期刊
INSECT BIOCHEMISTRY AND MOLECULAR BIOLOGY
卷 42, 期 3, 页码 193-202出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.ibmb.2011.11.010
关键词
Multicopper oxidase; Laccase; Substrate; Insect; Cuticle
资金
- NIH from the INBRE of the National Center for Research Resources [P20 RR-016464]
- National Institute of Allergy and Infectious Diseases [R01AI070864]
Laccase-2 is a highly conserved multicopper oxidase that functions in insect cuticle pigmentation and tanning. In many species, alternative splicing gives rise to two laccase-2 isoforms. A comparison of laccase-2 sequences from three orders of insects revealed eleven positions at which there are conserved differences between the A and B isoforms. Homology modeling suggested that these eleven residues are not part of the substrate binding pocket. To determine whether the isoforms have different kinetic properties, we compared the activity of laccase-2 isoforms from Tribolium castaneum and Anopheles gambiae. We partially purified the four laccases as recombinant enzymes and analyzed their ability to oxidize a range of laccase substrates. The predicted endogenous substrates tested were dopamine, N-acetyldopamine (NADA), N-beta-alanyldopamine (NBAD) and dopa, which were detected in T castaneum previously and in A. gambiae as part of this study. Two additional diphenols (catechol and hydroquinone) and one non-phenolic substrate (2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid)) were also tested. We observed no major differences in substrate specificity between the A and B isoforms. Dopamine, NADA and NBAD were oxidized with catalytic efficiencies ranging from 51 to 550 min(-1) mM(-1). These results support the hypothesis that dopamine, NADA and NBAD are endogenous substrates for both isoforms of laccase-2. Catalytic efficiencies associated with dopa oxidation were low, ranging from 8 to 30 min(-1) mM(-1): in comparison, insect tyrosinase oxidized dopa with a catalytic efficiency of 201 min(-1) mM(-1). We found that dopa had the highest redox potential of the four endogenous substrates, and this property of dopa may explain its poor oxidation by laccase-2. We conclude that laccase-2 splice isoforms are likely to oxidize the same substrates in vivo, and additional experiments will be required to discover any isoform-specific functions. (C) 2011 Elsevier Ltd. All rights reserved.
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