Calcium ions staphylococcal nuclease

In metalloproteins, the metal cofactor may be removed and replaced by a heavier atom with similar chemistry. The zinc in insulin was successfully replaced by cadmium or lead, the zinc in carboxypeptidase and carbonic anhydrase by mercury, the zinc in thermolysin by lanthanide ions or strontium or barium and the calcium in staphylococcal nuclease by barium. Success has most usually been achieved by soaking the crystals in a chelating agent and subsequently diffusing in the heavier atom. 

Cotton, F. A., Hazen, E. E., Jr., and Legg, M. J. (1979). Staphylococcal nuclease Proposed mechanism of action based on structure of enzyme-thymidine 3, 5 -bisphosphate-calcium ion complex at 1.5-A resolution. Proc. Natl. Acad. Sci. U.S.A. 76, 2551-2555. 

RA. Cotton, E.E. Elazen, M.J. Legg, Staphylococcal Nuclease - Proposed Mechanism of Action Based on Structure of Enzyme-Thymidine 3, 5 -Bisphophate-Calcium ion Complex at 1.5 A Resolution , Proc. Natl. Acad. Sci. USA, 76, 2551 (1979)... 

Figure 2. Schematic drawing of the active site of staphylococcal nuclease. Protein side chains are shown by light bonds, while the PdTp molecule is in dark. The Ca ion is shown as the large sphere below the inhibitor molecule. Also shown are the three inner sphere water ligands of the calcium ion and the water molecule bridging Glu-43 and the 5 -phosphate of the inhibitor (this bridging water is the putative nucleophile in the hydrolysis of phosphoesters) (from ref. 1).

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