Malic enzyme regulatory properties

In view of these problems with Ni2+, Mna+ has been used as a probe for Mg2+ with some success. However, it should be noted that there is a difference in radius which may be manifested in different biochemical behaviour (Mn2+, 0.80 A Mg2+, 0.65 A). Thus Mn2+ has been used to probe the Mg2+ site in pyruvate kinase.95 While the Mg2+-activated enzyme is inhibited by Ca2+ and Li+, the Mn2+-activated enzyme is inhibited by Ca2+ and not by Li+. There are also differences in the catalytic and regulatory properties of the NAD+-specific malic enzyme of E. coli,104 depending upon whether the divalent activator is Mn2+ or Mg2+. It is necessary, therefore, to express a cautionary note. These two cations may act in slightly different ways to bring about a similar final result. In the second example it appears that the metal cofactors stabilize two different conformational states of the enzyme. 

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