This dissertation describes investigations into the structure and function of selected laccase-like multi-copper oxidases (LMCOs). A plant LMCO from Acer pseudoplatanus has been expressed in tobacco BY2 cell ands compared to the native LMCO isolated from Acer pseudoplatanus cell suspension cultures. It was found to be nearly identical to the native enzyme except with respect to the Kms for certain substrates. A LMCO gene cloned from yellow-poplar (Liriodendron tulipifera) and expressed in transgenic tobacco cells showed ferroxidase activity. This marks the first report of ferroxidase activity associated with a plant LMCO, and suggests an alternative physiological function for these enzymes. Molecular modeling of the Fet3 LMCO gave insight into which amino acids could be involved in iron binding in LMCOs. Additionally, a new chromogenic substrate for laccases and peroxidases, 1,8-diaminonapthalene, was found to be more sensitive and less toxic than traditional substrates when used in SDS-PAGE zymograms.