Oxidative protein folding describes the process by which disulfide bonds are inserted into proteins as they fold into their native structure. This involves two distinct phases, an oxidation phase where these covalent linkages are first introduced, and an isomerization phase in which incorrectly placed disulfides are shuffled leading to the native pairings. In eukaryotes, disulfide bond formation can be catalyzed by a number of flavin-dependent sulfhydryl oxidases. This dissertation work investigates how a particular flavin-dependent sulfhydryl oxidase, Quiescin-sulfhydryl oxidase (QSOX), cooperates with protein disulfide isomerase (PDI) to generate native pairings in two unfolded reduced proteins: ribonuclease A (RNase A, four disulfide bonds and 105 disulfide isomers of the fully oxidized protein) and avian riboflavin binding protein (RfBP, nine disulfide bonds and more than 34 million corresponding disulfide pairings). This QSOX/PDI in vitro folding system involves no functional interaction between the two enzymatic components; QSOX inserts disulfide bonds into protein substrates while PDI isomerizes the misplaced pairs to the native ones. Rapid refolding does not require glutathione or glutathione-based redox buffers.
Refolding of RfBP is followed continuously by monitoring spectral changes experienced by the ligand, riboflavin, upon binding to the apoprotein. Efficient refolding of this protein only occurs with a large molar excess of reduced PDI over the folding client protein. These conditions likely mirror the environment of the endoplasmic reticulum lumen where small concentrations of nascent proteins are exposed to nearly mM levels of PDI. Subsequent studies performed in the absence of QSOX or redox buffers, explore the effectiveness of mixtures of oxidized and reduced PDI in refolding RfBP. Here, the fastest refolding of RfBP occurs with excess reduced PDI and just enough oxidized PDI to generate nine disulfides in the protein. The implications of these in vitro experiments for understanding oxidative folding processes in vivo are discussed.
Although unfolded proteins have been proven to be excellent substrates of QSOX, a recent proposal suggests that it can also function in the generation of inter-domain and inter-protein disulfide bridges, where the substrates are already substantially or completely folded. This suggestion has been tested using wild type and mutant Escherichia coli thioredoxin as a model substrate. These folded substrates are, by comparison, poorly oxidized by QSOX which is consistent with the expected stringent steric requirements for efficient thiol/disulfide exchange reactions.
|Commitee:||Colman, Roberta F., Koh, John T., McLane, Mary Ann|
|School:||University of Delaware|
|Department:||Department of Chemistry and Biochemistry|
|School Location:||United States -- Delaware|
|Source:||DAI-B 71/08, Dissertation Abstracts International|
|Keywords:||Disulfide bonds, Oxidative protein folding, Protein disulfide isomerase, Quiescin sulfhydryl oxidase, Substrate specificity|
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