Development of a scale for the comparison of metals in enzyme action

Formation constants (log K-1) for 26 ligands: with 16 metals of biochemical interest were compared to formation constants for EDTA. A correlation was found for each ligand suggesting a possible means for comparing metals. Experimental and calculated formation constants for phosphate, acetate, and ammonia were used to develop a set of constants to characterize the interaction of metals with these ligands. The constants were designated metal complex (MC) constants. Zn was selected as the reference metal and its value was set to 1.00. Values of the MC constants were in the order: K<Na<Li<Ba less than or equal to Sr<Mg less than or equal to Ca<Mn<Fe<Co<Zn<Al<Cd less than or equal to Ni<Cu<Fe(III), similar to the Irving-Williams series. The MC constants were found to statistically fit formation constants for the other ligands and other properties of metals and metal-ligand complexes. Correlation coefficients were no worse than 0.82 with no p values greater than 0.004. Examples of chemical and enzymatic reactions catalyzed by metals were found to be correlated by the MC scale, particularly well for transition metals. The use of these constants in comparing the activation of enzymes is discussed. (C) 1999 Elsevier Science Inc. All rights reserved.