Folding of horse cytochrome c in the reduced state.


Equilibrium and kinetic folding studies of horse cytochrome c in the reduced state have been carried out under strictly anaerobic conditions at neutral pH, 10 degrees C, in the entire range of aqueous solubility of guanidinium hydrochloride (GdnHCl). Equilibrium unfolding transitions observed by Soret heme absorbance, excitation energy transfer from the lone tryptophan residue to the ferrous heme, and far-UV circular dichroism (CD) are all biphasic and superimposable, implying no accumulation of structural intermediates. The thermodynamic parameters obtained by two-state analysis of these transitions yielded DeltaG(H2O)=18.8(+/-1.45) kcal mol(-1), and C(m)=5.1(+/-0.15) M GdnHCl, indicating unusual stability of reduced cytochrome c. These results have been used in conjunction with the redox potential of native cytochrome c and the known stability of oxidized cytochrome c to estimate a value of -164 mV as the redox potential of the unfolded protein. Stopped-flow kinetics of folding and unfolding have been recorded by Soret heme absorbance, and tryptophan fluorescence as observables. The refolding kinetics are monophasic in the transition region, but become biphasic as moderate to strongly native-like conditions are approached. There also is a burst folding reaction unobservable in the stopped-flow time window. Analyses of the two observable rates and their amplitudes indicate that the faster of the two rates corresponds to apparent two-state folding (U<-->N) of 80-90 % of unfolded molecules with a time constant in the range 190-550 micros estimated by linear extrapolation and model calculations. The remaining 10-20 % of the population folds to an off-pathway intermediate, I, which is required to unfold first to the initial unfolded state, U, in order to refold correctly to the native state, N (I<-->U<-->N). The slower of the two observable rates, which has a positive slope in the linear functional dependence on the denaturant concentration indicating that an unfolding process under native-like conditions indeed exists, originates from the unfolding of I to U, which rate-limits the overall folding of these 10-20 % of molecules. Both fast and slow rates are independent of protein concentration and pH of the refolding milieu, suggesting that the off-pathway intermediate is not a protein aggregate or trapped by heme misligation. The nature or type of unfolded-state heme ligation does not interfere with refolding. Equilibrium pH titration of the unfolded state yielded coupled ionization of the two non-native histidine ligands, H26 and H33, with a pK(a) value of 5.85. A substantial fraction of the unfolded population persists as the six-coordinate form even at low pH, suggesting ligation of the two methionine residues, M65 and M80. These results have been used along with the known ligand-binding properties of unfolded cytochrome c to propose a model for heme ligation dynamics. In contrast to refolding kinetics, the unfolding kinetics of reduced cytochrome c recorded by observation of Soret absorbance and tryptophan fluorescence are all slow, simple, and single-exponential. In the presence of 6.8 M GdnHCl, the unfolding time constant is approximately 300(+/-125) ms. There is no burst unfolding reaction. Simulations of the observed folding-unfolding kinetics by numerical solutions of the rate equations corresponding to the three-state I<-->U<-->N scheme have yielded the microscopic rate constants. Study holds ProTherm entries: 12227, 12228, 12229, 12230, 12231, 12232 Extra Details: oxidized and reduced cytochrome c; ferrocytochrome c; unfolded state;,heme axial ligation; ligand exchange dynamics

Submission Details

ID: 6MbyLsV74

Submitter: Connie Wang

Submission Date: April 24, 2018, 8:43 p.m.

Version: 1

Publication Details
Bhuyan AK;Udgaonkar JB,J. Mol. Biol. (2001) Folding of horse cytochrome c in the reduced state. PMID:11580255
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 UniProtKB Entries

Percent Identity Matching Chains Protein Accession Entry Name
100.0 Cytochrome c P00004 CYC_HORSE
99.0 Cytochrome c P68097 CYC_EQUAS
99.0 Cytochrome c P68096 CYC_EQUBU
97.1 Cytochrome c P62894 CYC_BOVIN
97.1 Cytochrome c P62895 CYC_PIG
97.1 Cytochrome c P62896 CYC_SHEEP
94.3 Cytochrome c P00007 CYC_HIPAM
95.2 Cytochrome c P68099 CYC_CAMDR
95.2 Cytochrome c P68100 CYC_ESCRO
95.2 Cytochrome c P68098 CYC_LAMGU
94.3 Cytochrome c P62897 CYC_MOUSE
94.3 Cytochrome c P00008 CYC_RABIT
94.3 Cytochrome c P62898 CYC_RAT
94.3 Cytochrome c P00011 CYC_CANLF
93.3 Cytochrome c P00014 CYC_MACGI
93.3 Cytochrome c P00013 CYC_MINSC
93.3 Cytochrome c Q52V09 CYC_CEPBA
93.3 Cytochrome c P00012 CYC_MIRLE
90.5 Cytochrome c Q52V10 CYC_SAISC
92.3 Cytochrome c P81280 CYC_ALLMI
92.2 Cytochrome c P00020 CYC_ANAPL
91.3 Cytochrome c P00021 CYC_COLLI
90.3 Cytochrome c B4USV4 CYC_OTOGA