the more likely in vivo activation mechanism for latent PPO, although proteolytic activation in vivo has also been demonstrated for red clover (Winters et al 2008).…”

“…PPO catalyses two distinct reactions; the o-hydroxylation of monophenols to produce o-diphenols (monophenolase or cresolase activity) and the subsequent oxidation of the o-diphenols to o-quinones (diphenolase or catecholase activity), both of which require the presence of molecular oxygen (López-Galvez et al 1996;Chazarra et al 2001;Toivonen and Brummell 2008). The initial monophenolase reaction is slow (therefore ratelimiting) and results in colourless products, while the diphenolase reaction is rapid and results in quinones and subsequently, coloured compounds (Toivonen and Brummell 2008).…”

“…The experiments by Leufken et al (2015) identified the linker domain between the proteolytically cleaved C-terminus and the N-terminal region of the protein as being associated with SDS sensitivity. Since the linker domain is not cleaved during proteolysis it should theoretically still be available for interaction with SDS however, many of the of these activation mechanisms appear to be associated with a conformational change in the PPO protein (Chazarra et al 2001;Sellés-Marchart et al 2007) and it may be that the C-terminal cleavage results in such a change meaning that either the SDS binding site on the linker domain is no longer physically available for interaction with SDS or that the configuration is now such that SDS binding no longer results in exposure of the enzyme active site.…”

Colour and phenolic compounds in sweet red wines from Merlot and Tempranillo grapes chamber-dried under controlled conditions

“…Since our drying temperature never exceeded 50°C, the Maillard reaction would have occurred to a much lesser extent than enzymatic browning reactions. Polyphenol oxidases (PPOs) oxidise certain phenolic compounds (hydroxycinnamic acids mainly) to brown compounds known as ''melanins'' via complex chemical reactions (Chazarra, Garcia-Carmona, & Cabanes, 2001). Since the concentrations of phenols and, specifically, hydroxycinnamic acids, are usually higher in red grapes, browning during the drying process was stronger than in Pedro Ximenez white grapes.…”

Two-year comparison of the biochemical properties of polyphenol oxidase from Turkish Alyanak apricot ( Prunus armenica L.)

“…Interaction of o-quinones with other molecules leads to formation of red, brown, or black pigments, which lower quality attributes and economic values of raw material and products. Enzymatic browning is regarded as a serious problem that can occur during harvesting, handling, processing, and storage of fruits and vegetables (Chararra, Carcia-Carmona, & Cabanes, 2001;Gauillard & Richard-Forget, 1997;Ramirez, Whitaker, & Virador, 2003;Zawistowski, Biliaderis, & Eskin, 1991). There are many reports related to purification and characterization of PPO from various fruits and vegetables, e.g.…”