Structure, function, and magnetism of metalloenzymes involved in the biological cycles of nitrogen and sulfur

CARLOS D. BRONDINO

Chascomús

Encuentro; Fourth Latin American Meeting on Biological Inorganic Chemistry (LABIC); 2014

INQUIMAE-Dpto de Química, FCEN, UBA

Structure, function, and magnetism of metalloenzymes involved in the biological cycles of nitrogen and sulfur Carlos D. Brondino Departamento de Física, Facultad de Bioquímica y Ciencias Biológicas, Universidad Nacional del Litoral, Campus Universitario, S3000ZAA Santa Fe, Argentina e-mail: brondino@fbcb.unl.edu.ar   The biological cycles of nitrogen and sulfur sustained by bacteria involve a group of redox reactions catalyzed by distinct proteins that contain in their structure metal or organic cofactors that are essential for catalysis. Most enzymes show a general reaction mechanism that implies the presence of an external electron donor/acceptor which transfers/receives electrons to/from the active site by means of an electron transfer reaction that can involve additional redox cofactors, which is exemplified in the figure (left) for mononuclear Mo-containing enzymes.1 It is postulated that this electron transfer process is through a chemical pathway in which the distinct redox cofactors are bridged by covalent and noncovalent bonds such as hydrogen bridge bonds and hydrophobic interactions. The redox centers may be paramagnetic in certain redox states of the protein and despite both the long intercenter distances and chemical pathway, they may present weak magnetic couplings produced by spin-spin interactions such dipole-dipole interaction and isotropic exchange, also known as superexchange interaction.2 The structural, kinetic, and spectroscopic characterization of these enzymes along with the study of the intercenter magnetic interactions give information on aspects such as the enzyme-substrate interaction mechanism, electron transfer processes, and conformational protein changes during the catalysis.               Aldehyde oxidoreductase from Desulfovibrio gigas (DgAOR) is a member of the xanthine oxidase (XO) family of mononuclear Mo-containing enzymes that catalyzes the oxidation of aldehydes to carboxylic acids.3 This protein is a homodimeric protein that contains besides the Mo active site two iron-sulfur clusters called FeS 1 and FeS 2 (see figure, right). The molybdenum site in the enzymes of the XO family shows a distorted square pyramidal geometry in which two ligands, a hydroxyl/water molecule (the catalytic labile site), and a sulfido ligand, have been shown to be essential for catalysis. In this talk we will discuss structural, kinetic, and EPR studies that led us to understand distinct aspects of DgAOR such us the EPR assignment the FeS clusters with those of the structure, the lack of the essential sulfido ligand in this enzyme, and the unusual reversible behavior towards irreversible inhibitors of most XO family members. 1. C. D. Brondino et al Curr. Op. Chem. Biol. 2006, 10, 109-114. 2. N. I. Neuman et al J. Phys. Chem. A 2012, 116, 12314-12320. 3. T. Santos Silva et al J. Am. Chem. Soc. 2009, 131, 7990-7998.