Pyruvate kinase magnesium ions

K ion is a cofactor for the aldolase reaction. Pyruvic kinase from all known sources also requires univalent cations, in addition to a divalent cation. This enzyme needs K, ammonium, or Rb ions as a cofactor. Mg ions serve an important function in photosynthetic processes in tobacco as it is an essential constituent of chlorophyll a and chlorophyll b. Some heavy metal and nonmetal ions are toxic to tobacco and can serve as metabolic inhibitors. The toxicity of fluoride, for example, is explained in part on the basis of the formation of a magnesium-fluorphosphate complex that inhibits the eno-lase reaction in glycolysis. Other enzymes are inhibited by substrate analogs, sulfhydryl complexing agents, and metal chelating agents. 

The function of magnesium in enzyme activity may either be to form a complex with the substrate, as in the magnesium-ATP complex formed in creatine kinase and phosphofructokinase, or to bind to the enzyme and either produce an allosteric activation or play a direct role in catalysis. If an enzyme is known to utilize a nucleotide as one of its substrates, it can be assumed that magnesium is also required for catalysis. The magnesium ion possibly acts as an electrostatic shield. The enzyme pyruvate kinase, described earlier, and shown in Figure 1, requires both magnesium and potassium ions for maximal activity. 

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