Kinetic and thermodynamic aspects of cellular thiol-disulfide redox regulation.

Regulation of intracellular thiol-disulfide redox status is an essential part of cellular homeostasis. This involves the regulation of both oxidative and reductive pathways, production of oxidant scavengers and, importantly, the ability of cells to respond to changes in the redox environment. In the cytosol, regulatory disulfide bonds are typically formed in spite of the prevailing reducing conditions and may thereby function as redox switches. Such disulfide bonds are protected from enzymatic reduction by kinetic barriers and are thus allowed to exist long enough to elicit the signal. Factors that affect the rate of thiol-disulfide exchange and stability of disulfide bonds are discussed within the framework of the underlying chemical foundations. This includes the effect of thiol acidity (pK(a)), the local electrostatic environment, molecular strain, and entropy. Even though a thiol-disulfide exchange reaction is thermodynamically favorable, it will only take place if the activation energy to form the transition state complex can be overcome. This is accomplished by enzymes, such as the oxidoreductases, that direct reactions in thermodynamically favorable directions by decreasing the activation energy barrier.