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Molybdenum: MPT-Containing Enzymes

  1. Charles G. Young

Published Online: 15 DEC 2011

DOI: 10.1002/9781119951438.eibc0136

Encyclopedia of Inorganic and Bioinorganic Chemistry

Encyclopedia of Inorganic and Bioinorganic Chemistry

How to Cite

Young, C. G. 2011. Molybdenum: MPT-Containing Enzymes. Encyclopedia of Inorganic and Bioinorganic Chemistry. .

Author Information

  1. University of Melbourne, Australia

Publication History

  1. Published Online: 15 DEC 2011

Abstract

Molybdenum, an essential trace element for all forms of life, is found in a wide range of Mo-MPT enzymes important in the metabolism of small molecules and anions of nitrogen, sulfur, carbon, chlorine, selenium, and arsenic, and in the global biogeochemical cycles of these elements. These enzymes feature active sites composed of a single (mononuclear) Mo atom coordinated by one or two MPT-based ligands, L (MPT = ‘molybdopterin′ or Metal-binding Pterin ene-1,2-diThiolate). These ligands contain a pyranopterin or simple pterin nucleus, an ene-1,2-dithiolate (dithiolene) unit, and a pendant phosphate group; in enzymes from prokaryotes, the phosphate is generally attached to a nucleotide, giving ‘dinucleotide’ forms of MPT. The dithiolene moiety functions as a bidentate S2-donor ligand to Mo. The Mo-MPT enzymes are classified into two broad functional types: oxotransferases and hydroxylases. The oxotransferases generally catalyze oxygen atom transfer reactions and fall into two families: the sulfite oxidase and dimethyl sulfoxide (DMSO) reductase families. Enzymes from these families contain square-pyramidal cis-[MoVIO2L(cysteinate)] and distorted trigonal prismatic [MoVIO(AA)L2] (AA = serinate, cysteinate, selenocysteinate, etc.) oxidized active sites, respectively. The hydroxylases all belong to the xanthine oxidase family and generally catalyze net insertion of oxygen into the C[BOND]H bonds of heterocyclic aromatic compounds and aldehydes. They contain biologically unique, square-pyramidal [MoVIOS(OH)L] oxidized active sites featuring a catalytically essential terminal sulfido ligand. The sulfido ligand can act in a bridging capacity to Cu, as recently demonstrated for CO dehydrogenase. In most reactions catalyzed by Mo-MPT enzymes, an oxygen atom is exchanged between substrate and water with concomitant consumption (for reductases) or production (for oxidases) of reducing equivalents (2e, 2H+). Synthetic analogue (model) studies have produced complexes replicating or informing the composition, structures, spectroscopy, and chemistry of the enzymes. This account focuses on the Mo bioinorganic chemistry of the Mo-MPT enzymes and selected synthetic analogues. It is divided into two main parts. Part 1 provides an overview of the enzyme systems, their distribution, roles, and essential chemistry, as well as a discussion of consensus structures and likely mechanisms. The genes and proteins involved in Mo trafficking and Mo-cofactor biosynthesis are also briefly described. Part 2 summarizes important developments in the production of synthetic analogues, with a focus on new dithiolene, scorpionate, and other complexes sharing the structural, spectroscopic, or chemical attributes of the enzymes.

Keywords:

  • molybdenum enzymes;
  • oxygen atom transfer (OAT);
  • coupled electron-proton transfer (CEPT);
  • pterin-dithiolene ligands;
  • sulfite oxidase;
  • nitrate reductase;
  • DMSO reductase;
  • xanthine oxidase/dehydrogenase;
  • dithiolene complexes;
  • trispyrazolylborate complexes