Date Published: December 1, 2008
Publisher: International Union of Crystallography
Author(s): Pascale Pirrat, Mark A. Smith, Arwen R. Pearson, Michael J. McPherson, Simon E. V. Phillips.
The structure of a xenon derivative of E. coli copper amine oxidase confirms the pathway of oxygen entry to the buried active site proposed for this class of enzymes.
Copper amine oxidases (CuAOs) catalyse the oxidation of a wide range of amine substrates to generate a product aldehyde, ammonia and hydrogen peroxide (Mure et al., 2002 ▶). They are ubiquitous in nature and in humans are involved in diverse processes including cell adhesion and signalling (Salmi et al., 1998 ▶; Elmore et al., 2002 ▶; Yu et al., 2003 ▶). In bacteria, they play a role in general nitrogen catabolism (Parrott et al., 1987 ▶; Hacisalihoglu et al., 1997 ▶). They contain a protein-derived cofactor that is generated autocatalytically by post-translation modification of a conserved endogenous tyrosine to generate 2,4,5-trihydroxylphenylalanine quinone (TPQ), requiring only the presence of copper and molecular oxygen (Cai & Klinman, 1994 ▶; Kim et al., 2002 ▶). TPQ is situated in the core of the protein at the end of a well defined substrate-access channel next to a type II copper coordinated by three histidines (Fig. 1 ▶; Parsons et al., 1995 ▶; Duff et al., 2006 ▶; Wilce et al., 1997 ▶; Lunelli et al., 2005 ▶; Kumar et al., 1996 ▶; Airenne et al., 2005 ▶; Li et al., 1998 ▶). The Escherichia coli enzyme (ECAO) shows a preference for small aromatic amine substrates such as phenethylamine and tyramine (Roh et al., 1994 ▶). The catalytic reaction proceeds via two half-reactions; the aldehyde product is released at the end of the reductive half-reaction before reduction of molecular oxygen in the oxidative half-reaction (Fig. 2 ▶).
After refinement, 11 xenon-binding sites were found in the structure (Fig. 4 ▶a). Although the occupancy of all sites was low (0.1–0.45), this is consistent with the xenon occupancies observed in other CuAO xenon complexes (Lunelli et al., 2005 ▶; Johnson et al., 2007 ▶; Duff et al., 2004 ▶). Xenons were numbered according to their occupancy (from high to low occupancy). The Xe atoms were bound in hydrophobic pockets located in either the amine substrate-entry channel or around the conserved β-sandwich oxygen-entry pathway (Table 3 ▶). Nearly identical sites were observed in both subunits of ECAO present in the asymmetric unit1.