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JXB Advance Access originally published online on June 27, 2005
Journal of Experimental Botany 2005 56(418):2203-2210; doi:10.1093/jxb/eri220
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Published by Oxford University Press [2005] on behalf of the Society for Experimental Biology.

RESEARCH PAPER

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

Jian-Feng Li1,2, Liang-Hu Qu2 and Ning Li1,*

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

* To whom correspondence should be addressed. Fax: +852 2358 1559. E-mail: boningli{at}ust.hk

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.

Key words: ACC formation, ACC synthase, catalysis, kinetic analysis, site-directed mutagenesis, tyrosine


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