Macmillan Learning
Temperature (°C)
Suppose that a Met residue buried in the folded, hydrophobic core of T4 lysozyme is replaced by mutation with a Lys residue.
How would the Met to Lys mutation affect the thermal denaturation of lysozyme at pH 3.0?
? It will not affect stability, but it will decrease the $T_m$ and shift the curve to lower temperatures.
? It will destabilize the protein and lower the $T_m$, shifting the curve to the left.
? It will stabilize the protein and raise the $T_m$, shifting the curve to higher temperatures.
? It will not change the $T_m$ at pH 3.0 because the Lys side chain $pK_a$ is 10.53.
Suppose that the thermal denaturation experiment on the protein with the Met to Lys mutation took place at pH 10.0.
Would the mutation have a greater or lesser impact on stability at pH 10.0 compared to pH 3.0?
? lesser, because a less acidic environment contributes to general protein stability
? lesser, because a greater fraction of Lys can remain buried in the core at pH 10.0
? no change, because the Lys side chain $pK_a$ is above 10.0 so the side chain remains charged
? greater, because high pH values destabilize proteins
