title: Early intermediates in the PDI-assisted folding of ribonuclease A.
abstract: The oxidative refolding of ribonuclease A has been investigated in several experimental conditions using a variety of redox systems. All these studies agree that the formation of disulfide bonds during the process occurs through a nonrandom mechanism with a preferential coupling of certain cysteine residues. We have previously demonstrated that in the presence of glutathione the refolding process occurs through the reiteration of two sequential reactions: a mixed disulfide with glutathione is produced first which evolves to form an intramolecular S-S bond. In the same experimental conditions, protein disulfide isomerase (PDI) was shown to catalyze formation and reduction of mixed disulfides with glutathione as well as formation of intramolecular S-S bonds. This paper reports the structural characterization of the one-disulfide intermediate population during the oxidative refolding of Ribonuclease A under the presence of PDI and glutathione with the aim of defining the role of the enzyme at the early stages of the reaction. The one-disulfide intermediate population occurring at the early stages of both the uncatalyzed and the PDI-catalyzed refolding was purified and structurally characterized by proteolytic digestion followed by MALDI-MS and LC/ESIMS analyses. In the uncatalyzed refolding, a total of 12 disulfide bonds out of the 28 theoretical possible cysteine couplings was observed, confirming a nonrandom distribution of native and nonnative disulfide bonds. Under the presence of PDI, only two additional nonnative disulfides were detected. Semiquantitative LC/ESIMS analysis of the distribution of the S-S bridged peptides showed that the most abundant species were equally populated in both the uncatalyzed and the catalyzed process. This paper shows the first structural characterization of the one-disulfide intermediate population formed transiently during the refolding of ribonuclease A in quasi-physiological conditions that mimic those present in the ER lumen. At the early stages of the process, three of the four native disulfides are detected, whereas the Cys26-Cys84 pairing is absent. Most of the nonnative disulfide bonds identified are formed by nearest-neighboring cysteines. The presence of PDI does not significantly alter the distribution of S-S bonds, suggesting that the ensemble of single-disulfide species is formed under thermodynamic control.
authors.affiliation: Dipartimento di Chimica Università degli Studi di Salerno, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Vinci
authors.forename: M
authors.initials: M
authors.lastname: Ruoppolo
authors.forename: P
authors.initials: P
authors.lastname: Pucci
authors.forename: R B
authors.initials: RB
authors.lastname: Freedman
authors.forename: G
authors.initials: G
authors.lastname: Marino
mesh_terms: D002384:Catalysis; D002853:Chromatography, Liquid; D004220:Disulfides / Q000737:chemistry; D005978:Glutathione / Q000737:chemistry; D013058:Mass Spectrometry; D010449:Peptide Mapping; D019704:Protein Disulfide-Isomerases / Q000737:chemistry*; D017510:Protein Folding; D012259:Ribonuclease, Pancreatic / Q000737:chemistry*
chemical_list: D004220:Disulfides; D012259:Ribonuclease, Pancreatic; D019704:Protein Disulfide-Isomerases; D005978:Glutathione