Least activation path for protein folding: investigation of staphylococcal nuclease folding by stopped-flow circular dichroism.


Is the pathway of protein folding determined by the relative stability of folding intermediates, or by the relative height of the activation barriers leading to these intermediates? This is a fundamental question for resolving the Levinthal paradox, which stated that protein folding by a random search mechanism would require a time too long to be plausible. To answer this question, we have studied the guanidinium chloride (GdmCl)-induced folding/unfolding of staphylococcal nuclease [(SNase, formerly EC; now called microbial nuclease or endonuclease, EC] by stopped-flow circular dichroism (CD) and differential scanning microcalorimetry (DSC). The data show that while the equilibrium transition is a quasi-two-state process, kinetics in the 2-ms to 500-s time range are triphasic. Data support the sequential mechanism for SNase folding: U3 <--> U2 <--> U1 <--> N0, where U1, U2, and U3 are substates of the unfolded protein and N0 is the native state. Analysis of the relative population of the U1, U2, and U3 species in 2.0 M GdmCl gives delta-G values for the U3 --> U2 reaction of +0.1 kcal/mol and for the U2 --> U1 reaction of -0.49 kcal/mol. The delta-G value for the U1 --> N0 reaction is calculated to be -4.5 kcal/mol from DSC data. The activation energy, enthalpy, and entropy for each kinetic step are also determined. These results allow us to make the following four conclusions. (i) Although the U1, U2, and U3 states are nearly isoenergetic, no random walk occurs among them during the folding. The pathway of folding is unique and sequential. In other words, the relative stability of the folding intermediates does not dictate the folding pathway. Instead, the folding is a descent toward the global free-energy minimum of the native state via the least activation path in the vast energy landscape. Barrier avoidance leads the way, and barrier height limits the rate. Thus, the Levinthal paradox is not applicable to the protein-folding problem. (ii) The main folding reaction (U1 --> N0), in which the peptide chain acquires most of its free energy (via van der Waals' contacts, hydrogen bonding, and electrostatic interactions), is a highly concerted process. These energy-acquiring events take place in a single kinetic phase. (iii) U1 appears to be a compact unfolded species; the rate of conversion of U2 to U1 depends on the viscosity of solution. (iv) All four relaxation times reported here depend on GdmCl concentrations: it is likely that none involve the cis/trans isomerization of prolines. Finally, a mechanism is presented in which formation of sheet-like chain conformations and a hydrophobic condensation event precede the main-chain folding reaction. Study holds ProTherm entries: 8651, 8652 Extra Details: activation barriers; kinetic pathways; hydrophobic condensation;,compact unfolded state

Submission Details

ID: mA3G3HrF4

Submitter: Connie Wang

Submission Date: April 24, 2018, 8:36 p.m.

Version: 1

Publication Details
Su ZD;Arooz MT;Chen HM;Gross CJ;Tsong TY,Proc. Natl. Acad. Sci. U.S.A. (1996) Least activation path for protein folding: investigation of staphylococcal nuclease folding by stopped-flow circular dichroism. PMID:8637910
Additional Information

Structure view and single mutant data analysis

Study data

No weblogo for data of varying length.
Colors: D E R H K S T N Q A V I L M F Y W C G P

Data Distribution

Studies with similar sequences (approximate matches)

Correlation with other assays (exact sequence matches)

Relevant PDB Entries

Structure ID Release Date Resolution Structure Title
4WRD 2014-10-23T00:00:00+0000 1.65 Crystal structure of Staphylcoccal nulease variant Delta+PHS V66E L125E at cryogenic temperature
2LKV 2011-10-21T00:00:00+0000 0 Staphylococcal Nuclease PHS variant
2M00 2012-10-14T00:00:00+0000 0 Solution structure of staphylococcal nuclease E43S mutant in the presence of ssDNA and Cd2+
2OXP 2007-02-20T00:00:00+0000 2.0 Crystal Structure of Staphylococcal Nuclease mutant V66D/P117G/H124L/S128A
3D4W 2008-05-15T00:00:00+0000 1.9 Crystal structure of Staphylococcal nuclease variant Delta+PHS A109R at cryogenic temperature
3D8G 2008-05-23T00:00:00+0000 1.99 Crystal structure of Staphylococcal nuclease variant Delta+PHS I72R at cryogenic temperature
3MVV 2010-05-04T00:00:00+0000 1.72 Crystal structure of Staphylococcal nuclease variant Delta+PHS F34A at cryogenic temperature
3QOJ 2011-02-10T00:00:00+0000 1.6 Cryogenic structure of Staphylococcal nuclease variant D+PHS/V23K
3QOL 2011-02-10T00:00:00+0000 1.9 Crystal structure of Staphylococcal nuclease variant D+PHS/V23E at pH 6 determined at 100 K
3R3O 2011-03-16T00:00:00+0000 1.9 Crystal structure of Staphylococcal nuclease variant Delta+PHS T62A at cryogenic temperature and with high redundancy

Relevant UniProtKB Entries

Percent Identity Matching Chains Protein Accession Entry Name
100.0 Thermonuclease P00644 NUC_STAAU
99.3 Thermonuclease Q5HHM4 NUC_STAAC
99.1 Thermonuclease Q99VJ0 NUC_STAAM
99.1 Thermonuclease Q7A6P2 NUC_STAAN
99.3 Thermonuclease Q6GB41 NUC_STAAS
99.3 Thermonuclease Q8NXI6 NUC_STAAW
99.3 Thermonuclease Q6GIK1 NUC_STAAR