[Campbell Biology P.131] The 'MAKE CONNECTIONS' section prompts us to consider the chemical ... | Practice Question

The 'MAKE CONNECTIONS' section prompts us to consider the chemical characteristics of valine and glutamic acid. Given that normal glutamic acid is replaced by valine in sickle-cell hemoglobin, and observing the 'Function' column for sickle-cell hemoglobin, what is the most likely chemical reason for the aggregation of sickle-cell hemoglobin proteins into fibers?

  • A: Glutamic acid is hydrophobic, and its replacement by hydrophilic valine causes protein aggregation.
  • B: Valine is a larger amino acid than glutamic acid, leading to steric hindrance and aggregation.
  • C: Valine is hydrophobic, and its substitution for the hydrophilic glutamic acid exposes a hydrophobic patch, leading to hydrophobic interactions and aggregation.
  • D: The substitution leads to the formation of new disulfide bonds, causing the proteins to clump together.

Explanation

Glutamic acid is a hydrophilic, negatively charged amino acid. Valine, on the other hand, is a hydrophobic amino acid. When hydrophilic glutamic acid is replaced by hydrophobic valine at the surface of the hemoglobin protein, this creates a hydrophobic patch. These hydrophobic patches on different hemoglobin molecules tend to interact with each other to avoid water, leading to the aggregation of hemoglobin into long fibers, as described in the 'Function' column for sickle-cell hemoglobin.