{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,6,10]],"date-time":"2026-06-10T16:56:14Z","timestamp":1781110574836,"version":"3.54.1"},"reference-count":0,"publisher":"National Academy of Sciences","issue":"10","content-domain":{"domain":["www.pnas.org"],"crossmark-restriction":true},"short-container-title":["Proc. Natl. Acad. Sci. U.S.A."],"published-print":{"date-parts":[[1978,10]]},"abstract":"\n Unfolded RNase A is known to contain an equilibrium mixture of two forms, a slow-folding form (U\n 1<\/jats:sub>\n ) and a fast-folding form (U\n 2<\/jats:sub>\n ). If U\n 1<\/jats:sub>\n is produced after unfolding by the slow\n cis-trans<\/jats:italic>\n isomerization of proline residues about X-Pro imide bonds, then the formation of U\n 1<\/jats:sub>\n should be catalyzed by strong acids. Therefore, the rate of formation of U\n 1<\/jats:sub>\n has been measured at different HClO\n 4<\/jats:sub>\n concentrations. After rapid unfolding of the native protein in concentrated HClO\n 4<\/jats:sub>\n at 0\u00b0, the slow formation of U\n 1<\/jats:sub>\n was measured by use of refolding assays. Catalysis of its formation was found at HClO\n 4<\/jats:sub>\n concentrations above 5 M. The uncatalyzed reaction follows apparent first-order kinetics but, in the acid-catalyzed range, two reactions are found. The faster reaction produces two-thirds of the slow-folding species and shows acid catalysis above 5 M HClO\n 4<\/jats:sub>\n . Catalysis of the slower reaction begins at 8 M HClO\n 4<\/jats:sub>\n . The faster reaction shows a 100-fold increase in rate at 10.6 M HClO\n 4<\/jats:sub>\n over the rate of the uncatalyzed reaction of 5 M. The activation enthalpy of the uncatalyzed reaction has been measured in two sets of unfolding conditions: \u0394H\n \u2021<\/jats:sup>\n is 21.5 kcal\/mol (1 kcal = 4.2 \u00d7 10\n 3<\/jats:sup>\n J) in 3.3 M HClO\n 4<\/jats:sub>\n and 21.0 kcal\/mol in 5 M guanidine HCl, pH 2.5.\n <\/jats:p>\n \n Both acid catalysis of the formation of U\n 1<\/jats:sub>\n and its high activation enthalpy are consistent with the rate-limiting step being\n cis-trans<\/jats:italic>\n isomerization either of X-Pro imide bonds or of peptide bond. The rate of the uncatalyzed reaction is in the range expected for proline isomerization and is 0.1% of that of peptide bond isomerization; thus, the simplest explanation for the formation of U\n 1<\/jats:sub>\n is proline isomerization. Earlier data, showing that the kinetic properties of the U\n 1<\/jats:sub>\n \u21c4 U\n 2<\/jats:sub>\n reaction in refolding conditions differ from those of proline isomerization, can be explained if there is kinetic coupling between early steps in the folding of U\n 1<\/jats:sub>\n and its conversion to U\n 2<\/jats:sub>\n .\n <\/jats:p>\n \n The existence of two acid-catalyzed reactions that are distinguished by the HClO\n 4<\/jats:sub>\n concentration at which catalysis begins suggests that at least two essential proline residues produce slow-folding species of RNase A by isomerization after unfolding. Because protonation of imide bonds is responsible for acid catalysis of proline isomerization, the slower reaction probably involves an imide bond with a low pK. It may be the bond connecting Lys-41 and Pro-42, because the positive charge on Lys-41 could make this bond more difficult to protonate.\n <\/jats:p>","DOI":"10.1073\/pnas.75.10.4764","type":"journal-article","created":{"date-parts":[[2006,5,31]],"date-time":"2006-05-31T03:37:50Z","timestamp":1149046670000},"page":"4764-4768","update-policy":"https:\/\/doi.org\/10.1073\/pnas.cm10313","source":"Crossref","is-referenced-by-count":162,"title":["Acid catalysis of the formation of the slow-folding species of RNase A: Evidence that the reaction is proline isomerization"],"prefix":"10.1073","volume":"75","author":[{"given":"Franz X.","family":"Schmid","sequence":"first","affiliation":[{"name":"Department of Biochemistry, Stanford University School of Medicine, Stanford, California 94305"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Robert L.","family":"Baldwin","sequence":"additional","affiliation":[{"name":"Department of Biochemistry, Stanford University School of Medicine, Stanford, California 94305"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"341","published-online":{"date-parts":[[1978,10]]},"container-title":["Proceedings of the National Academy of Sciences"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/pnas.org\/doi\/pdf\/10.1073\/pnas.75.10.4764","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2022,4,13]],"date-time":"2022-04-13T11:08:28Z","timestamp":1649848108000},"score":1,"resource":{"primary":{"URL":"https:\/\/pnas.org\/doi\/full\/10.1073\/pnas.75.10.4764"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[1978,10]]},"references-count":0,"journal-issue":{"issue":"10","published-print":{"date-parts":[[1978,10]]}},"alternative-id":["10.1073\/pnas.75.10.4764"],"URL":"https:\/\/doi.org\/10.1073\/pnas.75.10.4764","relation":{},"ISSN":["0027-8424","1091-6490"],"issn-type":[{"value":"0027-8424","type":"print"},{"value":"1091-6490","type":"electronic"}],"subject":[],"published":{"date-parts":[[1978,10]]},"assertion":[{"value":"1978-10-01","order":2,"name":"published","label":"Published","group":{"name":"publication_history","label":"Publication History"}}]}}