Quinone Catalysts in Mammalian Enzymes

Mammalian copper-dependent oxidases, including plasma amine oxidases (the semicarbazide-sensitive, chlorgyline-resistant, amine oxidases, not the flavin-dependent monoamine oxidases) and lysyl oxidase (which is involved in the cross-linking of collagen and elastin) have long been known to contain a reactive carbonyl group that is essential for activity. Although this was originally assumed to be pyridoxal phosphate, there is no evidence for its presence in these enzymes, and the apoenzymes cannot be reactivated with pyridoxal phosphate. In amine oxidases, the reactive quinone is topaquinone, whereas in lysyl oxidase it is lysyltopaquinone (see Figure 9.6).

Topaquinone and lysyltopaquinone are formed by postsynthetic modification of the precursor proteins of the active enzymes. A tyrosine residue in the enzyme undergoes autocatalytic oxidation in the presence of enzyme-bound copper and oxygen. Lysyltopaquinone in lysyl oxidase is believed to be synthesized by reaction between topaquinone and the e- amino group of a lysine residue to form the cross-linked imino adduct. This means that it is highly improbable that either topaquinone or lysyltopaquinone is a dietary essential, because there is no way in which preformed quinone could be incorporated into the precursor protein or a hypothetical apoenzyme.


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