Structure of an insect epsilon class glutathione S-transferase from the malaria vector Anopheles gambiae provides an explanation for the high DDT-detoxifying activity.
Glutathione S-transferases (GSTs), a major family of detoxifying enzymes, play a pivotal role in insecticide resistance in insects. In the malaria vector Anopheles gambiae, insect-specific epsilon class GSTs are associated with resistance to the organochlorine insecticide DDT [1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane]. Five of the eight class members have elevated expression levels in a DDT resistant strain. agGSTe2 is considered the most important GST in conferring DDT resistance in A. gambiae, and is the only member of the epsilon class with confirmed DDT-metabolizing activity. A delta class GST from the same species shows marginal DDT-metabolizing activity but the activity of agGSTe2 is approximately 350x higher than the delta class agGST1-6. To investigate its catalytic mechanism and the molecular basis of its unusually high DDT-metabolizing ability, three agGSTe2 crystal structures including one apo form and two binary complex forms with the co-factor glutathione (GSH) or the inhibitor S-hexylglutathione (GTX) have been solved with a resolution up to 1.4A. The structure of agGSTe2 shows the canonical GST fold with a highly conserved N-domain and a less conserved C-domain. The binding of GSH or GTX does not induce significant conformational changes in the protein. The modeling of DDT into the putative DDT-binding pocket suggests that DDT is likely to be converted to DDE [1,1-dichloro-2,2-bis-(p-chlorophenyl)ethylene] through an elimination reaction triggered by the nucleophilic attack of the thiolate group of GS(-) on the beta-hydrogen of DDT. The comparison with the less active agGST1-6 provides the structural evidence for its high DDT-detoxifying activity. In short, this is achieved through the inclination of the upper part of H4 helix (H4'' helix), which brings residues Arg112, Glu116, and Phe120 closer to the GSH-binding site resulting in a more efficient GS(-)-stabilizing hydrogen-bond-network and higher DDT-binding affinity.
Wang
Yujun
Y
Laboratory for Structural Biology, Department of Chemistry, Graduate Program of Biotechnology, University of Alabama in Huntsville, Huntsville, AL 35899, USA.
Qiu
Li
L
Ranson
Hilary
H
Lumjuan
Nongkran
N
Hemingway
Janet
J
Setzer
William N
WN
Meehan
Edward J
EJ
Chen
Liqing
L
D000818:Animals; D000852:Anopheles / Q000201:enzymology*; D001665:Binding Sites; D018360:Crystallography, X-Ray; D003634:DDT / Q000378:metabolism*; D004351:Drug Resistance; D005978:Glutathione / Q000737:chemistry; D005982:Glutathione Transferase / Q000737:chemistry* / Q000378:metabolism; D007303:Insect Vectors; D007313:Insecta; D007306:Insecticides / Q000378:metabolism; D008288:Malaria / Q000635:transmission; D011485:Protein Binding; D011487:Protein Conformation
D007306:Insecticides; D003634:DDT; D005982:Glutathione Transferase; D005978:Glutathione