Bi-Substrate Oxidation Reaction Catalyzed by H2O2-Dependent Lignin Peroxidase: A Good Case For Undergraduates to Fully Understand the Concept of an Inhibitor

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

Table of Contents

Abstract Volume 12 Issue 2 (2007) pp 59-60

Bi-Substrate Oxidation Reaction Catalyzed by H₂O₂-Dependent Lignin Peroxidase: A Good Case For Undergraduates to Fully Understand the Concept of an Inhibitor

Yun Zhang,^† Xirong Huang,^†,* Peiji Gaob^‡

^†Key Laboratory of Colloid & Interface Chemistry of the Education Ministry of China, Shandong University, Jinan 250100, P. R. China, xrhuang@sdu.edu.cn, ^‡State Key Laboratory of Microbial Technology of China, Shandong University, Jinan 250100, P. R. China.
Received June 20, 2006. Accepted December 11, 2006.

Published online: 1 April 2007

Abstract. An inhibitor is a basic concept of enzyme kinetics; it decreases the rate of an indicator reaction (for enzyme assay) catalyzed by an enzyme. For H₂O₂-dependent lignin peroxidase (LiP), a wrong conclusion that naphthol (NA) is a strong inhibitor of LiP could be drawn based on the incomplete statement above and the fact that the presence of small amounts of NA results in an obvious decrease in the initial rate of the LiP-catalyzed oxidation of veratryl alcohol (VA) (psychological substrate of LiP) by H₂O₂; however, NA, like VA, is also a substrate of LiP. A detailed kinetic analysis shows that the phenomenon was caused by VA^·+ (a cation radical) mediated oxidation of NA, which prevents the formation of veratraldehyde from VA^·+. The uniqueness of the present LiP-catalyzed bisubstrate oxidation reaction is the occurrence of a lag period in the kinetic curves of absorbance versus time. We think that the present case is a good demonstration for undergraduates to fully understand the concept of an inhibitor. It tells them that, for a compound that is an inhibitor, the negative effect on the initial rate of an indicator reaction is a necessary condition, but not a sufficient condition. Conversion of the compound itself by the enzyme or the reaction mechanism concerned is suggested to consult to draw a reliable conclusion.

Key Words: In the Classroom; biochemistry; enzyme kinetics; inhibitor

(*) Corresponding author. (E-mail: xrhuang@sdu.edu.cn)

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