The conserved histidine 106 of large thioredoxin reductases is likely to have a structural role but not a base catalyst function.
The catalytic activity of selenocysteine-containing thioredoxin reductases can be mimicked by cysteine-variants if the local environment at the C-terminal redox center supports thiol activation. This concept of a linear catalytic site was challenged by structural data suggesting that the invariant residue His106 functions as a base catalyst for the dithiol-disulphide exchange reaction between enzyme and substrate. As reported here, we changed His106 to asparagine, glutamine, and phenylalanine in various C-terminal mutants of Drosophila melanogaster thioredoxin reductase. The catalytic activity dropped considerably, yet pH-profiles did not reveal differences, rendering a function for His106 as a base catalyst unlikely. Interestingly, the phenylalanine-mutants, designed as negative controls were the most active mutants which suggests rather a structural role of His106.
Jacob
Judit
J
Biochemiezentrum der Universität Heidelberg (BZH), Im Neuenheimer Feld 504, D-69120 Heidelberg, Germany.
Schirmer
R Heiner
RH
Gromer
Stephan
S
D000595:Amino Acid Sequence; D000818:Animals; D002384:Catalysis; D004331:Drosophila melanogaster; D006639:Histidine / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D013880:Thioredoxin-Disulfide Reductase / Q000737:chemistry / Q000378:metabolism*
D006639:Histidine; D013880:Thioredoxin-Disulfide Reductase