Temperature Sensitive Mutations

Intoroduction:-

 These are mutations that decrease the stability of a protein so that the denaturation temperature is near 40°C. A single methylene group (–CH2–) involved in a hydrophobic interaction may contribute as much as -1.5 to -2 kcal/mol to the stability of a protein that is only stable by -10 kcal/mol. A single hydrogen bond might contribute as much as -1.5 to -3.5 kcal/mol. If a mutation disrupts interactions that stabilize the protein, the protein may be made just unstable enough to denature near body temperature. It might strike you as strange that we were talking earlier about how hydrogen bonds didn’t contribute much to the net stability of proteins and now I’m telling you they contribute -1.5 to -3.5 kcal/mol. Both statements are more or less right.

What we’re talking about now is messing up a protein by changing one amino acid for another by mutation. Here we’re destroying an interaction that’s present in the intact, folded protein. For any hydrogen-bonded group in the folded protein, there must be a complementary group. A donor must have an acceptor, and vice versa. Making a mutation that removes the donor of a hydrogen bond leaves the acceptor high and dry, missing a hydrogen bond.

Structure:-

 In the unfolded protein, the deserted acceptor can be accommodated by water; however, in the folded protein the loss of the donor by mutation hurts. It costs a hydrogen bond when the protein folds. The result: a loss in stability for the protein. Loss in stability means that the protein will denature at a lower temperature than before. Temperature-sensitive mutations usually arise from a single mutation’s effect on the stability of the protein.

 Temperature-sensitive mutations make the protein just unstable enough to unfold when the normal temperature is raised a few degrees. At normal temperatures , the protein folds and is stable and active. However, at a slightly higher temperature  the protein denatures and becomes inactive. The reason proteins unfold over such a narrow temperature range is that the folding process is very cooperative—each interaction depends on other interactions that depend on other interactions.

Analaysis:-

 For a number of temperature-sensitive mutations it is possible to find a seond mutation in the protein that will suppress the effects of the first mutation. For example, if the first mutation decreased the protein stability by removing a hydrogen-bond donor, a second mutation that changes the acceptor may result in a protein with two mutations that is just as stable as the native protein. The second  mutation is called a suppressor mutation.