Enolase
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  Rotate the molecule to observe the colored 2-phospho glycerate molecule.  This is the reactant which will be converted to phosphoenolpyruvate.  Essential to this reaction is the presence of two Mg2+ atoms. These magenta atoms are the magnesium atoms. Zoom in on the binding pocket.
The asp246, glu295, and asp320 residues coordinate with one of the magnesium atoms.  The other magnesium atom coordinates with one of the carboxylate oxygen of the substrate/product, ser39, and two water molecules.
It is believed that the presence of the magnesium atoms stabilizes the negative charge of the carbanion intermediate. These two amino acids are proposed by some to be the two main catalytic residues involved in the forward reaction which converts 2-phosphoglycerate to phosphoenolpyruvate.
The epsilon-amino group of lys345 serves as the catalytic base and the glu211 interacts with the 3-OH on 2-phosphoglycerate.
Now to observe the product (phosphoenolpyruvate) type: Now the phosphoenolpyruvate molecule is visible in the binding pocket.
 
Primary Citation:

Larsen, T. M., Wedekind, J. E., Rayment, I., Reed, G. H.: A carboxylate oxygen of the substrate bridges the magnesium ions at the active site of enolase: structure of the yeast enzyme complexed with the equilibrium mixture of 2-phosphoglycerate and phosphoenolpyruvate at 1.8 A resolution.. Biochemistry 35 pp. 4349 (1996)
 
 

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