Question

All of the following are ways in which peripheral proteins bind to membranes except A. binding to an integral protein. B. electrostatic binding between phospholipids and positive groups on the protein. C. by a short hydrophobic group at one end of the protein. D. attached by the charged carboxyl group at the carboxyl terminus of the protein. E. binding non-covalently to membrane phosphatidylinositol.

Short answer

Answer: C) By a short hydrophobic group at one end of the protein

Step-by-step solution

Option A: Binding to an integral protein

Peripheral proteins can associate with membranes by binding to integral proteins that span the membrane. Therefore, this option is a valid way for peripheral proteins to bind to membranes.

Option B: Electrostatic binding between phospholipids and positive groups on the protein

Peripheral proteins can bind to membranes by interacting electrostatically with the negatively charged phospholipids in the membrane. This creates a stable interaction, allowing the peripheral protein to associate with the membrane. Therefore, this option is a valid way for peripheral proteins to bind to membranes.

Option C: By a short hydrophobic group at one end of the protein

Peripheral proteins are typically hydrophilic, so it would be unusual for them to contain a hydrophobic group at one end. Moreover, the presence of a hydrophobic group in a protein would suggest that it is an integral membrane protein rather than a peripheral protein. Therefore, this option is NOT a valid way for peripheral proteins to bind to membranes.

Option D: Attached by the charged carboxyl group at the carboxyl terminus of the protein

Peripheral proteins can bind to membranes by interacting with membrane components through charged amino acid residues, such as the carboxyl group at the carboxyl terminus. Therefore, this option is a valid way for peripheral proteins to bind to membranes.

Option E: Binding non-covalently to membrane phosphatidylinositol

Peripheral proteins can bind non-covalently to membrane lipids, such as phosphatidylinositol, allowing them to associate with the membrane surface. Therefore, this option is a valid way for peripheral proteins to bind to membranes. In summary, the correct answer is option C, as it is not a valid way for peripheral proteins to bind to membranes.

Key concepts

Integral Membrane Proteins

Integral membrane proteins are essential components of the phospholipid bilayer found in cellular membranes. They differ significantly from peripheral proteins, mainly in their location and manner of association with the membrane. Integral proteins are embedded within the membrane itself and often span across it, serving as channels, receptors, or enzymes.

Due to their placement, they have unique structural features, including hydrophobic regions that interact with the fatty acid tails of the phospholipids, anchoring them firmly within the membrane. This attribute makes them distinct, as peripheral proteins lack these hydrophobic regions and instead associate more loosely with the membrane, often through interactions with integral proteins or membrane lipids.

Electrostatic Interactions

Electrostatic interactions are a type of non-covalent bonding that play a crucial role in the binding of proteins, such as peripheral proteins, to membranes. These interactions occur due to the attraction between oppositely charged regions on the protein and the membrane's phospholipids.

For example, the negatively charged phosphate groups in the phospholipid bilayer can attract and form stable interactions with positively charged amino acid side groups on a protein. This form of interaction emphasizes the importance of the protein's three-dimensional structure, as the surface charge distribution is vital for the protein to associate with the membrane effectively.

Phospholipid Membrane Association

The phospholipid membrane association entails how proteins, such as enzymatic or signaling molecules, attach to the cell's membrane. The primary structure of a cell's membrane is made up of a phospholipid bilayer, which has a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail.

Proteins can interact with the phospholipids in various ways, including hydrophobic interactions, electrostatic forces, and binding to specific lipid components like phosphatidylinositol, as mentioned in the exercise. This flexibility allows proteins to fulfill multiple roles and respond dynamically to cellular signals without permanently integrating into the membrane structure, as integral proteins do.

Hydrophobic Group in Proteins

Proteins can possess hydrophobic groups, which are parts of the molecule that preferentially associate with other non-polar substances like the hydrophobic tails of phospholipids. In the context of membrane proteins, these hydrophobic groups allow for the stable integration into the lipid bilayer.

Integral membrane proteins typically contain one or more stretches of hydrophobic amino acids that can span the membrane and interact with its core. In contrast, peripheral proteins usually lack these hydrophobic regions, which leads to different modes of membrane association that do not involve integration into the hydrophobic core of the bilayer.