The Biosynthesis of the Molybdenum Cofactor inEscherichia coliand Its Connection to FeS Cluster Assembly and the Thiolation of tRNA

Silke Leimkühler

doi:10.1155/2014/808569

Abstract

The thiolation of biomolecules is a complex process that involves the activation of sulfur. The L-cysteine desulfurase IscS is the main sulfur mobilizing protein inEscherichia colithat provides the sulfur from L-cysteine to several important biomolecules in the cell such as iron sulfur (FeS) clusters, molybdopterin (MPT), thiamine, and thionucleosides of tRNA. Various proteins mediate the transfer of sulfur from IscS to various biomolecules using different interaction partners. A direct connection between the sulfur-containing molecules FeS clusters, thiolated tRNA, and the molybdenum cofactor (Moco) has been identified. The first step of Moco biosynthesis involves the conversion of 5′GTP to cyclic pyranopterin monophosphate (cPMP), a reaction catalyzed by a FeS cluster containing protein. Formed cPMP is further converted to MPT by insertion of two sulfur atoms. The sulfur for this reaction is provided by the L-cysteine desulfurase IscS in addition to the involvement of the TusA protein. TusA is also involved in the sulfur transfer for the thiolation of tRNA. This review will describe the biosynthesis of Moco inE. coliin detail and dissects the sulfur transfer pathways for Moco and tRNA and their connection to FeS cluster biosynthesis.

Highlights

Molybdenum is the only second row transition metal essential for biological systems, which is biologically available as molybdate ion [1]
The different enzymes contain different forms of molybdenum cofactor (Moco) and were historically categorized into three families based on the ligands at the molybdenum atom, which are characteristic for each family (Figure 1): the xanthine oxidase (XO) family, the sulfite oxidase (SO) family, and the dimethyl sulfoxide (DMSO) reductase family [3, 7]
The XO family is characterized by an MPT-MoVIOS(OH) core in the oxidized state, with one MPT equivalent coordinated to the molybdenum atom, one oxo-group, one sulfido-group, and one hydroxogroup [3] (Figure 1)

Summary

H OH OH H

TMAO reductase A (TorA) TMAO reductase Z (TorZ) DMSO reductase (DmsABC) Nitrate reductase A (NarGHI) Nitrate reductase Z (NarZYV) Peripl. MoaA moaB moaC moaD moaE moa mobB mobA mob mocA moc moeB moeA moe mogA mog 1000 bp Figure 3: Organization of the genes involved in the biosynthesis of Moco in E. coli. During the reaction of the first sulfur transfer, the opening of the cyclic phosphate is proposed to shift the location of the intermediate within the protein so that the C1󸀠 position becomes more accessible to the attack by the second MoaD thiocarboxylate (Figure 4) This results in a second covalent intermediate that is converted to MPT via the elimination of a water molecule and hydrolysis of the thioester intermediate. It was shown that under physiological molybdate concentrations (1–10 μM), MogA is required in E. coli to form an MPT-AMP intermediate that facilitates molybdate insertion on the dithiolene sulfurs This reaction is not absolutely required under high molybdenum concentrations in vivo, since mogA− cells were rescued for molybdoenzyme activities by the addition of high molybdate concentrations to the medium [68]. After its formation, the Mo-MPT cofactor can be directly inserted into enzymes from the sulfite oxidase family without further modification [68], like into the E. coli YedY protein (Figure 1) [70]

Further Modification of Moco

C SH HS C

Mo-MPT MobA

The Distribution of Sulfur for SulfurContaining Biomolecules in Bacteria

Findings

CHNH2 C