title: Catalysis of oxidative protein folding by mutants of protein disulfide isomerase with a single active-site cysteine.
abstract: Protein disulfide isomerase (PDI), a very abundant protein in the endoplasmic reticulum, facilitates the formation and rearrangement of disulfide bonds using two nonequivalent redox active-sites, located in two different thioredoxin homology domains [Lyles, M. M., & Gilbert, H. F. (1994) J. Biol. Chem. 269, 30946-30952]. Each dithiol/disulfide active-site contains the thioredoxin consensus sequence CXXC. Four mutants of protein disulfide isomerase were constructed that have only a single active-site cysteine. Kinetic analysis of these mutants show that the first (more N-terminal) cysteine in either active site is essential for catalysis of oxidation and rearrangement during the refolding of reduced bovine pancreatic ribonuclease A (RNase). Mutant active sites with the sequence SGHC show no detectable activity for disulfide formation or rearrangement, even at concentrations of 25 microM. The second (more C-terminal) cysteine is not essential for catalysis of RNase disulfide rearrangements, but it is essential for catalysis of RNase oxidation, even in the presence of a glutathione redox buffer. Mutant active sites with the sequence CGHS show 12%-50% of the kcat activity of wild-type active sites during the rearrangement phase of RNase refolding but < 5% activity during the oxidation phase. In addition, mutants with the sequence CGHS accumulate significant levels of a covalent PDI-RNase complex during steady-state turnover while the wild-type enzyme and mutants with the sequence SGHC do not. Since both active-site cysteines are essential for catalysis of disulfide formation, the dominant mechanism for RNase oxidation may involve direct oxidation by the active-site PDI disulfide. Although it is not essential for catalysis of RNase rearrangements, the more C-terminal cysteine does contribute 2-8-fold to the rearrangement activity. A mechanism for substrate rearrangement is suggested in which the second active-site cysteine provides PDI with a way to "escape" from covalent intermediates that do not rearrange in a timely fashion. The second active-site cysteine may normally serve the wild-type enzyme as an internal clock that limits the time allowed for intramolecular substrate rearrangements.
authors.affiliation: Verna and Marrs McLean Department of Biochemistry, Baylor College of Medicine, Houston, Texas 77030, USA.
authors.forename: K W
authors.initials: KW
authors.lastname: Walker
authors.forename: M M
authors.initials: MM
authors.lastname: Lyles
authors.forename: H F
authors.initials: HF
authors.lastname: Gilbert
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D002384:Catalysis; D002417:Cattle; D003545:Cysteine / Q000737:chemistry; D017931:DNA Primers / Q000235:genetics; D004220:Disulfides / Q000737:chemistry; D066298:In Vitro Techniques; D007535:Isomerases / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D007700:Kinetics; D008956:Models, Chemical; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D017354:Point Mutation; D019704:Protein Disulfide-Isomerases; D017510:Protein Folding; D051381:Rats; D012260:Ribonucleases / Q000737:chemistry / Q000378:metabolism
chemical_list: D017931:DNA Primers; D004220:Disulfides; D012260:Ribonucleases; D007535:Isomerases; D019704:Protein Disulfide-Isomerases; D003545:Cysteine