Question

A transmembrane protein in the plasma membrane is glycosylated at two sites in the polypeptide sequence. Where in this protein would you expect these two sites to be? a. in transmembrane segments b. in hydrophilic regions that project into the extracellular environment c. in hydrophilic regions that project into the cytosol d. could be anywhere e. b and c only

Short answer

The correct answer to this problem is e. The glycosylation sites could be in hydrophilic regions that project into the extracellular environment or into the cytosol.

Step-by-step solution

Understanding Protein Structure

Recall that a transmembrane protein has different regions. It has a region that spans across the membrane, called transmembrane segments. Other parts of the protein are present on either side of the membrane and may be hydrophilic (water-attracting) or hydrophobic (water-repelling). The environment on either side of the transmembrane differs. One side projects into the extracellular environment and the other projects into the cytosol, which is the fluid inside the cell.

Applying Knowledge on Glycosylation

Glycosylation typically occurs in the hydrophilic regions of a protein. This is due to the nature of the carbohydrate group that is attached, as they are generally polar and hydrophilic.

Determining Glycosylation Sites

Having deduced that the glycosylation happens in hydrophilic regions, apply this to the transmembrane protein. The protein will generally have these hydrophilic regions exposed to environments that are compatible with such groups. These are the regions that project into the extracellular environment and into the cytosol. Therefore, the glycosylation sites could be in these hydrophilic regions of the protein.

Key concepts

Protein Structure

In the world of biology, proteins are crucial workhorses that perform a myriad of functions. A transmembrane protein, specifically, is an intriguing type of protein that spans the entirety of a cell membrane. It has distinct regions with specialized roles:

• Transmembrane segments: These are the parts of the protein that reside within the lipid bilayer of the membrane. These segments are typically insoluble in water (hydrophobic).
• Hydrophilic regions: Found on either side of the transmembrane segments, these regions are water-attracting and tend to interact with the watery environments outside and inside the cell.

Understanding the structure of a transmembrane protein is like unraveling a biological puzzle, where each piece (segment) has a distinct role in the protein's overall function.

Hydrophilic Regions

Hydrophilic regions of proteins are areas that are attracted to water. These regions typically contain amino acids with side chains that are polar, meaning they can form hydrogen bonds with water molecules. In a transmembrane protein, the hydrophilic regions are crucial:

• They ensure the protein maintains its proper orientation in the membrane.
• Allow for interactions with molecules that are water-soluble, such as ions and nutrients.

Because these regions love water, they are frequently the sites where modifications like glycosylation occur, making them essential for cellular communication and protein stability.

Extracellular Environment

The extracellular environment is the space outside the cell. It provides a rich milieu of molecules and signals that interact with cells.

In the context of a transmembrane protein, the extracellular-facing hydrophilic regions are often involved in important processes like:

• Signal reception: Proteins may act as receptors connecting to the outside signals.
• Cell communication: They engage in communication with neighboring cells.

This interaction is often facilitated by glycosylation—where carbohydrate groups are added to proteins, enhancing their ability to communicate and function effectively in the human body.

Cytosol

The cytosol is the fluidic environment inside the cell. Think of it as the bustling city within which cellular components float and function. For a transmembrane protein, having regions that interact with the cytosol is crucial:

• They help maintain protein orientation within the membrane.
• Enable interactions with intracellular molecules crucial for cellular processes.

Hydrophilic regions extending into the cytosol are also potential sites for glycosylation. This process is vital for intracellular processes like protein trafficking and signaling.

Plasma Membrane

The plasma membrane is a dynamic and complex barrier that encases the cell. It consists mainly of a lipid bilayer with embedded proteins, allowing selective interaction between the cell and its external environment. Within this structure:

• Transmembrane proteins serve as gateways and communicative bridges.
• Both hydrophobic and hydrophilic regions of these proteins allow them to interact effectively with the membrane's lipid and aqueous environments, respectively.

Its unique composition of lipids and proteins enables it to perform various roles such as regulating the entry and exit of substances, facilitating communication, and maintaining the internal environment of the cell.