Question

Protein molecules are usually quite large and yet often water-soluble. They are soluble in water because they can fold themselves into spherical shapes in which the polar (hydrophilic) side chains are on the surface and the nonpolar (hydrophobic) side chains are buried inside the sphere. Of the amino acids Ala, Ser, Phe, lle, Arg, and Asp, (a) Which would you expect to find on the surface of a water-soluble protein? (b) Which would you expect to find buried deep inside the protein structure? (c) Which might be in either location depending on the solution pH?

Short answer

(a) Ser, Arg, and Asp would be expected to be found on the surface of a water-soluble protein. (b) Ala, Phe, and Ile would be expected to be buried deep inside the protein structure. (c) Asp and Arg might be in either location depending on the solution pH.

Step-by-step solution

Categorize amino acids based on their polarity

First, let's identify the polar and nonpolar amino acids from the given set. This classification is based on properties of amino acid side chains: 1. Polar (hydrophilic) amino acids: Ser, Arg, and Asp 2. Nonpolar (hydrophobic) amino acids: Ala, Phe, and Ile Now let's move on to answering the questions.

Amino acids on the surface of a water-soluble protein

(a) Polar (hydrophilic) amino acids are likely to interact with water molecules and are therefore expected to be found on the surface of water-soluble proteins. So, we can expect Ser, Arg, and Asp to be on the surface of a water-soluble protein.

Amino acids buried inside the protein structure

(b) Nonpolar (hydrophobic) amino acids tend to avoid interaction with water molecules, thus will likely be buried deep inside the protein structure. As a result, we would expect Ala, Phe, and Ile to be found buried deep inside the protein structure.

Amino acids location depending on solution pH

(c) Depending on the solution pH, the charge of the amino acid side chains can change, affecting their properties. For example, the side chains of Asp and Arg are ionizable, and their charges may change with the pH of the solution. Hence, Asp and Arg might be found in either location within a protein structure depending on the pH of a solution. In conclusion, based on the properties of the amino acids - (a) Ser, Arg, and Asp would be expected to be found on the surface of a water-soluble protein. (b) Ala, Phe, and Ile would be expected to be buried deep inside the protein structure. (c) Asp and Arg might be in either location depending on the solution pH.

Key concepts

Protein Structure

Proteins play a critical role in biological processes, and their structures define their functions. The structure of a protein is organized into four levels. Primary structure refers to the sequence of amino acids in the polypeptide chain. Secondary structure involves patterns like \( \alpha \-helices \) and \( \beta \-sheets \) that arise from hydrogen bonding. Tertiary structure is the overall three-dimensional shape of a single protein molecule. Lastly, quaternary structure describes the arrangement of multiple polypeptide chains into a single complex.

Understanding how proteins fold into these structures provides insights into diseases where protein misfolding is a factor, such as Alzheimer's. To function properly, every protein must fold into the correct structure, which is determined by the interactions between its amino acids.

These interactions result from the properties of amino acids and determine the final shape of the protein, which is crucial to its specific biological role.

Polar and Nonpolar Amino Acids

Amino acids, the building blocks of proteins, can be categorized based on the chemical nature of their side chains. Polar amino acids, such as serine (Ser), arginine (Arg), and aspartate (Asp), have side chains that can form hydrogen bonds with water, making them hydrophilic or water-loving.

Nonpolar amino acids, including alanine (Ala), phenylalanine (Phe), and isoleucine (Ile), have hydrocarbon side chains that do not interact favorably with water but instead prefer associating with other nonpolar substances. This distinction between polar and nonpolar amino acids is critical for understanding why certain amino acids are found on the surfaces of proteins or tucked away in their cores.

Amino Acid Solubility

The solubility of amino acids is largely determined by their polarity. Polar amino acids are generally soluble in aqueous, or water-based, environments due to their ability to form hydrogen bonds or ionic interactions with water molecules. This characteristic means they often populate the outer regions of proteins in an aqueous environment.

In contrast, nonpolar amino acids are typically less soluble in water due to their lack of charge and inability to form hydrogen bonds. They are more compatible with lipid environments, leading to their presence in the inner regions of proteins, away from water. The solubility of amino acids can also be influenced by the pH of the solution, affecting the ionization state of certain amino acids.

Protein Folding

Protein folding is a complex process by which a protein structure assumes its functional shape or conformation. It is intricately linked to the amino acid composition of a protein; hydrogen bonding, hydrophobic interactions, ionic bonds, and disulfide bridges all contribute to the final folded structure. Folding is not a random event; it follows a specific pathway to reach the lowest-energy, most-stable conformation.

Proper folding is essential for protein functionality. Misfolded proteins can lead to aggregates that are often associated with diseases. Environmental factors like temperature and pH can influence protein folding. An understanding of how proteins fold helps in designing therapeutic drugs and in the study of genetic diseases caused by protein misfolding.