Metal ions in biological catalysts
The role of inorganic elements in enzymes is discussed. The first point of note is that strong retention of metal ions by a protein is limited to certain metal ions, mainly copper, zinc and molybdenum. A further group of metals can be retained but largely in coenzymes, iron, cobalt, nickel and magnesium. Many aquated metal ions are in intermediate or fast exchange, iron, manganese, magnesium, and calcium. The last group of ions act as controls as well as catalysts. In the light of the limitations and value of proteins as ligands for metal ion catalytic centres, the functional value of different metals is described under n—acids, a—acids, oxida— tion state ranges and potentials. It is concluded that biological systems have evolved so as to bring out the optimal value of metals in catalysis. This explains the use of different metals in very different catalytic steps and the nature of the peculiarities of the sites selected for each metal.