User:Sarah Abdalla/Thioredoxin Reductase: Difference between revisions
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{{STRUCTURE_1h6v|PDB=1h6v|SCENE=}} | {{STRUCTURE_1h6v|PDB=1h6v|SCENE=}} | ||
== '''Thioredoxin Reductase''' == | == '''Thioredoxin Reductase''' == | ||
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===Function and Mechanism=== | ===Function and Mechanism=== | ||
High molecular weight TRs catalyze the reduction of the redox active disulfide of thioredoxin, the enzyme’s cognate substrate. Together with thioredoxin and NADPH, TR forms the thioredoxin system which plays a major role in maintaining a reducing environment within cells. Studies on thioredoxin have provided a vast amount of information on the function and mechanism of TR. Although the enzyme reduces disulfide containing substrates, it has a broad substrate spectrum and also targets other nondisulfide substrates such hydrogen peroxide and selenite. The general mechanism of the enzyme is initiated upon transfer of electrons from NADPH via a bound FAD to the N-terminal redox active site. A second thiol-disulfide exchange step occurs resulting in the reduction of the C-terminal disulfide by the N-terminal redox center. Once reduced, the attacking nucleophile initiates attack on the disulfide of thioredoxin. | High molecular weight TRs catalyze the reduction of the redox active disulfide of thioredoxin, the enzyme’s cognate substrate. Together with thioredoxin and NADPH, TR forms the thioredoxin system which plays a major role in maintaining a reducing environment within cells. Studies on thioredoxin have provided a vast amount of information on the function and mechanism of TR. Although the enzyme reduces disulfide containing substrates, it has a broad substrate spectrum and also targets other nondisulfide substrates such hydrogen peroxide and selenite. The general mechanism of the enzyme is initiated upon transfer of electrons from NADPH via a bound FAD to the N-terminal redox active site. A second thiol-disulfide exchange step occurs resulting in the reduction of the C-terminal disulfide by the N-terminal redox center. Once reduced, the attacking nucleophile initiates attack on the disulfide of thioredoxin. [[Image:Thioredoxin Reductase.JPG | thumb]] | ||
===Links=== | ===Links=== | ||
Revision as of 21:54, 18 April 2009
Thioredoxin ReductaseThioredoxin Reductase
Thioredoxin reductase (TR) is a 55 kDa enzyme that belongs to the family of pyridine nucleotide disulfide oxidoreductases [1]. Also included in this family are lipoamide dehydrogenases and glutathione reductases, with which TRs share high homology. TR is ubiquitous from humans to archaea, however TR from higher eukaryotes is distinct from its prokaryotic counterpart and is thought to have evolved from gluthathione reductases due to similarities in the catalytic activity of both enzymes [2]. Compared with gluthathione reductases, TRs are unique in that they contain a 16 amino acid C-terminal extension which mediates the catalytic activity of the enzyme [3]. Mammalian TRs fall into the class of selenium containing enzymes due to the presence of its penultimate selenocysteine residue that has been shown to be essential for reduction of its cognate substrate, thioredoxin. The C-terminal redox center (which contains the selenocysteine residue) is notable because a number of high molecular weight TRs contain cysteine in place of at this site [4]. TR from Drosophila melanogaster falls under this category and has a vicinal cysteine dyad in the redox center. Mammalian TR is known for having decreased catalytic activity upon replacement of the selenocysteine residue with cysteine [5]. This aspect together with other findings not mentioned here suggests that selenocysteine plays a special role in the enzyme. For this reason, the role of selenocysteine in TR is controversial considering that cysteine is the functional residue in other forms of the enzyme.
StructureStructure
The functional unit of TR is a , typical of proteins in the family of glutathione reductases, with each subunit composed of mainly (yellow) and (blue). Each monomer is exhibits a three domain modular architecture, containing a NADP binding domain, a N-terminal FAD binding domain, and an interface domain. Both the binding domains have similar folds, and are variants of the Rossman fold, characterized by a β sheet linked by several alpha helices which in the enzyme is composed of 5 strands surrounded by helices. The two domains are positioned in a head to tail orientation allowing for electron transfer that leads to the reduction of the enzyme’s redox active center. The of the enzyme is located at the interface domain formed by two subunits, deeming the physiological significance of the dimeric form of the enzyme. This domain is composed of a five stranded β sheet flanked on either side by two helices. The C- terminal extension of TR runs parallel to the edge of the β sheet strand at the interface domain, with the last residues of the extension forming an arm that protrudes into the interface domain allowing for interaction with groups at the active site interface which is located at the N-terminus [6].
Function and MechanismFunction and Mechanism
High molecular weight TRs catalyze the reduction of the redox active disulfide of thioredoxin, the enzyme’s cognate substrate. Together with thioredoxin and NADPH, TR forms the thioredoxin system which plays a major role in maintaining a reducing environment within cells. Studies on thioredoxin have provided a vast amount of information on the function and mechanism of TR. Although the enzyme reduces disulfide containing substrates, it has a broad substrate spectrum and also targets other nondisulfide substrates such hydrogen peroxide and selenite. The general mechanism of the enzyme is initiated upon transfer of electrons from NADPH via a bound FAD to the N-terminal redox active site. A second thiol-disulfide exchange step occurs resulting in the reduction of the C-terminal disulfide by the N-terminal redox center. Once reduced, the attacking nucleophile initiates attack on the disulfide of thioredoxin.
LinksLinks
ReferencesReferences
- ↑ Sandalova T, Zhong L, Lindqvist Y, Holmgren A, Schneider G. Three-dimensional structure of a mammalian thioredoxin reductase: implications for mechanism and evolution of a selenocysteine-dependent enzyme. Proc Natl Acad Sci U S A. 2001 Aug 14;98(17):9533-8. Epub 2001 Jul 31. PMID:11481439 doi:10.1073/pnas.171178698
- ↑ Zhong L, Arner ES, Holmgren A. Structure and mechanism of mammalian thioredoxin reductase: the active site is a redox-active selenolthiol/selenenylsulfide formed from the conserved cysteine-selenocysteine sequence. Proc Natl Acad Sci U S A. 2000 May 23;97(11):5854-9. PMID:10801974 doi:10.1073/pnas.100114897
- ↑ Eckenroth BE, Lacey BM, Lothrop AP, Harris KM, Hondal RJ. Investigation of the C-terminal redox center of high-Mr thioredoxin reductase by protein engineering and semisynthesis. Biochemistry. 2007 Aug 21;46(33):9472-83. Epub 2007 Jul 28. PMID:17661444 doi:10.1021/bi7004812
- ↑ Sandalova T, Zhong L, Lindqvist Y, Holmgren A, Schneider G. Three-dimensional structure of a mammalian thioredoxin reductase: implications for mechanism and evolution of a selenocysteine-dependent enzyme. Proc Natl Acad Sci U S A. 2001 Aug 14;98(17):9533-8. Epub 2001 Jul 31. PMID:11481439 doi:10.1073/pnas.171178698
- ↑ Eckenroth BE, Rould MA, Hondal RJ, Everse SJ. Structural and biochemical studies reveal differences in the catalytic mechanisms of mammalian and Drosophila melanogaster thioredoxin reductases. Biochemistry. 2007 Apr 24;46(16):4694-705. Epub 2007 Mar 27. PMID:17385893 doi:10.1021/bi602394p
- ↑ Sandalova T, Zhong L, Lindqvist Y, Holmgren A, Schneider G. Three-dimensional structure of a mammalian thioredoxin reductase: implications for mechanism and evolution of a selenocysteine-dependent enzyme. Proc Natl Acad Sci U S A. 2001 Aug 14;98(17):9533-8. Epub 2001 Jul 31. PMID:11481439 doi:10.1073/pnas.171178698