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Mutational Analysis and Redesign of Alpha-class Glutathione Transferases for Enhanced Azathioprine Activity
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - BMC, Biochemistry.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Glutathione transferase (GST) A2-2 is the human enzyme most efficient in catalyzing azathioprine activation. Structure-function relationships were sought explaining the higher catalytic efficiency compared to other alpha class GSTs. By screening a DNA shuffling library, five recombined segments were identified that were conserved among the most active mutants. Mutational analysis confirmed the importance of these short segments as their insertion into low-active GSTs introduced higher azathioprine activity. Besides, H-site mutagenesis led to decreased azathioprine activity when the targeted positions belonged to these conserved segments and mainly enhanced activity when other positions were targeted. Hydrophobic residues were preferred in positions 208 and 213.

The prodrug azathioprine is today primarily used for maintaining remission in inflammatory bowel disease. Therapy leads to adverse effects for 30 % of the patients and genotyping of the metabolic genes involved can explain some of these incidences. Five genotypes of human A2-2 were characterized and variant A2*E had 3–4-fold higher catalytic efficiency with azathioprine, due to a proline mutated close to the H-site. Faster activation might lead to different metabolite distributions and possibly more adverse effects. Genotyping of GSTs is recommended for further studies.

Molecular docking of azathioprine into a modeled structure of A2*E suggested three positions for mutagenesis. The most active mutants had small or polar residues in the mutated positions. Mutant L107G/L108D/F222H displayed a 70-fold improved catalytic efficiency with azathioprine. Determination of its structure by X-ray crystallography showed a widened H-site, suggesting that the transition state could be accommodated in a mode better suited for catalysis.

The mutational analysis increased our understanding of the azathioprine activation in alpha class GSTs and highlighted A2*E as one factor possibly behind the adverse drug-effects. A successfully redesigned GST, with 200-fold enhanced catalytic efficiency towards azathioprine compared to the starting point A2*C, might find use in targeted enzyme-prodrug therapies.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. , 72 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1050
Keyword [en]
allelic variants, azathioprine, bioactivation, chimeric mutagenesis, directed evolution, DNA shuffling, enzyme engineering, glutathione transferase, GST, lysate screening, molecular docking, multiple alignment, multivariate analysis, polymorphism, principal component analysis, prodrug, prodrug activation, protein engineering, protein redesign, reduced amino acid alphabet, saturation mutagenesis, semi-rational enzyme engineering, site-directed mutagenesis, structure-activity relationship, structure-based redesign
National Category
Biochemistry and Molecular Biology Biocatalysis and Enzyme Technology
Research subject
Biochemistry
Identifiers
URN: urn:nbn:se:uu:diva-167332ISBN: 978-91-554-8685-3 (print)OAI: oai:DiVA.org:uu-167332DiVA: diva2:619112
Public defence
2013-06-05, B42, Biomedical Center (BMC), Husargatan 3, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2013-05-15 Created: 2012-01-25 Last updated: 2013-08-30Bibliographically approved
List of papers
1. Structural determinants of glutathione transferases with azathioprine activity identified by DNA shuffling of alpha class members
Open this publication in new window or tab >>Structural determinants of glutathione transferases with azathioprine activity identified by DNA shuffling of alpha class members
2008 (English)In: Journal of Molecular Biology, ISSN 0022-2836, E-ISSN 1089-8638, Vol. 375, no 5, 1365-1379 p.Article in journal (Refereed) Published
Abstract [en]

A library of alpha class glutathione transferases (GSTs), composed of chimeric enzymes derived from human (A1-1, A2-2 and A3-3), bovine (A1-1) and rat (A2-2 and A3-3) cDNA sequences was constructed by the method of DNA shuffling. The GST variants were screened in bacterial lysates for activity with the immunosuppressive agent azathioprine, a prodrug that is transformed into its active form, 6-mercaptopurine, by reaction with the tripeptide glutathione catalyzed by GSTs. Important structural determinants for activity with azathioprine were recognized by means of primary structure analysis and activities of purified enzymes chosen from the screening. The amino acid sequences could be divided into 23 exchangeable segments on the basis of the primary structures of 45 chosen clones. Segments 2, 20, 21, and 22 were identified as primary determinants of the azathioprine activity representing two of the regions forming the substrate-binding H-site. Segments 21 and 22 are situated in the C-terminal helix characterizing alpha class GSTs, which is instrumental in their catalytic function. The study demonstrates the power of DNA shuffling in identifying segments of primary structure that are important for catalytic activity with a targeted substrate. GSTs in combination with azathioprine have potential as selectable markers for use in gene therapy. Knowledge of activity-determining segments in the structure is valuable in the protein engineering of glutathione transferase for enhanced or suppressed activity.

Keyword
Azathioprine, DNA shuffling, glutathione transferase, protein modification, rational redesign, directed evolution, drug resistance, prodrug, SAR
National Category
Biochemistry and Molecular Biology Biocatalysis and Enzyme Technology
Identifiers
urn:nbn:se:uu:diva-97180 (URN)10.1016/j.jmb.2007.11.034 (DOI)000253098100016 ()18155239 (PubMedID)
Available from: 2008-04-29 Created: 2008-04-29 Last updated: 2017-12-14Bibliographically approved
2. Differences among allelic variants of human glutathione transferase A2-2 in the activation of azathioprine
Open this publication in new window or tab >>Differences among allelic variants of human glutathione transferase A2-2 in the activation of azathioprine
2010 (English)In: Chemico-Biological Interactions, ISSN 0009-2797, E-ISSN 1872-7786, Vol. 186, no 2, 110-117 p.Article in journal (Refereed) Published
Abstract [en]

Azathioprine has been clinically used for decades in connection with organ transplantation, autoimmune disease, and treatment of cancer. Toxic side-reactions are common and have been linked to the liberation of excessively high concentrations of 6-mercaptopurine and corresponding toxic metabolites. An allelic variant of thiopurine methyltransferase with low activity is associated with elevated concentrations of 6-mercaptopurine. However, other genetic markers remain to be identified in order to fully account for adverse reactions and efficacy failure. In the present study, we studied the five known allelic variants of human glutathione transferase A2-2 (GST A2-2) (EC2.5.1.18), abundantly expressed in liver and efficiently catalyzing the bioactivation of azathioprine to release 6-mercaptopurine. All five variants exhibited high activity with azathioprine, but allelic variant E of GST A2-2 displayed a 3-4-fold elevated catalytic efficiency compared to the other variants. High GST activity can lead to overproduction of 6-mercaptopurine, and the nature of the multiple forms of GSTs in a patient will obviously affect the metabolism of azathioprine. In addition to GST A2-2, the polymorphic GST M1-1 is also highly active with azathioprine. Considering our findings, it appears that the genotypic and phenotypic variations in the GST complement may influence the presentation of adverse reactions in patients treated with azathioprine. Clinical trials will be required to clarify the impact of the GST expression in comparison with the established biomarker thiopurine methyltransferase as predictors of adverse reactions.

Keyword
Azathioprine, Glutathione transferase, Bioactivation, Thiopurine methyltransferase, Allelic variants, Polymorphism
National Category
Biochemistry and Molecular Biology Biocatalysis and Enzyme Technology
Identifiers
urn:nbn:se:uu:diva-135839 (URN)10.1016/j.cbi.2010.04.028 (DOI)000279644900002 ()
Available from: 2010-12-08 Created: 2010-12-08 Last updated: 2017-12-11Bibliographically approved
3. Mutational analysis of human glutathione transferase A2-2 identifies structural elements supporting high activity with the prodrug azathioprine
Open this publication in new window or tab >>Mutational analysis of human glutathione transferase A2-2 identifies structural elements supporting high activity with the prodrug azathioprine
2012 (English)In: Protein Engineering Design & Selection, ISSN 1741-0126, E-ISSN 1741-0134, Vol. 25, no 4, 189-197 p.Article in journal (Refereed) Published
Abstract [en]

Glutathione transferase (GST) A2-2 is the human enzyme displaying the highest catalytic activity with the prodrug azathioprine (Aza). The reaction releases pharmacologically active 6-mercaptopurine by displacing the imidazole moiety from the Aza molecule. The GST-catalyzed reaction is of medical significance, since high rates of Aza activation may lead to adverse side effects in treated patients. The present study involves structureactivity relationships in GST A2-2 variants. Chimeric GSTs were previously generated by DNA shuffling and two peptide segments, one N-terminal and one C-terminal, were identified as primary determinants of Aza activity. The segments contain several residues of the substrate-binding H-site and their significance for supporting high Aza activity was investigated. Substitution of the corresponding two small regions in the low-activity human GST A3-3 or rat GST A3-3 by the human GST A2-2 segments generated chimeras with approximate to 10-fold enhanced Aza activity. The H-site residues Met208 and Leu213 in the C-terminal segment of GST A2-2 were mutated to produce a library with all possible residue combinations. At a calculated 93 library coverage, all of the 1880 mutants examined showed wild-type or decreased Aza activity, even though some retained activities with alternative substrates, further emphasizing the importance of this region for the targeted activity.

Keyword
directed evolution, DNA shuffling, GST, prodrug, structure-activity relationship, azathioprine, chimeric mutagenesis, glutathione transferase, prodrug activation, saturation mutagenesis
National Category
Biochemistry and Molecular Biology Biocatalysis and Enzyme Technology
Identifiers
urn:nbn:se:uu:diva-167324 (URN)10.1093/protein/gzs006 (DOI)000302021100006 ()
Available from: 2012-01-25 Created: 2012-01-25 Last updated: 2017-12-08Bibliographically approved
4. Structure-based redesign of GST A2-2 for enhanced catalytic efficiency with azathioprine
Open this publication in new window or tab >>Structure-based redesign of GST A2-2 for enhanced catalytic efficiency with azathioprine
2012 (English)In: Chemistry and Biology, ISSN 1074-5521, E-ISSN 1879-1301, Vol. 19, no 3, 414-421 p.Article in journal (Refereed) Published
Abstract [en]

Glutathione transferase (GST) A2-2 is the most efficient human enzyme in the biotransformation of the prodrug azathioprine (Aza). The activation of Aza has therapeutic potential for possible use of GSTs in targeted enzyme-prodrug treatment of diseases. Based on the assumed catalytic mechanism and computational docking of Aza to the active site of the enzyme, active-site residues were selected for construction of focused mutant libraries, which were thereafter screened for Aza activity. Mutants with elevated Aza activity were identified, DNA sequenced, and the proteins purified. The two most active mutants showed up to 70-fold higher catalytic efficiency than the parental GST A2-2. The structure of the most active triple mutant (L107G/L108D/F222H) enzyme was determined by X-ray crystallography demonstrating significant changes in the topography of the active site facilitating productive binding of Aza as a substrate. 

Keyword
glutathione transferase, azathioprine, structure-based redesign, semi-rational enzyme engineering, reduced amino acid alphabet, directed evolution
National Category
Biochemistry and Molecular Biology Biocatalysis and Enzyme Technology
Identifiers
urn:nbn:se:uu:diva-158395 (URN)10.1016/j.chembiol.2012.01.021 (DOI)000302588900014 ()
Available from: 2011-09-06 Created: 2011-09-06 Last updated: 2017-12-08Bibliographically approved

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