Histidine oxidation hypothesis

These homodimeric enzymes, that are present in both prokaryotic and eukaryotic organisms, contain one Cu ion and one redox-active cofactor topaquinone (TPQ) per monomer [5, 6]. They catalyze the oxidative deamination of primary amines [7-9]. The Cu(ll) ion is coordinated by three histidine residues and three water molecules (Fig. 11.1). The TPQ cofactor is not far from the Cu ion. The process can be divided into an initial reductive reaction followed by an oxidative step, based on the redox state of TPQ the Cu ion is thought to be involved in the formation of the TPQ semiquinone through reduction of Cu(II) to Cu(I). An alternative hypothesis has been recently proposed where the copper ion stays as Cu(II) and the one-electron reduction of O2 is carried out by a modified amino-resorcinol TPQ cofactor. The Cu(II) would provide electrostatic stabilization to the superoxide anion intermediate [10-12]. The reduction of molecular oxygen would result in weakly Cu-bormd hydroperoxide which is subsequently displaced by a water molecule, gets protonated and it is eliminated as hydrogen peroxide. 

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