Role of the glutaredoxin domain and FAD in the stabilization of thioredoxin glutathione reductase

Thioredoxin glutathione reductase (TGRsec) is a multi-domain flavoprotein that plays a principal role in redox homeostasis maintenance. We have previously demonstrated the role of selenocysteine in maintaining TGRsec structure-function, but the role of the glutaredoxin (Grx) domain and FAD is still unclear. In the present study, the urea-induced unfolding of recombinant Fasciola gigaraica TGRsec (FgTGRsec) and its N-terminal truncated variant (Delta NTD-FgTGRsec) were examined to understand the role of the Grx domain and FAD in the stabilization of FgTGRsec and Delta NTD-FgTGRsec. Our results showed that both proteins underwent unfolding in a three state manner. First, the protein undergoes a conformational transition rendering a near-native state with no FAD bound, and then full unfolding of the apo-dimer occurs without dissociation. The Grx domain stabilized the global FgTGRsec structure and positively regulated FgTGRsec activity, and alteration in the FAD micro-environment was directly proportional to the loss of thioredoxin reductase (TrxR) and glutathione reductase activities. Based on these results, we concluded that the Grx domain stabilizes the full-length FgTGRsec protein for efficient catalysis. Thus, we suggest that in platyhelminth parasites, during evolution, the Grx domain merged with the TrxR domain to confer higher catalytic activity and provide additional structural stability to the full-length TGR.