The oxidative pathway relies, just like the isomerization pathway, on a protein relay. The first member of this protein relay is a small periplasmic protein (21 kDa) called DsbA, which has two cysteine residues that have to be oxidized in order for it to be active. When in its oxidized state, the protein is able to form disulfide bonds between cysteine residues in newly synthesized, and yet unfolded proteins by the transfer of its own disulfide bond onto the folding protein. After the transfer of this disulfide bond, DsbA is in a reduced state and in order for it to act catalytically again, it has to be reoxidized. This is made possible by a 21 kDa inner membrane protein, called DsbB, which has two pairs of cysteine residues. A mixed disulfide is formed between a cysteine residue of DsbB and one of DsbA. Eventually, this cross-link between the two proteins is broken by a nucleophilic attack of the second cystein residue in the DsbA active site. On his turn, DsbB is reoxidized by transferring electrons to oxidized ubiquinone, which passes them to cytochrome oxidases, which finally reduce oxygen; this is in aerobic conditions. As molecular oxygen serves as the terminal electron acceptor in aerobic conditions, oxidative folding is conveniently coupled to it through the respiratory chain. In anaerobic conditions however, DsbB passes its electrons to menaquinone, followed by a transfer of electrons to fumarate reductase or nitrate reductase.