Binding of Cu2+ by 1-carboxymethylhistidine-119-ribonuclease was investigated by using diligand metal ion buffers. A single Cu2+-binding site was found over the Cu2+ concentration range studied. The binding constants for this site were 8·33×105 (±2%)m−1 and 1·57×104 (±6%)m−1 at pH7·0 and 6·1 respectively. An estimate of the pH-independent Cu2+-binding constant suggests that the most avid Cu2+-binding site has disappeared after carboxymethylation. This is consistent with an earlier report that binding of Cu2+ at the most avid site is associated with the loss of enzymic activity.
