title: Disulfide bond formation involves a quinhydrone-type charge-transfer complex.
abstract: The chemistry of disulfide exchange in biological systems is well studied. However, the detailed mechanism of how oxidizing equivalents are derived to form disulfide bonds in proteins is not clear. In prokaryotic organisms, it is known that DsbB delivers oxidizing equivalents through DsbA to secreted proteins. DsbB becomes reoxidized by reducing quinones that are part of the membrane-bound electron-transfer chains. It is this quinone reductase activity that links disulfide bond formation to the electron transport system. We show here that purified DsbB contains the spectral signal of a quinhydrone, a charge-transfer complex consisting of a hydroquinone and a quinone in a stacked configuration. We conclude that disulfide bond formation involves a stacked hydroquinone-benzoquinone pair that can be trapped on DsbB as a quinhydrone charge-transfer complex. Quinhydrones are known to be redox-active and are commonly used as redox standards, but, to our knowledge, have never before been directly observed in biological systems. We also show kinetically that this quinhydrone-type charge-transfer complex undergoes redox reactions consistent with its being an intermediate in the reaction mechanism of DsbB. We propose a simple model for the action of DsbB where a quinhydrone-like complex plays a crucial role as a reaction intermediate.
authors: Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, MI 48109-1048, USA.
authors: James
authors: J
authors: Regeimbal
authors: Stefan
authors: S
authors: Gleiter
authors: Bernard L
authors: BL
authors: Trumpower
authors: Chang-An
authors: CA
authors: Yu
authors: Mithun
authors: M
authors: Diwakar
authors: David P
authors: DP
authors: Ballou
authors: James C A
authors: JC
authors: Bardwell
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism; D004220:Disulfides / Q000737:chemistry* / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D006873:Hydroquinones / Q000737:chemistry; D008565:Membrane Proteins / Q000378:metabolism; D008954:Models, Biological; D010084:Oxidation-Reduction; D019704:Protein Disulfide-Isomerases / Q000378:metabolism; D021241:Spectrometry, Mass, Electrospray Ionization; D013053:Spectrophotometry
chemical_list: D001426:Bacterial Proteins; D004220:Disulfides; C079503:DsbB protein, Bacteria; D029968:Escherichia coli Proteins; D006873:Hydroquinones; D008565:Membrane Proteins; D019704:Protein Disulfide-Isomerases; C004601:quinhydrone