Reaction of triosephosphate isomerase with L-glyceraldehyde 3-phosphate and triose 1,2-enediol 3-phosphate.

Triosephosphate isomerase catalyzes the isomerization and/or racemization reactions of L-glyceraldehyde 3-phosphate (LGAP), the enantiomer of the physiological substrate. The reaction is inhibited by the active site directed reagent glycidol phosphate. The amount of protonation product formation catalyzed by a fixed enzyme concentration is nearly independent of increasing steady-state concentrations of triose 1,2-enediol 3-phosphate caused by buffer catalysis of LGAP deprotonation. Therefore, enzymatic protonation of the enediol or enediolate, which could account for the observed enzymatic catalysis of LGAP isomerization and/or racemization, is at best a minor reaction. Instead LGAP reacts directly at the enzyme active site. Triosephosphate isomerase catalysis of the protonation of triose 1,2-enediol 3-phosphate was expected because of the strong evidence supporting an enediol reaction intermediate for the overall reaction catalyzed by isomerase. The most reasonable explanation for the failure to observe enzymatic protonation is that in solution the enediol undergoes beta elimination of phosphate (t 1/2 is estimated to be 10(-6) s) faster than it can diffuse to and form a complex with isomerase.