Mechanism
The mechanism involves the intermediate formation of an "enediol". The changes in free energy for each step, including the transition states, have been calculated, and are displayed in the figure.
The structure of TPI facilitates the conversion between dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP). The nucleophilic glutamate 165 residue of TPI deprotonates the substrate, and the electrophilic histidine 95 residue donates a proton to form the enediol intermediate. When deprotonated, the enediolate then collapses and, abstracting a proton from protonated glutamate 165, forms the GAP product. Catalysis of the reverse reaction proceeds analogously, forming the same enediol but with enediolate collapse from the oxygen at C2.
TPI is diffusion-limited. In terms of thermodynamics, DHAP formation is favored 20:1 over GAP production. However, in glycolysis, the use of GAP in the subsequent steps of metabolism drives the reaction toward its production. TPI is inhibited by sulfate, phosphate, and arsenate ions, which bind to the active site. Other inhibitors include 2-phosphoglycolate, a transition state analog, and D-glycerol-1-phosphate, a substrate analog.
Read more about this topic: Triosephosphate Isomerase
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