Copper-PQQ Interactions in Amine Oxidases

Abstract

Interactions between copper and PQQ in several amine oxidases have been probed by a variety of physical methods. A Cu(I)-semiquinone (PQQ^●) state can be generated by substrate-reduction in the presence of ligands that stabilize Cu(I). This state has now been characterized in detail for amine oxidases from bovine plasma, porcine kidney, pea seedlings, and Arthrobacter P1. The properties of the semiquinone are independent of the enzyme, substrate, or copper ligand. It is possible that a Cu(I)-PQQ^● state may be a catalytic intermediate in the oxidation of the substrate-reduced enzyme by O₂. Additional substrate-reduced forms are observed for some amine oxidases; the distribution among these forms is sensitive to copper ligation. Resonance Raman spectroscopy has been used to characterize both native and metal-depleted amine oxidases, which have been derivatized by chromophoric carbonyl reagents. 2-Hydrazinopyridine reacts with all the amine oxidases (and PQQ) to form a hydrazone derivative with λ_max ≅ 415 nm; subsequently this converts, in a copper-dependent reaction, to a form with an absorption band at ≃ 520 nm.