The melting temperature of a human protein is 33 °C in 100 mM sodium
phosphate buffer at pH 6.5. What should the predominant conformation of the
protein in the same buffer at physiological temperature conditions be?
disordered
α helical
β sheet
Mixture of alpha helix and beta sheet.
Physiological temperature for humans is around 37°C, which exceeds the protein’s melting temperature (Tm) of 33°C in 100 mM sodium phosphate buffer at pH 6.5. The predominant conformation is disordered because proteins unfold above their Tm, losing native structure.
Understanding Protein Melting Temperature
Protein melting temperature, or Tm, marks the midpoint where half the protein population transitions from folded (native) to unfolded (denatured) states. Below Tm, thermodynamic stability favors the folded form with functional secondary structures like helices or sheets. Above Tm, entropy drives unfolding into a disordered random coil, as hydrophobic interactions and hydrogen bonds weaken. This two-state model applies here, predicting >50% unfolded at 37°C.
Explanation of All Conformation Options
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Disordered: Correct choice. At 37°C > 33°C Tm, the protein predominantly unfolds into a flexible, non-native random coil lacking stable secondary elements, impairing function. This matches biophysical definitions where elevated temperature disrupts stabilizing forces.
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α helical: Incorrect. Alpha helices are stabilized secondary structures requiring intra-chain hydrogen bonds, prevalent in native folded proteins below Tm, but absent post-unfolding.
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β sheet: Incorrect. Beta sheets form via inter-strand hydrogen bonds in native states, but thermal denaturation above Tm eliminates these, yielding no sheets.
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Mixture of alpha helix and beta sheet: Incorrect. Native proteins often feature mixed helices and sheets, but this describes folded conformations stable only below Tm; at physiological temperature, unfolding prevents any mixture.
