User:Sarah Abdalla/Thioredoxin Reductase: Difference between revisions

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===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 <ref>PMID: 11481439</ref>. 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 <ref>PMID: 17385893</ref>. One of the two cysteine residues in the N-terminal redox center forms a charge transfer complex with the FAD, while the other initiates a thiol-disulfide exchange step resulting in the reduction of the C-terminal disulfide/selenylsulfide of one TR monomer by the N-terminal redox center of the opposite monomer. Once the C-terminus is reduced, the attacking nucleophile initiates attack on the disulfide of thioredoxin. The model below depicts the mechanism of a cysteine TR during catalysis with the thioredoxin substrate. The cysteines of the N-terminal redox center are labeled charge transfer (CT) and interchange (IC). According to Arner et al, the concept of the TR catalytic principle can be rationalized in terms of “yin and yang dualism” (Ref Arner). This view was expressed as both subunits are aligned in a head to tail fashion and are required for electron transfer leading to thioredoxin reduction as shown in the figure below (ying yang depiction). The N-terminal dithiol-containing active site on one subunit in TR reduces the C-terminal selenosulfide on the opposite subunit as shown below. The resulting selenothiol motif may subsequently reduce thioredoxin and several other substrates. The Sec containing motif constitutes the active site of the enzyme and further explains the selenium dependency of TR. Mutation of this residue to a serine or truncation of the region containing the Sec residue leads to the inability of the enzyme to reduce thioredoxin, while substitution of the Sec residue with Cys leads to decreased activity towards substrate reduction. These findings raise questions concerning the role of the Sec residue in the enzyme given that it provides a catalytic advantage over Cys containing enzymes. The current notion behind Sec failing to replace Cys in enzymes is perhaps the cumbersome recoding mechanism used in translating the Sec encoding UGA codon as a sense codon for selenocysteine, which is further complicated by the fact the both eukaryotes and prokaryotes have different UGA recoding mechanisms (ref Hondal and Arner). The intricacy behind this process suggests that Sec plays an important role in the enzyme that a Cys residue is unable to fulfill, therefore a number of rationales have been purported based on the fact that Sec is implicated in what is thought to be the rate determining step of the enzyme. Currently, the conventional wisdom is that Sec exists due to its ability to act as a superior nucleophile based on the data described earlier. At neutral pH, Sec has a lower pKa (5.5) compared to Cys (pKa-8.00), implying that Sec is significantly more nucleophilic, however, since individual rate constants for the TR mechanism have never been measured, experimental evidence supporting this notion of nucleophility is nonexistent. Another school of thought is the better leaving group potential of a selenol (SecH) relative to a thiol (SH) due to the significantly lower pKa (5.5) of Sec, which confers stability upon selenolate (S-) in the absence of a proton which would typically be required as the selenosulfide bond is being broken. Furthermore, the acidity of selenol is much greater than that of a typical thiol due the low pka. This view suggests that the rate limiting step involves the transfer of electrons from the N-terminal redox center to the C-terminal selenosulfide site. This was determined by studying a truncated form of the enzyme missing the last 8 residues and using a synthetic peptide containing the selenosulfide in both the cyclic and linear form as a substrate for the enzyme. Based on these studies, ring geometry was found to be more important in the Cys containing enzymes versus the Sec containing mammalian enzyme. In addition, the interpretation of selenolate not requiring protonation prior to breakage of the selenothol bond was attributed to the fact that Sec was found to be more important in flow of electrons from the N-terminal redox center to the Sec containing motif, therefore Se is needed in the step before nucleophilic attack on thioredoxin. Recently, other views have been proposed with regard to the role of selenium in TR based on the fact that Sec in a selenosulfide bond expresses a much stronger electrophilic character compared to Se in a disulfide bond. [[Image:Ying Yang TR.png|thumb|400px200px|right|BBA 2009 1790]] [[Image:TR mech.png|500px200px|~~center~~|Possible functions of Sec in TR]]

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 <ref>PMID: 11481439</ref>. 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 <ref>PMID: 17385893</ref>. One of the two cysteine residues in the N-terminal redox center forms a charge transfer complex with the FAD, while the other initiates a thiol-disulfide exchange step resulting in the reduction of the C-terminal disulfide/selenylsulfide of one TR monomer by the N-terminal redox center of the opposite monomer. Once the C-terminus is reduced, the attacking nucleophile initiates attack on the disulfide of thioredoxin. The model below depicts the mechanism of a cysteine TR during catalysis with the thioredoxin substrate. The cysteines of the N-terminal redox center are labeled charge transfer (CT) and interchange (IC). According to Arner et al, the concept of the TR catalytic principle can be rationalized in terms of “yin and yang dualism” (Ref Arner). This view was expressed as both subunits are aligned in a head to tail fashion and are required for electron transfer leading to thioredoxin reduction as shown in the figure below (ying yang depiction). The N-terminal dithiol-containing active site on one subunit in TR reduces the C-terminal selenosulfide on the opposite subunit as shown below. The resulting selenothiol motif may subsequently reduce thioredoxin and several other substrates. The Sec containing motif constitutes the active site of the enzyme and further explains the selenium dependency of TR. Mutation of this residue to a serine or truncation of the region containing the Sec residue leads to the inability of the enzyme to reduce thioredoxin, while substitution of the Sec residue with Cys leads to decreased activity towards substrate reduction. These findings raise questions concerning the role of the Sec residue in the enzyme given that it provides a catalytic advantage over Cys containing enzymes. The current notion behind Sec failing to replace Cys in enzymes is perhaps the cumbersome recoding mechanism used in translating the Sec encoding UGA codon as a sense codon for selenocysteine, which is further complicated by the fact the both eukaryotes and prokaryotes have different UGA recoding mechanisms (ref Hondal and Arner). The intricacy behind this process suggests that Sec plays an important role in the enzyme that a Cys residue is unable to fulfill, therefore a number of rationales have been purported based on the fact that Sec is implicated in what is thought to be the rate determining step of the enzyme. Currently, the conventional wisdom is that Sec exists due to its ability to act as a superior nucleophile based on the data described earlier. At neutral pH, Sec has a lower pKa (5.5) compared to Cys (pKa-8.00), implying that Sec is significantly more nucleophilic, however, since individual rate constants for the TR mechanism have never been measured, experimental evidence supporting this notion of nucleophility is nonexistent. Another school of thought is the better leaving group potential of a selenol (SecH) relative to a thiol (SH) due to the significantly lower pKa (5.5) of Sec, which confers stability upon selenolate (S-) in the absence of a proton which would typically be required as the selenosulfide bond is being broken. Furthermore, the acidity of selenol is much greater than that of a typical thiol due the low pka. This view suggests that the rate limiting step involves the transfer of electrons from the N-terminal redox center to the C-terminal selenosulfide site. This was determined by studying a truncated form of the enzyme missing the last 8 residues and using a synthetic peptide containing the selenosulfide in both the cyclic and linear form as a substrate for the enzyme. Based on these studies, ring geometry was found to be more important in the Cys containing enzymes versus the Sec containing mammalian enzyme. In addition, the interpretation of selenolate not requiring protonation prior to breakage of the selenothol bond was attributed to the fact that Sec was found to be more important in flow of electrons from the N-terminal redox center to the Sec containing motif, therefore Se is needed in the step before nucleophilic attack on thioredoxin. Recently, other views have been proposed with regard to the role of selenium in TR based on the fact that Sec in a selenosulfide bond expresses a much stronger electrophilic character compared to Se in a disulfide bond. [[Image:Ying Yang TR.png|thumb|400px200px|right|BBA 2009 1790]] [[Image:TR mech.png|500px200px|right|Possible functions of Sec in TR]]

Revision as of 02:23, 26 April 2010 view source Sarah Abdalla (talk \| contribs) 162 edits No edit summary ← Older edit		Revision as of 02:25, 26 April 2010 view source Sarah Abdalla (talk \| contribs) 162 edits No edit summary Newer edit →
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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 <ref>PMID: 11481439</ref>. 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 <ref>PMID: 17385893</ref>. One of the two cysteine residues in the N-terminal redox center forms a charge transfer complex with the FAD, while the other initiates a thiol-disulfide exchange step resulting in the reduction of the C-terminal disulfide/selenylsulfide of one TR monomer by the N-terminal redox center of the opposite monomer. Once the C-terminus is reduced, the attacking nucleophile initiates attack on the disulfide of thioredoxin. The model below depicts the mechanism of a cysteine TR during catalysis with the thioredoxin substrate. The cysteines of the N-terminal redox center are labeled charge transfer (CT) and interchange (IC). According to Arner et al, the concept of the TR catalytic principle can be rationalized in terms of “yin and yang dualism” (Ref Arner). This view was expressed as both subunits are aligned in a head to tail fashion and are required for electron transfer leading to thioredoxin reduction as shown in the figure below (ying yang depiction). The N-terminal dithiol-containing active site on one subunit in TR reduces the C-terminal selenosulfide on the opposite subunit as shown below. The resulting selenothiol motif may subsequently reduce thioredoxin and several other substrates. The Sec containing motif constitutes the active site of the enzyme and further explains the selenium dependency of TR. Mutation of this residue to a serine or truncation of the region containing the Sec residue leads to the inability of the enzyme to reduce thioredoxin, while substitution of the Sec residue with Cys leads to decreased activity towards substrate reduction. These findings raise questions concerning the role of the Sec residue in the enzyme given that it provides a catalytic advantage over Cys containing enzymes. The current notion behind Sec failing to replace Cys in enzymes is perhaps the cumbersome recoding mechanism used in translating the Sec encoding UGA codon as a sense codon for selenocysteine, which is further complicated by the fact the both eukaryotes and prokaryotes have different UGA recoding mechanisms (ref Hondal and Arner). The intricacy behind this process suggests that Sec plays an important role in the enzyme that a Cys residue is unable to fulfill, therefore a number of rationales have been purported based on the fact that Sec is implicated in what is thought to be the rate determining step of the enzyme. Currently, the conventional wisdom is that Sec exists due to its ability to act as a superior nucleophile based on the data described earlier. At neutral pH, Sec has a lower pKa (5.5) compared to Cys (pKa-8.00), implying that Sec is significantly more nucleophilic, however, since individual rate constants for the TR mechanism have never been measured, experimental evidence supporting this notion of nucleophility is nonexistent. Another school of thought is the better leaving group potential of a selenol (SecH) relative to a thiol (SH) due to the significantly lower pKa (5.5) of Sec, which confers stability upon selenolate (S-) in the absence of a proton which would typically be required as the selenosulfide bond is being broken. Furthermore, the acidity of selenol is much greater than that of a typical thiol due the low pka. This view suggests that the rate limiting step involves the transfer of electrons from the N-terminal redox center to the C-terminal selenosulfide site. This was determined by studying a truncated form of the enzyme missing the last 8 residues and using a synthetic peptide containing the selenosulfide in both the cyclic and linear form as a substrate for the enzyme. Based on these studies, ring geometry was found to be more important in the Cys containing enzymes versus the Sec containing mammalian enzyme. In addition, the interpretation of selenolate not requiring protonation prior to breakage of the selenothol bond was attributed to the fact that Sec was found to be more important in flow of electrons from the N-terminal redox center to the Sec containing motif, therefore Se is needed in the step before nucleophilic attack on thioredoxin. Recently, other views have been proposed with regard to the role of selenium in TR based on the fact that Sec in a selenosulfide bond expresses a much stronger electrophilic character compared to Se in a disulfide bond. [[Image:Ying Yang TR.png\|thumb\|400px200px\|right\|BBA 2009 1790]] [[Image:TR mech.png\|500px200px\|~~center~~\|Possible functions of Sec in TR]]		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 <ref>PMID: 11481439</ref>. 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 <ref>PMID: 17385893</ref>. One of the two cysteine residues in the N-terminal redox center forms a charge transfer complex with the FAD, while the other initiates a thiol-disulfide exchange step resulting in the reduction of the C-terminal disulfide/selenylsulfide of one TR monomer by the N-terminal redox center of the opposite monomer. Once the C-terminus is reduced, the attacking nucleophile initiates attack on the disulfide of thioredoxin. The model below depicts the mechanism of a cysteine TR during catalysis with the thioredoxin substrate. The cysteines of the N-terminal redox center are labeled charge transfer (CT) and interchange (IC). According to Arner et al, the concept of the TR catalytic principle can be rationalized in terms of “yin and yang dualism” (Ref Arner). This view was expressed as both subunits are aligned in a head to tail fashion and are required for electron transfer leading to thioredoxin reduction as shown in the figure below (ying yang depiction). The N-terminal dithiol-containing active site on one subunit in TR reduces the C-terminal selenosulfide on the opposite subunit as shown below. The resulting selenothiol motif may subsequently reduce thioredoxin and several other substrates. The Sec containing motif constitutes the active site of the enzyme and further explains the selenium dependency of TR. Mutation of this residue to a serine or truncation of the region containing the Sec residue leads to the inability of the enzyme to reduce thioredoxin, while substitution of the Sec residue with Cys leads to decreased activity towards substrate reduction. These findings raise questions concerning the role of the Sec residue in the enzyme given that it provides a catalytic advantage over Cys containing enzymes. The current notion behind Sec failing to replace Cys in enzymes is perhaps the cumbersome recoding mechanism used in translating the Sec encoding UGA codon as a sense codon for selenocysteine, which is further complicated by the fact the both eukaryotes and prokaryotes have different UGA recoding mechanisms (ref Hondal and Arner). The intricacy behind this process suggests that Sec plays an important role in the enzyme that a Cys residue is unable to fulfill, therefore a number of rationales have been purported based on the fact that Sec is implicated in what is thought to be the rate determining step of the enzyme. Currently, the conventional wisdom is that Sec exists due to its ability to act as a superior nucleophile based on the data described earlier. At neutral pH, Sec has a lower pKa (5.5) compared to Cys (pKa-8.00), implying that Sec is significantly more nucleophilic, however, since individual rate constants for the TR mechanism have never been measured, experimental evidence supporting this notion of nucleophility is nonexistent. Another school of thought is the better leaving group potential of a selenol (SecH) relative to a thiol (SH) due to the significantly lower pKa (5.5) of Sec, which confers stability upon selenolate (S-) in the absence of a proton which would typically be required as the selenosulfide bond is being broken. Furthermore, the acidity of selenol is much greater than that of a typical thiol due the low pka. This view suggests that the rate limiting step involves the transfer of electrons from the N-terminal redox center to the C-terminal selenosulfide site. This was determined by studying a truncated form of the enzyme missing the last 8 residues and using a synthetic peptide containing the selenosulfide in both the cyclic and linear form as a substrate for the enzyme. Based on these studies, ring geometry was found to be more important in the Cys containing enzymes versus the Sec containing mammalian enzyme. In addition, the interpretation of selenolate not requiring protonation prior to breakage of the selenothol bond was attributed to the fact that Sec was found to be more important in flow of electrons from the N-terminal redox center to the Sec containing motif, therefore Se is needed in the step before nucleophilic attack on thioredoxin. Recently, other views have been proposed with regard to the role of selenium in TR based on the fact that Sec in a selenosulfide bond expresses a much stronger electrophilic character compared to Se in a disulfide bond. [[Image:Ying Yang TR.png\|thumb\|400px200px\|right\|BBA 2009 1790]] [[Image:TR mech.png\|500px200px\|right\|Possible functions of Sec in TR]]