165. A protein sequence has twenty consecutive hydrophobic amino acids followed by a few hydrophilic residues and this pattern repeats itself five to seven times. Which kind of a protein is described here?
A. A secretory protein
B. A transmembrane protein
C. A cytoplasmic soluble enzyme
D. A ribosomal protein

Introduction Transmembrane Proteins Based on Their Sequence Patterns

Proteins exhibit diverse structures and functions within a cell, and one way to classify them is by their sequence composition. The sequence of amino acids in a protein often gives insight into its location and function. When examining a protein sequence with a specific pattern of hydrophobic and hydrophilic amino acids, it can help in predicting the type of protein it might be. In this case, we are dealing with a protein sequence consisting of consecutive hydrophobic amino acids followed by hydrophilic residues, repeating multiple times.

Characteristics of Hydrophobic and Hydrophilic Amino Acids

• Hydrophobic amino acids are nonpolar and tend to avoid water, making them likely to be located in the interior of proteins or embedded within membranes.

• Hydrophilic amino acids are polar and interact well with water, often found on the exterior of proteins or in aqueous environments.

Understanding the Sequence Pattern

The sequence described in the question contains:

• Twenty consecutive hydrophobic amino acids followed by a few hydrophilic residues.

• This pattern repeats five to seven times.

Such a pattern is a typical feature of transmembrane proteins, which span across biological membranes.

Analysis of the Options

1. A. A secretory protein

   • Incorrect. Secretory proteins are typically water-soluble and not embedded in membranes. They usually do not have long hydrophobic stretches like those described in the sequence. Secretory proteins are designed to be secreted outside the cell or into organelles, not to cross membranes.

2. B. A transmembrane protein

   • Correct. Transmembrane proteins have regions composed of hydrophobic amino acids that help anchor the protein into the lipid bilayer of the membrane. The hydrophobic stretches are typically organized in alpha-helical structures that span the membrane. The repeated pattern of hydrophobic residues followed by hydrophilic residues is characteristic of transmembrane proteins, as the hydrophilic regions interact with the aqueous environment inside and outside the membrane.

3. C. A cytoplasmic soluble enzyme

   • Incorrect. Cytoplasmic soluble enzymes are usually water-soluble proteins found in the cytoplasm. They do not typically have long hydrophobic regions, as they function in the aqueous environment of the cytosol.

4. D. A ribosomal protein

   • Incorrect. Ribosomal proteins are involved in protein synthesis and are generally associated with the ribosome, not the membrane. They typically do not have the hydrophobic stretches described in the sequence.

Conclusion

The correct answer is:

B. A transmembrane protein

This is because transmembrane proteins are known to have hydrophobic regions that interact with the lipid bilayer, allowing them to span the membrane. The sequence described matches the typical characteristics of a transmembrane protein, where hydrophobic amino acids are aligned to interact with the membrane, and hydrophilic regions are exposed to the aqueous environments on either side.