Functional plasticity of alkyl hydroperoxide reductase

In Escherichia coli, the glutathione/glutaredoxin and thioredoxin pathways are essential for the reduction of cytoplasmic protein disulfide bonds, including those formed in the essential enzyme ribonucleotide reductase during its action on substrates. In addition to the primary ribonucleotide reductase, E. coli has two alternative enzymes, used during oxidative stress and anaerobic growth. We investigate the requirement of the thioredoxin and glutaredoxin pathways for the functioning of these alternative ribonucleotide reductases. Aerobically, double mutants lacking thioredoxin reductase (trxB) and glutathione reductase (gor) or glutathione biosynthesis (gshA) cannot grow. Growth of Delta gor DeltatrxB strains is restored by a mutant (ahpC*) of the peroxiredoxin AhpC. We find that AhpC* exhibits an enhanced reductase activity towards mixed disulfides between glutathione and glutaredoxin, consistent with the in vivo requirements for these components. These studies show that ahpC* also restores growth to a DeltagshB DeltatrxB strain, which lacks glutathione and accumulates only its precursor gamma-glutamylcysteine, by allowing accumulation of reduced gamma-glutamylcysteine, which can substitute for glutathione. Surprisingly, new ahpC suppressor mutations arose in a DeltagshA DeltatrxB strain lacking glutathione and gamma-glutamylcysteine. Some of these mutant AhpC proteins channel electrons into the disulfide reducing pathways via either the thioredoxins or the glutaredoxins without, evidently, the intermediary of glutathione. Our results reveal surprising plasticity of a peroxidase, as different mutant versions of AhpC can channel electrons into the disulfide-reducing pathways by at least four distinct routes. Peroxiredoxins are linked evolutionarily to the thioredoxin and glutaredoxin pathways, thus isolation of mutants in AhpC that suppress defects in these pathways may reflect the evolution of AhpC. The potential evolutionary significance is amplified by the finding that some bacteria exhibit more than one homologue of AhpC, with one version being very close to the E. coli AhpC and a second, more distant one, being altered in some of same residues that are altered in our suppressors. Some of these AhpC homologues appear naturally to have disulfide reductase activity, suggesting that AhpC may have an additional role in cellular redox pathways. These findings suggest that this type of functional genomic analysis may provide a novel means of predicting protein function.