replacement of threonine-55 with glycine decreases the reduction rate of ostrx20 by glutathione

thioredoxins (trxs) are small ubiquitous oxidoreductase proteins with two redox-active cys residues in a conserved active site (wcg/ppc) that regulate numerous target proteins via thiol/disulfide exchanges in the cells of prokaryotes and eukaryotes. the isoforms ostrx23 with a typical active site (wcgpc) and ostrx20 with an atypical active site (wctpc) are two  trx h- type isoforms in rice that were previously found to be reduced by nadph-dependent thioredoxin reductase and gsh/grx system, respectively. in the present work the reduction of mutants g41tostrx23, t55gostrx20, k48dostrx20 and t55g-k48d ostrx20 as well as wild types ostrx23 and ostrx20 were tested in the reaction containing either nadph/ntr or glutathione (gsh). the results revealed that reduction rate of t55gostrx20 was remarkably decreased by gsh as compared to wtostrx20 highlighting the critical role of thr-55 in interaction of ostrx20 with gsh. on the other hand a significant decrease in the reduction rate of g41tostrx23 was observed in reaction containing nadph-dependent thioredoxin reductase as compared with readuction rate of wtostrx23. these results suggest that first residue after n-terminal active site cys is one of the critical residue in determination of system that trxs can be reduced in.