A PDI-catalyzed thiol-disulfide switch regulates the production of hydrogen peroxide by human Ero1

Oxidative folding in the endoplasmic reticulum (ER) involves ER oxidoreductin 1 (Ero1)-mediated disulfide formation in protein disulfide isomerase (PDI). In this process, Ero1 consumes oxygen (O-2) and releases hydrogen peroxide (H2O2), but none of the published Ero1 crystal structures reveal any potential pathway for entry and exit of these reactants. We report that additional mutation of the Cys(208)-Cys(241) disulfide in hyperactive Ero1 alpha (Ero1 alpha-C104A/C131A) potentiates H2O2 production, ER oxidation, and cell toxicity. This disulfide clamps two helices that seal the Flavin cofactor where O-2 is reduced to H2O2. Through its carboxyterminal active site, PDI unlocks this seal by forming a Cys(208)/cy-(241) - dependent mixed-disulfide complex with Ero1 alpha. The H2O2-detoxifying glutathione peroxidase 8 also binds to the Cys(208)/Cys(241) loop region. Supported by O-2 diffusion simulations, these data describe the first enzymatically controlled O-2 access into a flavoprotein active site, provide molecular-level understanding of En:Act regulation and H2O2 production/detoxification, and establish the deleterious consequences of constitutive Ero1 activity. (C) 2015 Elsevier Inc. All rights reserved.