Modifications of the active center of T4 thioredoxin by site-directed mutagenesis.

The active site sequence of T4 thioredoxin, Cys-Val-Tyr-Cys, has been modified in two positions to Cys-Gly-Pro-Cys to mimic that of Escherichia coli thioredoxin. The two point mutants Cys-Gly-Tyr-Cys and Cys-Val-Pro-Cys have also been constructed. The mutant proteins have similar reaction rates with T4 ribonucleotide reductase as has the wild-type T4 thioredoxin. Mutant T4 thioredoxins with Pro instead of Tyr at position 16 in the active site sequence have three to four times lower apparent KM with E. coli ribonucleotide reductase than wild-type T4 thioredoxin. The KM values for these mutant proteins which do not have Tyr in position 16 are thus closer to E. coli thioredoxin than to the wild-type T4 thioredoxin. The bulky tyrosine side chain probably prevents proper interactions to E. coli ribonucleotide reductase. Also the redox potentials of these two mutant thioredoxins are lower than that of the wild-type T4 thioredoxin and are thereby more similar to the redox potential of E. coli thioredoxin. Mutations in position 15 behave more or less like the wild-type protein. The kinetic parameters with E. coli thioredoxin reductase are similar for wild-type and mutant T4 thioredoxins except that the apparent kcat is lower for the mutant protein with Pro instead of Tyr in position 16. The active site sequence of T4 thioredoxin has also been changed to Cys-Pro-Tyr-Cys to mimic that of glutaredoxins. This change does not markedly alter the reaction rate of the mutant protein with T4 ribonucleotide reductase or E. coli thioredoxin reductase, but the redox potential is lower for this mutant protein than for wild-type T4 thioredoxin.