METAL COORDINATION OF SEVERAL DIVALENT CATIONS: Mg2+,Ca2+,Be2+, AND Zn2+. Jenny P. Glusker+, Amy Kaufman Katz+, Charles W. Bock[[daggerdbl]], The Institute for Cancer Research, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111, USA (+), The Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 19144, USA ([[daggerdbl]])
We have examined the coordination preferences of the divalent ions Mg2+, Ca2+, Be2+, and Zn2+ by analyses of crystal structures in the Cambridge Structural Database (CSD)1, the Protein Databank (PDB)2, and ab initio molecular orbital calculations that provide energetics data on the binding of water molecules to these cations.
Our findings are that magnesium has a firm coordination number of 6 with an overwhelming preference for water or other oxygen-containing ligands.3 Beryllium4 has the same preference for oxygen, but with a coordination number of 4. The energy consequences of changing the coordination number of Mg2+ from 6 to any other number are high. Zinc ions, with the same size charge, are softer, and also bind sulfur, and nitrogen in addition to oxygen. Their coordination number can readily vary between 4 and 6 with little energetic penalty. Therefore zinc is better able than magnesium to mediate chemical reactions in enzymes.5 Calcium ions appear to bind with a higher coordination number and values of 7-9 are observed in crystal structures. Oxygen is the preferred ligand. There is not much energetic change between Ca2+ with 7 or 8 bound water molecules.6
In protein crystal structures magnesium ions tend to bind two water molecules (ideal for displacement by a binding substrate in an enzyme), while calcium more often binds only one water molecule. The geometry of binding of enzymatic carboxylate groups is also different between calcium and magnesium in that calcium may share both oxygen atoms of the carboxylate group while magnesium only binds one. The results of these surveys will be illustrated.
Supported by grant CA-10925 from National Institutes of Health.
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6. Katz A. K., Bock, c. w., Glusker J. P. J. Amer. Chem. Soc. (in press)