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JXB Advance Access published online on June 27, 2005
Journal of Experimental Botany, doi:10.1093/jxb/eri220
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Published by Oxford University Press [2005] on behalf of the Society for Experimental Biology.
Received January 20, 2005
Accepted May 5, 2005

RESEARCH PAPER

Tyr152 plays a central role in the catalysis of 1-aminocyclopropane-1-carboxylate synthase

Jian-Feng Li 1, Liang-Hu Qu 2, and Ning Li 3*

1 Department of Biology, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong SAR, China; Key Laboratory of Gene Engineering of the Ministry of Education, Zhongshan University, Guangzhou 510275, China
2 Key Laboratory of Gene Engineering of the Ministry of Education, Zhongshan University, Guangzhou 510275, China
3 Department of Biology, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong SAR, China

* To whom correspondence should be addressed.
Ning Li, E-mail: boningli{at}ust.hk


  Abstract

1-Aminocyclopropane-1-carboxylate (ACC) synthase is a key enzyme in the regulation of ethylene biosynthesis in higher plants. To investigate the catalytic significances of two conserved tyrosine residues, Tyr151 and Tyr152, of a tomato ACC synthase isozyme (LeACS2), five ACC synthase mutants (Y151F, Y151G, Y152F, Y152G, and Y151F/Y152F) were constructed and over-expressed in Escherichia coli. Subsequent kinetic analysis indicated that these point mutations in mutants Y152F, Y152G, and Y151F/Y152F, either reduced the catalytic efficiency more than 98% or fully inactivated ACC synthase, while Y151F and Y151G mutants reduced the enzymatic activities by 27% and 83%, respectively. It is therefore concluded that Tyr152, especially its hydroxyl group, plays an essential role in the catalysis of ACC synthase. Thus, a revised catalytic model is hereby proposed for functional ACC synthase.

Keywords: ACC formation; ACC synthase; catalysis; kinetic analysis; site-directed mutagenesis; tyrosine.
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