Redox-active disulfide/dithiol (D/D) couples in proteins play important regulatory roles within cells. D/D redox reactions of proteins found in the purple photosynthetic bacteria R. sphaeroides and R. capsulatus are used to regulate the expression of photosystem components in response to the presence oxygen and light. D/D redox reactions of regulatory proteins found in the yeast S. cerevisiae regulate the production of peroxide-scavenging proteins. AppA and PpsR are present in R. sphaeroides regulate gene expression in response to oxygen. It has been proposed that AppA reduces PpsR, causing PpsR to lose its ability to bind DNA. Redox titrations of the D/D couples in PpsR and AppA were carried out at pH 7.0 and the two proteins were shown to be isopotential, having Em values of -320 mV. Yap1 is a key regulator of gene expression in S. cerevisiae in response to peroxides. Gpx3 and Trx2 are two additional components. Em values for the two disulfide bonds in Yap1 have been determined (Em1 = -330 mV and Em2 = -155 mV), as has an Em value of -315 mV for Gpx3. Trx2 has an Em of -275 mV, which is capable of reducing the disulfide in Yap1 that corresponds to Em2, but not Em1.

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