New features of the steady-state rate related with the initial concentration of substrate in the diphenolase and monophenolase activities of tyrosinase

• Published in: Journal of mathematical chemistry Vol. 48; no. 2; pp. 347 - 362
• Main Authors: Muñoz-Muñoz, Jose Luis, Garcia-Molina, Francisco, Varon, Ramón, Tudela, Jose, Garcia-Cánovas, Francisco, Rodríguez-López, Jose N.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01.08.2010; Springer
• Subjects: Chemistry; Chemistry and Materials Science; Exact sciences and technology; General and physical chemistry; Math. Applications in Chemistry; Original Paper; Physical Chemistry; Theoretical and Computational Chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Monophenolase activity; Tyrosinase; Kinetically preferred pathway; Slow pathway; Diphenolase activity; Action; Mathematical chemistry; Oxygen; Enzyme; Curve; Activity; Concentration; Steady state; Graph; Oxidation; Inhibition
• ISSN: 0259-9791; 1572-8897
• DOI: 10.1007/s10910-010-9675-5

Tyrosinase has two types of enzymatic activities: the hydroxylation of monophenols to o- diphenols (monophenolase activity) and oxidation of o- diphenols to o- quinones (diphenolase activity). The action on o- diphenols involves two substrates: oxygen and o- diphenol, while the mechanism proposed is a Uni Uni Bi Bi ping-pong. In this contribution, we demonstrate experimentally that there is a kinetically preferred pathway, which translates into the appearance of curves of initial velocity vs . initial diphenol concentration shows inhibition by an excess of substrate, while sigmoid curves are obtained when the initial velocity vs . initial oxygen concentration are graphed. However, the action mechanism of the enzyme on monophenols, which is more complex because it involves three substrates (monophenol, oxygen and o- diphenol), does behave differently from the hyperbolic behaviour as regards the initial velocity vs . initial monophenol concentration, results that can be explained if the limiting step in the action of tyrosinase is the hydroxylation of monophenol to o- diphenol.