Abstract
Over the recent decade, a number of unique cofactors derived from amino acid residues have been discovered in various enzyme proteins, including 2,4,5-trihydroxyphenylalanine quinone (topa quinone; TPQ) of copper amine oxidase and tryptophan tryptophylquinone (TTQ) of bacterial amine dehydrogenase. These quinone cofactors temporarily store the reducing equivalents derived from the substrate before passing them on to exogenous 1- or 2-electron acceptors such as a cytochrome, a cupredoxin, or molecular oxygen. Both of TPQ and TTQ are encoded as normal amino acids (Tyr or Trp) in the enzyme genes and thus are synthesized by post-translational modification. Using the inactive precursor forms of recombinant copper amine oxidases from Arthrobacter globiformis overproduced in Escherichia coli, we previously demonstrated that TPQ is generated through self-processing of the enzyme proteins with the participation of the bound copper ion. X-ray crystallographic studies of the Cu/TPQ-less inactive enzyme (apoenzyme) as well as the Cu/TPQ-containing active form (holoenzyme) revealed the structural difference only in the active site. Furthermore, we have recently identified a novel quinone cofactor, cysteine tryptophylquinone (CTQ) in quinohemoprotein amine dehydrogenases from Paracoccus denitrificans and Pseudomonas putida. CTQ is the forth quinone cofactor derived from amino acids, following TPQ, TTQ, and lysine tyrosylquinone (LTQ) found in lysyl oxidase.
