A protein can fold into three different conformations. A) If the states are iso-

Question

A protein can fold into three different conformations.

A) If the states are iso-energetic, and the system has 1 mole of protein equally mixed between the three states, what is the entropy change of forcing it all into a single confirmation on a per mole basis? (This might occur for example during the process of crystallization.)

B) Instead imagine that states B and C have a +1 kbT and +2 kbT energy increase compared to state A, respectively. What is the probability of finding the protein in each of the states?

C) For the system in part (B), now what’s the entropy change of forcing the equilibrium mixture all to state A?

Explanation / Answer

The spatial arrangement of atoms in a protein is called a conformation. The term conformation refers to a structural state that can, without breaking any covalent bonds, interconvert with other structural states. A change in conformation could occur, for example, by rotation about single bonds. Of the innumerable conformations that are theoretically possible in a protein containing hundreds of single bonds, one generally predominates. This is usually the conformation that is thermodynamically the most stable, having the lowest Gibbs' free energy (G). Proteins in their functional conformation are called native proteins.

The covalent backbone of proteins is made up of hundreds of individual bonds. If free rotation were possible around even a fraction of these bonds, proteins could assume an almost infinite number of threedimensional structures. Each protein has a specific chemical or structural function, however, strongly suggesting that each protein has a unique three-dimensional structure.

The simple fact that proteins can be crystallized provides strong evidence that this is the case. The ordered arrays of molecules in a crystal can generally form only if the molecular units making up the crystal are identical.

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