Mechanism
E. coli thioredoxin reductase mechanism: In E. coli ThxR the spatial orientation of the FAD and NADPH domains are such that the redox-active rings of FAD and NADPH are not in close proximity to each other. When the FAD domain of E. coli is rotated 66 degrees with the NADPH domain remaining fixed the two prosthetic groups move into close contact allowing electrons to pass from NADPH to FAD and then to the active site disulfide bond. The conserved active site residues in E. coli are -Cys-Ala-Thr-Cys-.
Mammalian thioredoxin reductase mechanism: Mammalian TrxRs have a much higher sequence homology with glutathione reductase than E. coli. The active-site Cys residues in the FAD domain and bound NADPH domain are in close proximity removing the necessity for a 66 degree rotation for electron transfer found in E. coli. An additional feature of the mammalian mechanism is the presence of a selenocystine residue at the C-terminal end of the protein which is required for catalytic activity. The conserved residues in mammalian active site are -Cys-Val-Asn-Val-Gly-Cys-.
Read more about this topic: Thioredoxin Reductase
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