Question

describe the chemical nature of the non-covalent bonding interactions between peptides and lipids.

Answer 1

There are integral proteins which are buried in the lipid membrane of the cell. These lipids are made up of mainly phospholipids. Phospholipids contain two long chain of fatty acids which are non polar. As cell membrane is bilayered, the interior of the cell membrane is completely non polar in nature. The integral protein which passed through this membrane has to interact with the fatty acid chains (non polar) of membrane. Therefore, this integral protein has hydrophobic amino acid residues in the region which passes through the membrane. As hydrophobic residue is also non polar in nature, they can easily interact with the non polar fatty acid chain of the membrane by hydrophobic interactions.

The hydrophobic interaction is not a force between two non polar moieties. When non polar molecules mix with polar solvent like water, they break the hydrogen bonds formed between the water molecules, which takes up the energy form the system and, require the surrounding to provide the energy. However, non polar molecules do not provide any compensation for the loss of energy. This causes gain in enthalpy (change in enthalpy is positive) of water molecules.The water molecules surround the non polar molecules in highly ordered manner. This causes the loss in entropy of water molecules, which means change in entropy is negative.

As we know that, delta G = delta H – T x delta S

In above scenario delta S (change in entropy) is negative, and delta H (change in enthalpy) is positive. Therefore, delta G (change in Gibb’s free energy) is positive, which is unfavorable. Hence, water tries to minimize the interaction with non polar molecules like fatty acid chain of cell membrane.

When two non polar molecules interact with water individually, the net surface area which is interacting with water is very high. However, when two non polar molecules associate with each other the net surface area which is exposed to the surrounding is minimized, and hence, the water gets least non polar area to interact with. Thus, association of hydrophobic/non polar molecules is favored in polar solvent like water. This is the basis of hydrophobic interaction. This is how, non polar residues of protein associates with non polar fatty acid chain of cell membrane by hydrophobic interaction.