Study of Sterospecificity in Mushroom Tyrosinase

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Study of stereospecificity in mushroom tyrosinase and the inhibiting effects of thiourea, cinnamic acid and benzoic acid

BIOL/BIOC 393 L03
Dr. Judit Moldovan

Submitted: Nov. 22nd, 2010
By: Jackie Minnick (Partners Amanda Verwoerd & Kersti Ojamaa)
Study of stereospecificity in mushroom tyrosinase and the inhibiting effects of thiourea, cinnamic acid and benzoic acid.

Jackie Minnick

This paper reports experiments on the stereospecificity observed in the monophenolase and diphenolase activities of mushroom tyrosinase and the inhibiting effects of thiourea, cinnamic acid and benzoic acid. The enantiomer L-DOPA and D-DOPA were assayed. The Vmax and Km values obtained for each were different. Thus mushroom tyrosinase showed stereospecificity in its higher affinity towards L-DOPA than D-DOPA. The same Km values were obtained for cinnamic acid and benzoic acid as compared to the uninhibited L-DOPA. However, the Vmax values were different. Therefore, both cinnamic acid and benzoic acid exhibit non-competitive inhibition for tyrosinase. Thiourea showed different results for Vmax and Km, still indicating that thiourea is a non-competitive inhibitor of tyrosinase. It was found that cinnamic acid is the strongest inhibitor of the three.

Introduction

Tyrosinase, also commonly called polyphenol oxidase, catalyses two reactions: ortho-hydroxylation of monophenols and aerobic oxidation of ortho-diphenols. Equations 1 and 2: Monohydroxyphenol + O2 + AH2 → o-dihydroxyphenol + H2O +A (1) o-dihydroxyphenol + ½ O2 → o-quinone + H2O (2)
Reaction 1 and 2 are referred to as the monohydroxyphenolase and o-dihydroxyphenolase activities of tyrosinase, respectively (Andrawis & Kahn, 1986).

Tyrosinase is a copper containing enzyme found in many plant and animal cells. It plays a central role in the enzymatic browning of some fruits and vegetables, as well as such processes as vertebrate pigmentation. The reaction is localized in pigment cells and produces the melanin



Cited: Anderson, J.W. (1968) Phytochemistry 7, 1973-1988 Andrawis, A Boon, J. (2007) UBCO Biology 311 Lecture Notes, Enzyme Kinetics 1 & 2 Chemfinder, (2007) Espin, J.C., Garcia-Ruiz, P.A., Tudela, J. & Garcia-Canova, F. (1998) Biochem. J. 331, 547-551 Hanlon, D.P Huang, H., Liu, X.S., & Chen, Q.X. (2003) Journal of Xiamen University (Natural Science) 42, 5, 97-101 Robb, D.A Shi, Y., Chen, Q.X., Wang, Q., Song, K.K. & Qui, L. (2005) Food Chemistry 92, 4, 707-712 Sonesson, B., Eide, S., Rorsman, H Winder, A.J. & Harris, H. (1991) Eur. J. Biochem. 198, 317-325

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