Electrostatics in Macromolecular Solutions - Food Colloids: Self-Assembly and Material Science - page 134

Table 2 Shift in the stoichiometric calcium-binding constant K s as a function of
salt concentration c s for the chelator BAPTA. The case of 2 mM salt
has been taken as the reference point, and the shifts are calculated
relative this value

D pK Exp
s

DpK Sim
s

DpK DH
s

c s (mM)

2 0.00 0.00 0.00

10 0.26 0.32 0.32

25 0.64 0.60 0.59

50 0.89 0.85 0.84

100 1.20 1.12 1.11

300 1.58 1.58 1.59

500 1.77 1.79 1.81

1000 1.97 2.05 2.09

Figure 3 Comparison of experimental and theoretical-binding constant shifts for the
calcium-binding protein calbindin D 9k . The electrostatic interactions have been
modified by adding salt in the range 2-150 mM and by mutating (neutralizing)
charge residues in the protein. 10 Spheres are simulated data and squares are
calculated using the TK approach. Filled symbols denote the addition of KCl
and open symbols the addition of K 2 SO 4 . The dashed line corresponds to perfect
agreement. The shifts are calculated relative to the native protein at 2 mM salt
concentration

particle. That is, when the charge reaches a certain level, the electrostatic
response is no longer linear, but rather it approaches an asymptotic value. This
means that the binding becomes 'saturated', and, e.g., a further increase of
negatively charged residues in the protein does not lead to an increased binding
of calcium.