FIGURE 4.

A, residues observed after 30 min of H/D exchange in calcium-free VhACP A75H are shown in gold, while additional residues observed in the Ca²⁺-bound protein are shown in blue. B, H/D exchange rates as observed in Ca²⁺-bound (black) and Ca²⁺-free (gray) VhACP A75H, with lower values indicative of slower exchange. Note the vastly slower rates of the Ca²⁺-bound protein in the helical regions. Residues denoted by stars indicate peaks were present in too few spectra to reliably determine any rates. Binding of Mg²⁺ has a similar effect on the H/D exchange rates as Ca²⁺ (not shown).