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Laccases are a family of multicopper oxidases that bear a close resemblance, particularly in amino acid sequence, to ascorbate oxidase (found in cucumbers) and mammalian plasma protein ceruloplasmin. First isolated in 1883 from Rhus venicifera, the Japanese lacquer tree, laccases are also commonly found in fungi such as the lignolytic white-rot fungus Trametes versicolor. They have also recently been discovered in bacteria. Their physiological function is still under intense investigation although they are implicated in the synthesis and/or degradation of the biopolymer lignin, wound response mechanisms and the morphogenesis of microorganisms.
They have a broad specificity that enables them to catalyse the one-electron oxidation of a range of substrates. These substrates, usually phenolic in nature, provide electrons for the concomitant four electron reduction of atmospheric O2 to water. Catalysis takes place via the four copper centres which are classified according to their electronic properties. For example, the type I (T1) Cu shows a characteristic sulfur to Cu(II) charge transfer absorption band at around 610 nm.
Currently the catalytic properties of laccases are being exploited for a range of technological applications such as the bioremediation of soils and water and the development of environmentally friendly processes in the pulp and paper industry. Much work has gone into the sequencing, cloning and mutagenesis of laccase genes from microrganisms. Recently several crystal structures containing the full complement of Cu atoms have been reported.
Protein film voltammetry allows the interrogation of laccase to discern information on electroactive enzyme coverage on the electrode, turnover number and the kinetics of the turnover process. The roles that the different redox centres play in catalysis can also be explored. Fungal laccases are particularly interesting for electrochemical study since they readily adsorb to pyrolytic graphite edge electrodes and possess a generally higher T1 Cu redox potential than those found in plant laccases.
Trametes versicolor growing on a stump outside the Conway Institute at University College Dublin.
A ribbon representation of the X-ray-determined crystal structure of laccase III from Trametes versicolor (PDB code 1KYA)
Photos and illustrations © 2007 C.F. Blanford.
- “A stable electrode for high-potential, electrocatalytic O2 reduction based on rational attachment of a blue copper oxidase to a graphite surface.” C.F. Blanford, R.S. Heath and F.A. Armstrong. Chem. Commun. (17), 1710–1712 (2007). [PDF]
- “Laccases: structure, reactions, distribution.” H. Claus. Micron 35, 93–96 (2004). [PDF]
- “Crystal Structure of a Laccase from the Fungus Trametes versicolor at 1.90 Å Resolution Containing a Full Complement of Coppers.” K. Piontek, M. Antorini and T. Choinowski. J. Biol. Chem. 277(40), 37663–37669 (2002). [PDF]
- “Electrochemical properties of some copper-containing oxidases.” A.I. Yaropolov, A.N. Kharybin, J. Emneus, G. MarkoVarga and L. Gorton. Bioelectrochemistry and Bioenergetics 40(1), 49–57 (1996). [PDF]