Lysyl-tyrosyl quinone

Figure 18 Tyrosine-derived quinone cofactors, (a) TPQ, 2,4,5-tri-hydroxyphenylalanine quinone (b) LTQ, lysyl tyrosyl quinone...

Lysyl oxidase contains a single Type 2 copper center (see Copper Proteins with Type 2 Sites) at the active site, bound by three histidine residues. The metal ion appears not to be redox active during turnover, and the oxidation chemistry is instead performed by a covalently attached cofactor, lysine tyrosyl quinone, derived from a tyrosine residue in the protein (vide infra. Section 3.1). The overall reaction involves oxidation of the primary amine group of a peptide-bound lysine residue, reduction of dioxygen to hydrogen peroxide, and release of ammonium ion (equation 4). 

Structural characterization (by mass spectrometry) has shown that in lysyl oxidase, a lysine side chain adds to the quinocofactor forming a covalent cross-link within the protein. This novel cofactor, which has been given the trivial name lysine tyrosyl quinone (LTQ) (13), is irrunobilized by this linkage, and exhibits an altered pattern of reactivity, forming an o-quinone (equation 11) rather than the p-quinonoid sttucture stabilized by TPQ (equation 12). 

Fig. 7.7. Some of the covalent links which occur in structural proteins. ABCD represents one protein, EFB C G another. The N-terminal amino acid A is linked to an o-quinone which is also linked to a lysyl residue F(=quinone tanning) BB represents two tyrosyl residues coupled by a biphenyl linkage CC represents two cysteinyl residues coupled by a cystine linkage (as in keratin) D, E and G are not cross-linked. (After Brunet, 1967.)...

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