Magnesium-utilizing enzymes

Figure 7. A metal ion-water-carboxylate motif found in many magnesium-utilizing enzymes. This motif probably helps to orient the fairly rigid coordination octahedron of magnesium.

Magnesium is bound in the active site of D-xylose isomerase (Carrell et al., 1984, 1989 Farber et al., 1987 Key etal., 1988 Henrick a/., 1989). Here, the site at which two metals [from among Mg(II), Mn(ll), and Co(II)] bind is similar to that found for Fe(ll) in ribonucleotide reductase (Nordlund et al., 1990). The active site of xylose isomerase is shown in Fig. 28. Magnesium ions are preferred in ATP-utilizing enzyme reactions (Mildvan, 1987). 

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 phosphofhictokinase, 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 pyravate kinase, described earlier, and shown in Figure 1, requires both magnesium and potassium ions for maximal activity. 

The model has numerous short-comings . Chiefly, it fails to account for the fact that U- and R-enzymes frequently have other phosphoryl-acceptor and -donor substrates, and the model does not explain how fluctuations in these cosubstrate concentrations may alter the so-called responsiveness of enzymes. Moreover, the concentration of orthophosphate is wholly omitted from the model, despite the fact that key regulatory steps in ATP utilization and regeneration are exquisitely sensitive to changes in orthophosphate concentration. The energy-charge model also fails to consider magnesium ion com-... 

Preliminary rate measurements should allow one to make a plot of initial velocity Vq versus [metal ion], and this should provide information on the optimal metal ion concentration. (For many MgATP -dependent enzymes, the optimum is frequently 1-3 mM uncomplexed magnesium ion.) Then, by utilizing pubhshed values for formation constants (also known as stability constants) defining metal ion-nucleotide complexation, one can readily design experiments to keep free metal ion concentration at a fixed level. To compensate properly for metal ion complexation in ATP-dependent reactions, one must chose a buffer for which a stability constant is known. For example, in 25 mM Tris-HCl (pH 7.5), the stability constant for MgATP is approximately 20,000 M Thus, one can write the following equation ... 

The justification for these constituents can be seen in the structures of the primary metabolites of fungi. Many metabolic processes utilize phosphates, often with a magnesium ion. It has been estimated that a quarter to a third of all enzymes contain a metal ion as a functional component. Iron is particularly important in the cytochrome P450 oxidases which are involved in the biosynthesis of many fungal metabolites. Other metal ions, particularly zinc, play a... 

---