Triose phosphate isomerase catalyzes the following reaction:

Glyceraldehyde-3-Phosphate (G3P) <=> cis-enediol intermediate <=> Dihydroxyacetone Phosphate (DHAP)

As the reaction is written, consider G3P as the substrate and DHAP as the product. The enediol intermediate is unstable and normally has a much more positive free energy than either G3P or DHAP.

The active enzyme is a dimer of two identical subunits. Each has the configuration shown in Figure 11.9a. The active site (the place on the enzyme where catalysis occurs) can accommodate either G3P or DHAP (Figure 11.9b). At the active site, a glutamate residue (Glu 165) and a histidine (His 95) are essential for function of the enzyme. Steps in the process of binding and catalysis, as they relate to these residues, are shown here. After binding, an enzyme "lid" closes down on the substrate to provide a cage that protects the enediol intermediate. In the absence of the lid, the enediol intermediate is lost and catalytic efficiency decreases by a factor of 100,000. Conversion of Glu 165 to Asp retains the negative charge, but reduces efficiency by a factor of 1000. The reaction can be broken down into the following steps:

E + G3P <=> E-G3P (Binding of G3P)

E-G3P <=> E-ed (Conversion to enediol)

E-ed <=> E-DHAP (Conversion to DHAP)

E-DHAP <=> E + DHAP (Release of DHAP)

Like other enzymes, triose phosphate isomerase lowers the energy barriers of the transition states (Figure 11.10).

Triose phosphate isomerase is an extremely efficient enzyme. (See here).