Question

A particulara-helix is known to cross the cell membrane. Which of these amino acids is most likely to be found in the transmembrane portion of the helix? (A) Glutamate (B) Lysine (C) Phenylalanine (D) Aspartate

Short answer

Phenylalanine (C).

Step-by-step solution

- Understanding Transmembrane Helices

Transmembrane helices usually span the hydrophobic central part of the cell membrane. This means that they typically contain non-polar amino acids, which interact favorably with the lipid bilayer.

- Analyze Each Amino Acid

Check the properties of each listed amino acid: (A) Glutamate - Polar, negatively charged (B) Lysine - Polar, positively charged (C) Phenylalanine - Non-polar, aromatic (D) Aspartate - Polar, negatively charged

- Identify the Most Likely Amino Acid

Since transmembrane portions are typically non-polar, the amino acid must be non-polar or hydrophobic. Of the list, Phenylalanine (C) is the non-polar amino acid.

Key concepts

hydrophobic amino acids

Hydrophobic amino acids play a vital role in proteins that interact with the cell membrane. These amino acids have non-polar side chains that do not interact well with water, making them hydrophobic. Instead, they prefer to associate with other non-polar molecules and environments.
In transmembrane helices, hydrophobic amino acids are crucial because they can interact favorably with the lipid bilayer of the cell membrane. This lipid bilayer is also hydrophobic, making it an ideal environment for these amino acids to reside.
Some key examples of hydrophobic amino acids include:

• Leucine
• Isoleucine
• Valine
• Phenylalanine

Recognizing these amino acids helps understand why they are often found in membrane-spanning regions of proteins.

cell membrane

The cell membrane is a protective barrier that surrounds the cell. It is primarily composed of a double layer of phospholipids, which create a hydrophobic (water-repelling) environment in its core. This membrane structure is essential for separating the cell's internal contents from the external environment.
Within the cell membrane, proteins can span across from one side to the other. These proteins are often involved in transporting molecules and signaling.
Transmembrane helices are parts of these proteins that traverse the cell membrane. The hydrophobic central region of the membrane is key to understanding which amino acids are found in these helices. Hydrophobic, non-polar amino acids are favored because they can interact well with the lipid bilayer, making the helix stable within the membrane.

amino acid properties

Amino acids are the building blocks of proteins. Each amino acid has specific properties that determine how it interacts with its environment and other molecules.
Amino acids can be categorized based on the properties of their side chains. They can be:

• Non-polar (hydrophobic)
• Polar (hydrophilic)
• Positively charged (basic)
• Negatively charged (acidic)

For example:

• Glutamate and Aspartate are negatively charged and polar.
• Lysine is positively charged and polar.
• Phenylalanine is non-polar and hydrophobic.

Understanding these properties is essential for predicting how proteins like transmembrane helices will behave in different environments, such as the hydrophobic core of the cell membrane.

non-polar interactions

Non-polar interactions are crucial for the stability and function of proteins within the cell membrane. These interactions occur between non-polar (hydrophobic) amino acids and the hydrophobic lipid bilayer.
In a transmembrane helix, non-polar amino acids are strategically positioned to engage in these interactions, allowing the helix to anchor within the cell membrane. This is because the hydrophobic central region of the membrane repels water but accommodates non-polar molecules.
Examples of non-polar interactions include:

• Van der Waals forces between hydrophobic side chains
• Hydrophobic interactions that drive the folding of proteins

These non-polar interactions ensure that transmembrane helices remain stable and functional, contributing to the overall integrity of membrane proteins.