Lecture 34 Protein Unfolding Thermodynamics

Transcription

1 Physical Principles in Biology Biology 3550 Fall 2018 Lecture 34 Protein Unfolding Thermodynamics Wednesday, 21 November c David P. Goldenberg University of Utah goldenberg@biology.utah.edu

2 Clicker Question #1 What holiday falls on Thursday, 22 November 2018? A) Halloween B) Veterans Day C) Thanksgiving D) Christmas E) Kwanza F) Chanukah

3 Protein Unfolding: A Simplified Summary For small, single-domain proteins ( 100 amino acids), unfolding is well described as a two-state process: N U Partially folded molecules are rarely detected at equilibrium. Free energy profile for unfolding and refolding:

4 Entropy Change for Protein Unfolding For now, focus only on the polypeptide chain itself: The native protein is a (relatively) unique structure. The unfolded state is an ensemble of rapidly interconverting structures. From Boltzmann: S = k ln Ω (for a single molecule) For the native state, assume Ω N = 1, S N = 0. (A questionable assumption, but it turns out to not be so bad.) What about the unfolded state?

5 Rotatable Bonds in an Amino Acid Residue Amino acid residue Amino acid residue: Part of amino acid left in a polypeptide. Assume that each residue can take on 10 conformations in the unfolded state.

6 Why This isn t an Absurd Way to Estimate the Entropy Change Rotational isomers (rotamers) In both native and unfolded states, dihedral angles fluctuate around energy minima. In native state, most bonds are restricted to one minimum. In unfolded state, bonds can sample two or three minima. The rotational isomeric state approximation

7 Entropy Change for Protein Unfolding From Boltzmann: S = k ln Ω For the native state, assume Ω N = 1, S N = 0. For the unfolded state assume that each residue can take on 10 possible conformations. For two residues, Ω U = 10 2 For three residues, Ω U = 10 3 For n residues, Ω U = 10 n S for unfolding S = S U S N = k ln Ω U k ln Ω N = k ln Ω U Ω N = k ln 10 n

8 Clicker Question #2 Estimate S for a protein 100 amino-acid residues long. A) J/K B) J/K C) J/K D) J/K k = J/K

9 Entropy Change for Unfolding S = k ln 10 n = J/K ln ln = 100 ln 10 S = J/K 100 ln J/K

10 Entropy Change for Protein Unfolding From the previous slides: S = k ln 10 n n is the number of amino acid residues. Assumes 1 conformation for the native state and 10 conformations for each residue in the unfolded state. On a molar basis for n = 100 S = R ln = J/(mol K) ln = J/(mol K) Because it only considers polypeptide conformation, call this S conf. Corresponding free energy change at 298 K: T S conf = J/mol = 570 kj/mol Compare with the overall free energy change for unfolding, on the order of 30 kj/mol

11 Could We Measure the Conformational Entropy Change for Unfolding a Protein? Recall reversible isothermal compression of a gas: Weight Weight Weight Thermal Reservoir Thermal Reservoir Thermal Reservoir To measure entropy: Measure work required for reversible process, w rev. Since E = 0 and E = q + w, q rev = w rev S sys = q rev /T = w rev /T. For a protein, measure the work to go from the unfolded ensemble to a single conformation (at constant temperature).

12 Stretching an Unfolded Protein Entropically, the single stretched-out conformation is equivalent to the single folded conformation. Folded Unfolded ensemble Single conformation Stretched conformation is equivelent to unfolded ensemble, with respect to stabilizing interactions. Have to stretch very slowly, to ensure reversibility. Have to measure very small forces as a function of distance. w rev = Fdx

13 An Atomic Force Microscope (AFM) Usual purpose is to make images of surfaces. Very fine tip (a few nm in radius) held on flexible cantilever. Sample is scanned below probe. Movement of cantilever is monitored optically. Cantilever can be calibrated to measure force as a function of displacement. (spring constant)

14 Stretching an Unfolded Protein with AFM Stage is moved very slowly, as deflection of cantilever is monitored. Deflection represents force as a function of distance. Force integrated over distance gives w rev. S conf = w rev /T This experiment is anti-trivial! Results are consistent with calculation based on rotational isomers. Thompson, J. B., Hansma, H. G., Hansma, P. K. & Plaxco, K. W. (2002). J. Mol. Biol., 322,

15 Observed Thermodynamics for Protein Folding For a typical single-domain protein of 100 amino-acid residues at room temperature (300 K): G u : 5 kj/mol to 50 kj/mol H u : 0 kj/mol to 200 kj/mol S u : G u = H u T S u S u = H u G u T = 100 kj/mol 30 kj/mol 300 T S u = 230 J/(mol K)

16 Observed Thermodynamics for Protein Folding For our (hypothetical) example at room temperature (300 K): Measured experimentally for unfolding: G u = 30 kj/mol H u = 100 kj/mol S u = 230 J/(mol K) Estimated change in conformational entropy: S conf = J/(mol K) T S conf = 570 kj/mol What we need to explain: Why is S u S conf? Why is G u T S conf?

17 Thermodynamics of Transfer of a Non-polar Molecule to Water CH 4 CH 4 Methane in a non-polar solvent Methane in Water G tr = H tr T S tr G tr is positive because S tr is negative! (an entropically driven process). Water molecules become more ordered when a non-polar molecule is introduced. Non-polar groups buried in the interior of folded proteins become exposed to water on unfolding.

18 Transfer Free Energy versus Accessible Surface Area 10 8 Hydrocarbons Nonpolar amino-acid sidechains Polar amino-acid side chains Figure adapted from: F. M. Richards. Areas, volumes, packing and protein structure. Annu. Rev. Biophys. Bioeng., 6: ,

19 Thermodynamics of Non-polar Surface Transfer to Water At 300 K H tr = A np 7 J/mol S tr = A np 0.3 J/(mol K) T S tr = A np 90 J/mol G tr = A np 97 J/mol A np : Non-polar surface area (Å 2 ) transferred from non-polar environment to water. How does the surface area exposed to water change when a protein unfolds? Estimates are from transfer measurements summarized in: Baldwin, R. L. (1986). Proc. Natl. Acad. Sci., USA, 83, and Spolar, R. S., Livingstone, J. R. & Record, T. M. (1992). Biochemistry, 31,

20 Folded Structure of a Small Protein: Ribonuclease A

21 Solvent-accessible Surface of Folded Ribonuclease A

22 Solvent-accessible Surface of Unfolded Ribonuclease A (one representative conformation)

23 Change in Accessible Surface Area for Unfolding Folded (Å 2 ) Unfolded (Å 2 ) Difference (Å 2 ) Total 7,000 14,700 7,700 Non-polar 3,800 8,800 5,000 Polar 3,200 5,900 2,700

24 Thermodynamic Consequence of Non-polar Surface Area Exposed Upon Unfolding (Hydrophobic Effect) For 5, 000 Å 2 at 300 K H hyd = 35 kj/mol S hyd = 1, 500 J/(mol K) G hyd = 480 kj/mol

25 Contributions to Protein Unfolding Thermodynamics For protein of 100 amino-acid residues at 300 K: H kj/mol S J/(mol K) G kj/mol Conformational entropy 2, Hydrophobic effect 35-1, Other Overall Increase in conformational entropy is largely compensated for by decrease in water entropy associated with hydrophobic effect. What might other contributions to H be? Breaking protein hydrogen bonds. Exposure of polar surface area to water.