Role of cations in stability of acidic protein Desulfovibrio desulfuricans apoflavodoxin

Apoflavodoxin from the sulfate reducing bacteria Desulfovibrio desulfuricans is a small, acidic protein with a net charge of −19 at neutral pH. Here, we show that monovalent cations in biologically relevant amounts have dramatic effects on apoflavodoxin stability. The effect is largest for Gdm+ and decreases as a function of increased cation charge density (Gdm+ > NH4+ ⩾ K+ ∼ Cs+ ∼ Na+ > Li+). A linear correlation of stabilizing effects with cation hydration properties suggests an important role of dehydration in efficient cation interaction with the protein. The effects on stability are due to preferential binding of one cation to native apoflavodoxin and results in an increase in thermal midpoint of 20 °C and the free energy of unfolding (at 20 °C) increases fivefold. Tuning of biophysical properties (such as folding and ligand/cofactor binding) of acidic proteins by cation binding may be important in vivo.