Metal-Ion-Specific Screening of Charge Effects in Protein Amide H/D Exchange and the Hofmeister Series

In this study, protein charge ladders and mass spectrometry were used to quantify how metal cations in the Hofmeister series (Na+, K+, Li+, Mg2+, and Ca2+) permute the effects of lysine acetylation on the rate of amide H/D exchange in a representative protein (myoglobin, Mb). The successive acetylation of up to 18 Lys-epsilon-NH3(+) groups in Mb caused a linear decrease in its global rate of amide H/D exchange (as measured by mass spectrometry), despite also decreasing the thermostability of Mb by >10 degrees C. The ability of a metal cation to screen kinetic electrostatic effects during H/D exchange-and to abolish the protective effect of acetylation against H/D exchange-was found to depend on the position of the cation in the Hofmeister series. Na+ and K+ cations did not fully equalize the rates of H/D exchange among each "rung" of the charge ladder, whereas Mg2+ and Ca2+ did equalize rates without eliminating the hydrophobic core of the protein (i.e., without unfolding Mb); Li+ exhibited intermediate effects. The ability of Mg2+ and Ca2+ to completely screen electrostatic effects associated with the H/D exchange of charge isomers of Mb suggests that Mg2+ or Ca2+ (but not Na+ or K+) can be used to quantify the magnitude by which electrostatic charge contributes to the observed rates of amide H/D exchange in proteins.