Studies of the competing rates of catechol oxidation and suicide inactivation of tyrosinase

Abstract

Tyrosinase oxidation of catechols to ortho-quinones is accompanied by suicide inactivation of the enzyme. The rates of these competing processes vary and depend on the nature of ring substituents. For a series of 4-substituted catechols the relationships between structure and reaction rates have been examined using multiple regression. Significant but different structurerate relationships were found for each process. The oxidation rate (k 1 ) is greatest for short hydrophobic substituents; there is an optimum substituent hydrophobicity (π ~ 0.7) for the rate of inactivation (k 2 ).

Jack of all trades: versatile catechol crosslinking mechanisms

“…7, in which k 1 is the oxidation rate and k 2 is the rate of inactivation of the tyrosinase. 43 The oxidase activity (log k 1 ) can be approximated using the following empirical relationship (1), in which p describes the hydrophobicity of the substituent and L describes its length, i.e., whether the substituent is long or short. log k 1 = 0.364(AE0.097)p À 0.297(AE0.077)L + 2.369 (1) By investigating the oxidation kinetics of catechols with different substituents on the 4-position, Ramsden and Riley demonstrated that shorter (smaller L) or more hydrophobic substituents (larger p) yielded faster oxidation rates; and larger (larger L) or more hydrophilic substituents (smaller p) resulted in a slower oxidation.…”

“…log k 1 = 0.364(AE0.097)p À 0.297(AE0.077)L + 2.369 (1) By investigating the oxidation kinetics of catechols with different substituents on the 4-position, Ramsden and Riley demonstrated that shorter (smaller L) or more hydrophobic substituents (larger p) yielded faster oxidation rates; and larger (larger L) or more hydrophilic substituents (smaller p) resulted in a slower oxidation. 43 The electronic properties of the substituents also affect the oxidation rates. With substituents that are more electronwithdrawing (such as NO 2 , CN, CF 3 ), catechols become difficult to oxidize.…”

“…With substituents that are more electronwithdrawing (such as NO 2 , CN, CF 3 ), catechols become difficult to oxidize. 43,52 In contrast, substituents that are more electrondonating (such as -OMe, -Me), stabilize the radicals, resulting in an opposite effect. 53 These effects of the substituents may also be applicable to other oxidants with different oxidation mechanisms.…”

Mussel-Inspired Self-Healing Double-Cross-Linked Hydrogels by Controlled Combination of Metal Coordination and Covalent Cross-Linking

“…Alternatively, the gels can be designed to resist oxidation, thereby preserving their dynamic mechanical properties because they are completely held together by reversible pH-responsive bonds. Catechols with electron-withdrawing substituents such as -Cl, -NO 2 , -CN, and -CF 3 are more difficult to oxidize, whereas it is the opposite for catechols with electron-donating substituents such as -OMe and -Me. , Recently, Menyo et al demonstrated how to prevent oxidation in mussel-inspired gels by the use of intelligently designed polymers equipped with a chelating HOPO functionality instead of DOPA (the structure of 3-hydroxy-4-pyridinone (HOPO) is shown in Scheme A) . HOPO has a lower susceptibility to oxidation than DOPA as electron density is withdrawn from the aromatic ring through inductive and resonance effects, resulting in reduced phenolic p K a values (HOPO: p K a1 = 3.6, p K a2 = 9.9; DOPA: p K a1 = 9.1, p K a2 = 14) .…”

Considerations about the kinetic mechanism of tyrosinase in its action on monophenols: A review

“…Several models have been proposed to explain the suicide inactivation of tyrosinase, the two oldest proposals are: i) the so-called "cresolase presentation" in which o-diphenol is presented in the active center of the enzyme as a monophenol and the enzyme hydroxylates it in ortho position [89][90][91][92][93], ii) the one proposed by our group that fundamentally consists of the transfer of a proton to hydroperoxide instead of to a base.…”