Suppose that you are a hydrophobic protein with a certain configuration of 10 su

Question

Suppose that you are a hydrophobic protein with a certain configuration of 10 surface polar groups. You have just been synthesized in the cell cytoplasm. In order to enter a membrane and attain a lower free-energy state you must give up the water molecules H-bonded to your surface polar groups at a total cost to you of 50 kcal/mol. In the mitochondrial membrane there are hydrophobic structural protein surfaces with complementary configurations of polar groups that will allow your surface polar groups to reform H-bonds. In the ER, there are only hydrophobic proteins with non-complementary configurations of polar groups. At your temperature the net negative free-energy change you can bring about when you enter either membrane and increase the entropy of the aqueous phase by removing your hydrophobic surface from it is 48 kcal/mol.

A) Ignoring the small positive delta H of transferring your hydrophobic groups from the aqueous to the lipid phase is it energetically feasible for you to go into the ER? How many kcal/mol will you gain/lose by doing so?

B) How many kcal/mol will you gain/lose by going into the mitochondrial membrane?

Explanation / Answer

The energy of a hydrogen bond depending on the distance between the donor and acceptor and the angle between them is in the range of 2kcal/mol to 10kcal/mol.

A. Transferring the hydrophobic protein group from the aqueous to the lipid phase is energetically feasible as they are not soluble in water and would interact with water molecules and there is the gain in free energy. Since the distance and angle is negligible and not given. The hydrophobic polar end of protein will gain about 2kcal/mol free energy.

B. While the protein enters the mitochondrial membrane there is no loss or gain or energy as protein up to 10,000 daltons can pass through the mitochondrial membrane.

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