The formation of protein disulphide bonds in newly synthesised secretory proteins within the endoplasmic reticulum (ER) depends on an electron transfer system in which oxidising equivalents are passed from molecular oxygen through several carriers within the oxidase Ero1 to protein disulphide isomerase (PDI) and then to the reduced protein substrate. Recent articles published in The EMBO Journal highlight a crucial and surprising feature of the control of this process in mammalian cells; the activity of the key oxidase Ero1 is downregulated in oxidising conditions through the formation of a stable non-catalytic disulphide by a cysteine residue that participates in one of the redox active sites within the oxidase (Appenzeller-Herzog et al, 2008; Baker et al, 2008).

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