Modeling novel quinocofactors: An overview

In recent years, three novel quinonoid cofactors, topaquinone (TPQ), tryptophan tryptophyl-quinone (TTQ), and lysine tyrosylquinone (LTQ) were identified in copper amine oxidase, methylamine dehydrogenase, and lysyl oxidase, respectively. The novel quinocofactors all derive through posttranslational modification of amino acid residues in the backbone of the enzymes, whereas the previously known quinonoid coenzyme, pyrroloquinoline quinone (PQQ), is noncovalently bound to several prokaryotic dehydrogenases. The identification of these new redox cofactors stimulated numerous studies aimed at characterizing their properties and their role in substrate oxidation. Many efforts have been made to shed light on specific points, including (i) the mechanism of posttranslational modification leading to these covalently bound quinonoid coenzymes, (ii) the structural characterization of the copper-binding site and the relation to quinonoid cofactor (TPQ or LTQ), (iii) catalytic mechanism, (iv) modulation of quinocofactor reactivity by the enzyme matrix. In all these cases, recent model studies, disclosing the basic chemical and physicochemical properties of compounds closely resembling the novel quinocofactors, have greatly contributed to answering specific questions, and have offered a frame of reference for interpretation of studies of TPQ, TTQ, and LTQ in enzymatic systems. This minireview is an updated, comprehensive account of these contributions.