Characterization of the Preprocessed Copper Site Equilibrium in Amine Oxidase and Assignment of the Reactive Copper Site in Topaquinone Biogenesis

Replacement of Tyrosines by Unnatural Amino Acid Aminophenylalanine Leads to Metal-Mediated Aniline Free Radical Formation in a Copper Amine Oxidase

Copper amine oxidases (CAOs) catalyze the oxidative deamination of primary amines to aldehyde, ammonia, and hydrogen peroxide as products and are widely distributed in bacteria, plants, and eukaryotes. These enzymes initiate the single turnover, post-translational conversion of an active site tyrosine to the redox cofactor 2,4,5-trihydroxyphenylalanine quinone (TPQ), subsequently employing TPQ to catalyze steady-state amine oxidation. The mechanisms of TPQ biogenesis and steady-state amine oxidation have been studied extensively, with consensus mechanisms proposed for both reactions. One unresolved issue has been whether the Cu2+ center must undergo formal reduction to Cu1+ in the course of the reaction. Herein, we investigate the properties of the active site of a yeast (Hansenula polymorpha) amine oxidase (HPAO) that has undergone site-specific insertion of a para-aminophenylalanine (pAF) into the position of either the precursor tyrosine to TPQ (Y405) or the two strictly conserved neighboring tyrosines (Y305 and Y407). While our original intention was to interrogate cofactor biogenesis using a precursor unnatural amino acid (UAA) of altered redox potential and pKa, we instead observe an unanticipated reaction assigned to an intramolecular electron transfer from pAF to the active site copper ion. We establish the generality of the observed active site chemistry using exogenously added, aniline-containing substrates under conditions that prevent side chain amine oxidation. The results support previous proposals that the activation of the TPQ precursor occurs in the absence of a formal valence change at the active site copper site. The described reaction of pAFs with the active site redox Cu2+ center of HPAO provides a prototype for either the engineering of the enzymatic oxidation of exogenous anilines or the insertion of site-specific free radical probes within proteins.

Diversity of oxidation state in copper complexes with phenolate ligands

The phenoxyl radical binding copper complexes have been widely developed and their detailed geometric and electronic structures have been clarified. While many one-electron oxidized CuII-phenolate complexes have been reported previously, recent studies of the Cu-phenolate complexes proceed toward elucidation of the complexes with other oxidation states, such as the phenoxyl radical binding CuI complexes and CuIV-phenolate complexes in the formal oxidation state. This Perspective focuses on new aspects of the properties and reactivities of various Cu-phenolate and Cu-phenoxyl radical complexes with emphasis on the relationship between geometric and electronic structures.

Insight into the Spatial Arrangement of the Lysine Tyrosylquinone and Cu2+ in the Active Site of Lysyl Oxidase-like 2

Lysyl oxidase-2 (LOXL2) is a Cu²⁺ and lysine tyrosylquinone (LTQ)-dependent amine oxidase that catalyzes the oxidative deamination of peptidyl lysine and hydroxylysine residues to promote crosslinking of extracellular matrix proteins. LTQ is post-translationally derived from Lys653 and Tyr689, but its biogenesis mechanism remains still elusive. A 2.4 Å Zn²⁺-bound precursor structure lacking LTQ (PDB:5ZE3) has become available, where Lys653 and Tyr689 are 16.6 Å apart, thus a substantial conformational rearrangement is expected to take place for LTQ biogenesis. However, we have recently shown that the overall structures of the precursor (no LTQ) and the mature (LTQ-containing) LOXL2s are very similar and disulfide bonds are conserved. In this study, we aim to gain insights into the spatial arrangement of LTQ and the active site Cu²⁺ in the mature LOXL2 using a recombinant LOXL2 that is inhibited by 2-hydrazinopyridine (2HP). Comparative UV-vis and resonance Raman spectroscopic studies of the 2HP-inhibited LOXL2 and the corresponding model compounds and an EPR study of the latter support that 2HP-modified LTQ serves as a tridentate ligand to the active site Cu². We propose that LTQ resides within 2.9 Å of the active site of Cu²⁺ in the mature LOXL2, and both LTQ and Cu²⁺ are solvent-exposed.

Non-innocent redox behavior of CuII-p-dimethylaminophenolate complexes: formation and characterization of the CuI-phenoxyl radical species

Cu complexes with p-dimethylaminophenolate ligands were synthesized by the reaction of Cu^II ions with the ligands under inert gas atmosphere and characterized. The complexes showed a valence state change from Cu^II-phenolate to Cu^I-phenoxyl radical on loss of the coordinated solvent. The Cu^I-phenoxyl radical species showed the characteristic properties and reactivities.

Plant Copper Metalloenzymes As Prospects for New Metabolism Involving Aromatic Compounds

Copper is an important transition metal cofactor in plant metabolism, which enables diverse biocatalysis in aerobic environments. Multiple classes of plant metalloenzymes evolved and underwent genetic expansions during the evolution of terrestrial plants and, to date, several representatives of these copper enzyme classes have characterized mechanisms. In this review, we give an updated overview of chemistry, structure, mechanism, function and phylogenetic distribution of plant copper metalloenzymes with an emphasis on biosynthesis of aromatic compounds such as phenylpropanoids (lignin, lignan, flavonoids) and cyclic peptides with macrocyclizations via aromatic amino acids. We also review a recent addition to plant copper enzymology in a copper-dependent peptide cyclase called the BURP domain. Given growing plant genetic resources, a large pool of copper biocatalysts remains to be characterized from plants as plant genomes contain on average more than 70 copper enzyme genes. A major challenge in characterization of copper biocatalysts from plant genomes is the identification of endogenous substrates and catalyzed reactions. We highlight some recent and future trends in filling these knowledge gaps in plant metabolism and the potential for genomic discovery of copper-based enzymology from plants.

Dual Role for Astroglial Copper-Assisted Polyamine Metabolism during Intense Network Activity

Astrocytes serve essential roles in human brain function and diseases. Growing evidence indicates that astrocytes are central players of the feedback modulation of excitatory Glu signalling during epileptiform activity via Glu-GABA exchange. The underlying mechanism results in the increase of tonic inhibition by reverse operation of the astroglial GABA transporter, induced by Glu-Na⁺ symport. GABA, released from astrocytes, is synthesized from the polyamine (PA) putrescine and this process involves copper amino oxidase. Through this pathway, putrescine can be considered as an important source of inhibitory signaling that counterbalances epileptic discharges. Putrescine, however, is also a precursor for spermine that is known to enhance gap junction channel communication and, consequently, supports long-range Ca²⁺ signaling and contributes to spreading of excitatory activity through the astrocytic syncytium. Recently, we presented the possibility of neuron-glia redox coupling through copper (Cu⁺/Cu²⁺) signaling and oxidative putrescine catabolism. In the current work, we explore whether the Cu⁺/Cu²⁺ homeostasis is involved in astrocytic control on neuronal excitability by regulating PA catabolism. We provide supporting experimental data underlying this hypothesis. We show that the blockade of copper transporter (CTR1) by AgNO₃ (3.6 µM) prevents GABA transporter-mediated tonic inhibitory currents, indicating causal relationship between copper (Cu⁺/Cu²⁺) uptake and the catabolism of putrescine to GABA in astrocytes. In addition, we show that MnCl₂ (20 μM), an inhibitor of the divalent metal transporter DMT1, also prevents the astrocytic Glu-GABA exchange. Furthermore, we observed that facilitation of copper uptake by added CuCl₂ (2 µM) boosts tonic inhibitory currents. These findings corroborate the hypothesis that modulation of neuron-glia coupling by copper uptake drives putrescine → GABA transformation, which leads to subsequent Glu-GABA exchange and tonic inhibition. Findings may in turn highlight the potential role of copper signaling in fine-tuning the activity of the tripartite synapse.