Which of the following amino acids is most likely to be found on the outside of a soluble protein, and which of them is more likely to be found on the inside? Explain each answer. (Hint: Consider the effect of the amino acid side chain in each case and that the protein is folded up into its globular form.)
a. Valine

Verified step by step guidance

1. Step 1: Understand the context of the problem. Proteins in their globular form fold in such a way that hydrophilic (water-attracting) amino acid side chains are typically found on the outside of the protein, where they can interact with water, while hydrophobic (water-repelling) side chains are usually buried on the inside, away from water.
2. Step 2: Analyze the amino acid in question, valine. Valine is a nonpolar amino acid with a hydrophobic side chain consisting of a branched alkyl group (isopropyl group). This makes valine less likely to interact with water.
3. Step 3: Apply the principle of hydrophobicity. Since valine has a hydrophobic side chain, it is more likely to be found on the inside of a soluble protein, where it can avoid contact with the aqueous environment.
4. Step 4: Consider the overall structure of the protein. The folding of the protein into its globular form is driven by the hydrophobic effect, which minimizes the exposure of hydrophobic residues like valine to water, stabilizing the protein structure.
5. Step 5: Conclude that valine is more likely to be found on the inside of a soluble protein due to its hydrophobic nature, and explain that hydrophilic amino acids (not valine) would be more likely to be found on the outside, interacting with the aqueous environment.

Key Concepts

Amino Acid Properties

Amino acids have distinct properties based on their side chains (R groups), which can be classified as hydrophobic, hydrophilic, acidic, or basic. Hydrophobic amino acids, like valine, tend to avoid water and are usually found in the interior of proteins, while hydrophilic amino acids are more likely to be on the exterior, interacting with the aqueous environment.

Protein Folding

Proteins fold into specific three-dimensional shapes, a process driven by the interactions between amino acid side chains. This folding results in a globular structure where hydrophobic residues are buried inside to minimize exposure to water, while hydrophilic residues are exposed on the surface, facilitating interactions with the surrounding environment.

Solubility of Proteins

The solubility of a protein in water is influenced by the distribution of hydrophilic and hydrophobic amino acids. Soluble proteins typically have a higher proportion of hydrophilic amino acids on their surface, allowing them to interact favorably with water, while hydrophobic amino acids are sequestered within the protein's core.