Journal
JOURNAL OF MATHEMATICAL CHEMISTRY
Volume 48, Issue 2, Pages 347-362Publisher
SPRINGER
DOI: 10.1007/s10910-010-9675-5
Keywords
Tyrosinase; Kinetically preferred pathway; Slow pathway; Diphenolase activity; Monophenolase activity
Funding
- Ministerio de Educacion y Ciencia (Madrid, Spain) [BIO2009-12956]
- Fundacion Seneca (CARM, Murcia, Spain) [08856/PI/08, 08595/PI/08]
- Consejeria de Educacion (CARM, Murcia, Spain) BIO-BMC [06/01-0004]
- FISCAM [PI-2007/53]
- Consejeria de Salud y Bienestar Social de la Junta de Comunidades de Castilla La Mancha
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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.
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