Tarantula Hemocyanin Shows Phenoloxidase Activity*
- From the Institute for Molecular Biophysics, University of Mainz, Welder Weg 26, D-55128 Mainz, Germany
Abstract
An enzyme generally catalyzes one well defined reaction with high specificity and efficiency. We report here in contrast that the copper protein hemocyanin of the tarantulaEurypelma californicum exhibits two different functions. These occur at the same active site. While hemocyanin usually is an oxygen carrier, its function can be transformed totally to monophenoloxidase and o-diphenoloxidase activity after limited proteolysis with trypsin or chymotrypsin.N-acetyldopamine (NADA) is more effectively oxidized thanl-dopa or dopamine. This irreversible functional switch of tarantula hemocyanin function is limited to the two subunitsb and c of its seven subunit types. A conserved phenylalanine in the hemocyanin molecule acts as a placeholder for other substrates that are phenylalanine derivatives. The proteolytic cleavage removes an N-terminal fragment, including the critical phenylalanine residue, which opens an entrance for substrates. Therefore no new arrangement of the active site, with its two copper atoms and the μ − η2:η2 bound O2 molecule, is necessary to develop the catalytic function.
Footnotes
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↵* This work was supported by the Deutsche Forschungsgemeinschaft and the Naturwissenschaftlich-Medizinisches Forschungszentrum of the University Mainz.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ To whom correspondence should be addressed. Tel.: 49-06131-393570; Fax: 49-06131-393557; E-mail: decker{at}biophysik.biologie.uni-mainz.de.
- Abbreviations:
- NADA
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N-acetyldopamine.
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- Received May 15, 1998.
- Revision received July 9, 1998.
- The American Society for Biochemistry and Molecular Biology, Inc.
