Questions are based on the following passage: The homodimeric DNA-binding protein Ssh10b, from Sulfolobus shibattae, is conserved in the majority of currently sequenced thermophilic and hyperthermophilic archaeal genomes. Denaturation of Ssh10b by urea, a chaotropic agent, was tested at five different pH values (3, 5, 7, 9, and 10) and eight different NaCl salt concentrations (0 M [black squares], 0.01 M [open squares], 0.02 M [black circles], 0.05 M [open circles], 0.1 M [black triangles], 0.2 [open triangles], 0.5 M [inverted black triangles], and 1 M [inverted open triangles]). The... Show more

Questions are based on the following passage:

The homodimeric DNA-binding protein Ssh10b, from Sulfolobus shibattae, is conserved in the majority of currently sequenced thermophilic and hyperthermophilic archaeal genomes. Denaturation of Ssh10b by urea, a chaotropic agent, was tested at five different pH values (3, 5, 7, 9, and 10) and eight different NaCl salt concentrations (0 M [black squares], 0.01 M [open squares], 0.02 M [black circles], 0.05 M [open circles], 0.1 M [black triangles], 0.2 [open triangles], 0.5 M [inverted black triangles], and 1 M [inverted open triangles]).

The results are shown in Figure 1.

The structure of urea is shown in Figure 2.

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MCAT Biological and Biochemical Foundations of Living Systems: Passage 1

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4 Questions

1. In the presence of salt, what effect does pH have on the unfolding free energy (∆G) of Ssh10b?

The unfolding ∆G increases as pH increases.The unfolding ∆G decreases at low pH and increases at high pH.The unfolding ∆G increases as pH decreases.The unfolding ∆G increases at low pH and decreases at high pH.

2. The pKa values for four buffers at room temperature are shown in the table.

Which buffer would be most appropriate for obtaining the denaturation curves displayed in Figure 1, panel c?

Sodium acetateGlycine-HClHEPESAmmonium acetate

3. Based on the results obtained, what effect do the tested salt concentrations have on protein stability?

At the tested pH, increased salt concentration consistently increases protein stability.At high pH, increased salt concentration results in increased protein stability, but at low pH, protein stability is independent of salt concentration.At the tested pH, increased salt concentration consistently decreases protein stability.At low pH, increased salt concentration results in increased protein stability, but at high pH, protein stability is independent of salt concentration.

4. What is the most likely mode of protein denaturation by urea, based on its structure?

Promotion of peptide bond hydrolysis to break the amino acid chainDisruption of crosslink bonds between monomersDisruption of the hydrogen-bonding network in the proteinBreakage of disulfide bonds within the tertiary structure of the protein