Escherichia coli thioredoxin contains two tryptophan residues (Trp28 and Trp31) situated close to the active site disulfide/dithiol. In order to probe the structural and functional roles of tryptophan in the mechanism of E. coli thioredoxin (Trx), we have replaced Trp28 with alanine using site-directed mutagenesis and expressed the mutant protein W28A in E. coli. Changes in the behavior of the mutant protein compared with the wild-type protein have been monitored by a number of physical and spectroscopic techniques and enzyme assays. As expected, removal of a tryptophan residue causes profound changes in the fluorescence spectrum of thioredoxin, particularly for the reduced protein (Trx-(SH)2), and to a lesser extent for the oxidized protein (Trx-S2). These results show that the major contribution to the strongly quenched fluorescence of Trx-S2 in both wild-type and mutant proteins is from Trp31, whereas the higher fluorescence quantum yield of Trx-(SH)2 in the wild-type protein is dominated by the emission from Trp28. The fluorescence, CD, and 1H NMR spectra are all indicative that the mutant protein is fully folded at pH 7 and room temperature, and, despite the significance of the change, from a tryptophan in close proximity to the active site to an alanine, the functions of the protein appear to be largely intact. W28A Trx-S2 is a good substrate for thioredoxin reductase, and W28A Trx-(SH)2 is as efficient as wild-type protein in reduction of insulin disulfides. DNA polymerase activity exhibited by the complex of phage T7 gene 5 protein and Trx-(SH)2 is affected only marginally by the W28A substitution, consistent with the buried position of Trp28 in the protein. However, the thermodynamic stability of the molecule appears to have been greatly reduced by the mutation: guanidine hydrochloride unfolds the protein at a significantly lower concentration for the mutant than for wild type, and the thermal stability is reduced by about 10 degrees C in each case. The stability of each form of the protein appears to be reduced by the same amount, an indication that the effect of the mutation is identical in both forms of the protein. Thus, despite its close proximity to the active site, the Trp28 residue of thioredoxin is not apparently essential to the electron transfer mechanism, but rather contributes to the stability of the protein fold in the active site region.