The cupredoxin fold is found in the soluble CuA and CyoA domains of two terminal oxidases.

The CD spectra of the CuA domain from subunit II of Paracoccus cytochrome c oxidase and the CyoA domain of subunit II from E. coli quinol oxidase have been recorded in the wavelength region 260-185 nm. A computer program based on a set of CD spectra of proteins with known structures, and employing the statistical method of variable selection, has been used to estimate the distribution of five forms of secondary structure. The analysis was improved by including the CD spectra of azurin and plastocyanin in the basis set. For the CuA domain, an estimate from the primary structure was also made. The results show that the soluble domains have the cupredoxin fold, with very little helical structure and a predominance of beta-strands. The CyoA domain is very similar to azurin, but the beta-structure in the CuA protein resembles that in plastocyanin.