ABSTRACT
Lysyl oxidase (EC 1.4.3.13) is an extracellular enzyme that catalyzes the oxidation of lysyl side chains in structural proteins such as collagen and elastin, as the initial step in formation of cross-linkages. Purification of lysyl oxidase from mammalian tissue has proved to be very difficult [109] and requires an initial urea extraction step. Although the refolded purified protein is catalytically active, it is uncertain whether this corresponds to the full in vivo activity. The purified protein has a monomer molecular mass of 32 kDa and contains one mole-equivalent of copper [1 1 0 ]. The carbonyl cofactor has been shown to be a novel quinone, termed lysine tyrosyl quinone (LTQ), involving post-translational cross-linking between a lysine and a mod ified tyrosine residue [111]. Lysyl oxidase cDNA has been sequenced to reveal a 1233bp region coding for a 411-amino-acid, 46.6-kDa protein [112] that is further pro cessed to give the 32-kDa secreted protein. Attempts to express the lo gene to give a soluble form of the protein, suitable for crystallization have not been successful. We have no information on the 3-D structure of lysyl oxidase. Since it is a completely different protein in terms of its primary structure and molecular weight from the amine oxidases whose structures have been discussed in Sec. 2 , homology modeling of the lysyl oxidase 3-D structure is not possible.
