A mathematical model for metal affinity protein partitioning

A mathematical model of metal affinity partitioning has been derived and used to describe protein partitioning in Cu (II)PEG/dextran systems. A working model has been extended to account for inhibition, which for metal affinity extraction is the inhibition of protein‐metal binding by hydrogen ion. PEG/dextran partitioning experiments were performed on four proteins, tuna heart cytochrome c, Candida krusei cytochrome c, horse myoglobin, and sperm whale myoglobin. The partition coefficients for these proteins are increased by the addition of Cu (II)PEG‐IDA, due to the affinity between the chelated copper atom and metal‐coordinating histidine residues on the protein surface. The results of experiments to determine the effects of the number of binding sites on the protein, the copper concentration, and pH on partitioning are all well‐described by the mathematical model. The pK_a value of the metal binding site was determined to be 6.5, which is in the range of pK_a values commonly observed for surface histidines. The average association constant for the binding of Cu (II)PEG‐IDA to accessible histidines was found to be 4.5 × 10^3. This value is comparable to stability constants measured by conventional potentiometry techniques for analogous small complexes.