The conserved histidine 106 of large thioredoxin reductases is likely to have a structural role but not a base catalyst function

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Abstract
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.