Protein-Derived Cofactors Revisited: Empowering Amino Acid Residues with New Functions

A protein-derived cofactor is a catalytic or redox-active site in a protein that is formed by post-translational modification of one or more amino acid residues. These post-translational modifications are irreversible and endow the modified amino acid residues with new functional properties. This Perspective focusses on the following advances in this area that have occurred during recent years. The biosynthesis of the tryptophan tryptophylquinone (TTQ) cofactor is catalyzed by a di-heme enzyme, MauG. A bis-Fe(IV) redox state of the hemes performs three two-electron oxidations of specific Trp residues via long-range electron transfer. In contrast, a flavoenzyme catalyzes the biosynthesis of the cysteine tryptophylquinone (CTQ) cofactor present in a newly discovered family of CTQ-dependent oxidases. Another carbonyl cofactor, the pyruvoyl cofactor found in classes of decarboxylases and reductases, is formed during an apparently autocatalytic cleavage of a precursor protein at the N-terminus of the cleavage product. It has been show that in at least some cases, the cleavage is facilitated by binding to an accessory protein. Tyrosylquinonine cofactors, topaquinone (TPQ) and lysine tyrosylquinone (LTQ) are found in copper-containing amine oxidases and lysyl oxidases, respectively. The physiological roles of different families of these enzymes in humans has been more clearly defined and shown to have significant implications towards human health. There has also been continued characterization of the roles of covalently cross-linked amino acid side-chains that influence the reactivity of redox-active metal centers in proteins. These include Cys-Tyr species in galactose oxidase and cysteine dioxygenase, and the Met-Tyr-Trp species in the catalase-peroxidase KatG.