Figure 4. Factors which determine susceptibility to thiol modification and cellular thiol targets.

Thiol residues have different susceptibilities to being modified by thiol-reactive agents. One important factor is accessibility (within the cell as within a protein) and the pK_a value of the thiol target. (A) The pH of the environment is different, depending on the subcellular location. As shown, the average pH of the mitochondrial matrix is 8–8.5, whereas lysosomes are much more acidic, averaging a pH less than 5.5. The pH, along with the thiol pK_a value determines whether a thiol is deprotonated to form thiolate anion. (B) The local protein environment is a very important determinant of thiol reactivity. For example, an inaccessible, high pK_a protein thiol would be considered the least prone to modification. However, a low pK_a accessible thiol would be a highly sensitive target.