Full text loading...
Chapter 15 : The Biosynthesis of the Molybdenum Cofactor and Its Incorporation into Molybdoenzymes
Category: Bacterial Pathogenesis; Microbial Genetics and Molecular Biology
Ebook: Choose a downloadable PDF or ePub file. Chapter is a downloadable PDF file. File must be downloaded within 48 hours of purchase
Molybdenum cofactor (Moco) biosynthesis is an ancient, ubiquitous, and highly conserved pathway leading to the biochemical activation of molybdenum. ModE enhances the transcription of molybdenum-dependent enzymes like DMSO reductase or nitrate reductase A and also of the molybdenum cofactor biosynthesis operon moaABCDE. The biosynthesis of Moco can be divided into four steps in Escherichia coli: (i) formation of precursor Z, (ii) formation of molybdopterin (MPT) from precursor Z, (iii) insertion of molybdenum to form Moco, and (iv) additional modification of Moco with the attachment of GMP, forming the MPT-guanine dinucleotide cofactor (MGD). It was shown that in most E. coli molybdoenzymes the molybdenum atom is coordinated by the dithiolene groups of two MGD molecules, forming the bis-MGD cofactor. It was shown that in E. coli the proteins for the biosynthesis of Moco are located in the cytoplasm and that the insertion of Moco occurs before the translocation of molybdoenzymes either to the membrane or the periplasmic space. The biosynthesis of Moco is conserved in all organisms and genes for Moco biosynthesis are found in bacteria, archaea, fungi, plants, and animals. In humans, possessing solely three molybdoenzymes, a defect in Moco biosynthesis is lethal because of the loss of sulfite oxidase activity. The elucidation of Moco biosynthesis, the analysis of assembly of molybdoenzymes, insertion of Moco into the apoproteins, and translocation of the mature proteins to different compartments are fascinating topics of ongoing research.
Synthesis of precursor Z from GTP. All carbon atoms of the GTP are found within precursor Z. The C8 atom transferred as formyl is inserted between the C2’ and C3’ atoms of the ribose. Precursor Z is shown in the tetrahydropyrano form and in a hydrated product with a geminal diol at the C1’ position.
Active site structures of mononuclear molybdenum enzymes. (A) The three molybdenumcontaining enzyme families are divided into the xanthine oxidase, sulfite oxidase, and DMSO-reductase families according to their active site structures. (B) Representative models of subunit and cofactor composition of molybdenum-containing enzymes, from which the X-ray structures have been solved: bovine milk XDH/XO ( Enroth et al., 2000 ), chicken liver sulfite oxidase ( Kisker et al., 1997b ), and R. sphaeroides DMSO reductase ( Schindelin et al., 1996 ).
Proposed localization of E. coli molybdoenzymes that are part of respiratory systems. FdhGHI is a component of the nitrate respiratory pathway, in which formate oxidation is coupled to nitrate reduction (NarGHI) via lipid-soluble quinone/hydroquinone (Q/QH2).TMAO is reduced to TMA by at least two respiratory systems, TorCAD and DmsABC (also reducing DMSO to dimethyl sulfide [DMS]). NapABCGH is produced under nitrate-limiting conditions and are believed to be involved in redox balancing. The physiological substrates for YedY, a member of the sulfite-oxidase family binding the MPT form of Moco, are not known to date.