Experimental Investigation of Initial Steps of Helix Propagation in Model Peptides

Abstract

It is not certain whether the helix propagation parameters s(n)() (i.e., the equilibrium constants between (n - 1)- and n-residue long alpha-helices) determined from numerous studies of rather long model peptides are applicable for description of the initial steps of the helix formation during the protein folding process. From fluorescence, NMR, and calorimetric studies of a series of model peptides, containing the La(3+)-binding sequence nucleating the helix (Siedlecka, M., Goch, G., Ejchart, A., Sticht, H., and Bierzynski, A. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 903-908), we have determined, at 25 degrees C, the average values of the enthalpy DeltaH(n)() and of the helix growth parameters s(n)() describing the first four steps of helix propagation in polyalanine. The absolute values of the C-cap parameters, describing the contribution of the C-terminal residues to the helix free energy, have also been estimated for alanine (1.2 +/- 0.5) and NH(2) group (1.6 +/- 0.7). The initial four steps of the helix growth in polyalanine can be described by a common propagation parameter s = 1.54 +/- 0.04. The enthalpy DeltaH(n)() is also constant and equals -980 +/- 100 cal mol(-)(1).

Energetics of Protein Folding

“…64 Titration calorimetry shows that alanine helix formation is enthalpy-driven. The value for ΔH is − 3.8( ± 0.4) kJ/mol per residue over the temperature range 5°C to 45°C [65][66][67] and ΔCp is too small to measure. Helix enthalpy data have been obtained recently for several amino acids by Makhatadze and co-workers.…”

Enthalpy of helix–coil transition: Missing link in rationalizing the thermodynamics of helix-forming propensities of the amino acid residues

“…3. (24,26,32). Because La 3ϩ binding leads to the folding of the P2X peptides, the La 3ϩ -binding affinities of the peptides will include contribution from the helix formation, thus allowing estimates of the thermodynamic helix propensity scale.…”

“…4 and 5 assume a two-state process. There is an indication that in a peptide system similar, but not identical, to P2X, there is some fraying at the helix end; however, it appears that the correction due to fraying is rather small, Ͻ5% (32). The values of F ␣ Ϫ (P2X) that are used in Eq.…”

Weak Interactions in Protein Folding: Hydrophobic Free Energy, van der Waals Interactions, Peptide Hydrogen Bonds, and Peptide Solvation

“…When their predictions are compared with values for an alanine peptide helix, the results are numerically different (not surprising, in view of the structural difference) but qualitatively the same. Ben-Tal et al omit the loss in translational and rotational entropy for forming the NMA dimer and their DG value of À5 kJ mol À1 [122] may be compared with the measured DH (À4.0 kJ mol À1 [123,124]) for forming the alanine peptide helix.…”

“…In principle, Scheme 6.2 may be adapted to predict the enthalpy change for forming an alanine peptide helix, whose experimental value is known [123,124], by assigning provisional values to the different steps.…”