Triosephosphate Isomerase Has Approached Evolutionary Perfection

Triosephosphate isomerase catalyzes the interconver- sion of dihydroxyacetone phosphate and 3-phospho-glyceraldehyde. 

The interconversion of the two triosephosphates is an essential step in the catabolism of carbohydrates (see chapter 12). Examining the catalytic mechanism of triosephosphate isomerase is instructive. It is among the enzymes that ap- 

The probable reaction mechanism of triosephosphate isomerase. The y-carboxylate group of Glu 165 acts as a general base to remove a proton from C-l of the substrate, dihydroxyacetone phosphate (DHAP). This generates a planar ene-diolate intermediate that has two 

Additional support for this mechanism comes from experiments in which the enzymatic reaction is run in D20. Deuterium is incorporated stereospecifically onto carbon 2 of the product. 

The hydrogen-bonding pattern in the crystal and studies of the pH dependence of the reaction suggest that the histidine and lysine residues in the active site do not act as general acids in the catalytic pathway but rather serve to stabilize the negative charge on an ene-diolate intermediate by electrostatic effects (see fig. 8.21). 

---